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  • 1.
    Abidin, Aysajan
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Pacher, Christoph
    Austrian Institute of Technology, Austria.
    Lorünser, Thomas
    Austrian Institute of Technology, Austria.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Peev, Momtchil
    Austrian Institute of Technology, Austria.
    Quantum cryptography and authentication with low key-consumption2011In: Proceedings of SPIE - The International Society for Optical Engineering, 2011, p. 818916-Conference paper (Refereed)
    Abstract [en]

    Quantum Key Distribution (QKD - also referred to as Quantum Cryptography) is a technique for secret key agreement. It has been shown that QKD rigged with Information-Theoretic Secure (ITS) authentication (using secret key) of the classical messages transmitted during the key distribution protocol is also ITS. Note, QKD without any authentication can trivially be broken by man-in-the-middle attacks. Here, we study an authentication method that was originally proposed because of its low key consumption; a two-step authentication that uses a publicly known hash function, followed by a secret strongly universal2 hash function, which is exchanged each round. This two-step authentication is not information-theoretically secure but it was argued that nevertheless it does not compromise the security of QKD. In the current contribution we study intrinsic weaknesses of this approach under the common assumption that the QKD adversary has access to unlimited resources including quantum memories. We consider one implementation of Quantum Cryptographic protocols that use such authentication and demonstrate an attack that fully extract the secret key. Even including the final key from the protocol in the authentication does not rule out the possibility of these attacks. To rectify the situation, we propose a countermeasure that, while not informationtheoretically secure, restores the need for very large computing power for the attack to work. Finally, we specify conditions that must be satisfied by the two-step authentication in order to restore informationtheoretic security.

  • 2.
    Aguilar, Edgar A.
    et al.
    Univ Gdansk, Poland.
    Farkas, Mate
    Univ Gdansk, Poland.
    Martinez, Daniel
    Univ Concepcion, Chile.
    Alvarado, Matias
    Univ Concepcion, Chile.
    Carine, Jaime
    Univ Concepcion, Chile.
    Xavier, Guilherme B
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering. Univ Concepcion, Chile.
    Barra, Johanna F.
    Univ Concepcion, Chile.
    Canas, Gustavo
    Univ Bio Bio, Chile.
    Pawlowski, Marcin
    Univ Gdansk, Poland.
    Lima, Gustavo
    Univ Concepcion, Chile.
    Certifying an Irreducible 1024-Dimensional Photonic State Using Refined Dimension Witnesses2018In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 120, no 23, article id 230503Article in journal (Refereed)
    Abstract [en]

    We report on a new class of dimension witnesses, based on quantum random access codes, which are a function of the recorded statistics and that have different bounds for all possible decompositions of a high-dimensional physical system. Thus, it certifies the dimension of the system and has the new distinct feature of identifying whether the high-dimensional system is decomposable in terms of lower dimensional subsystems. To demonstrate the practicability of this technique, we used it to experimentally certify the generation of an irreducible 1024-dimensional photonic quantum state. Therefore, certifying that the state is not multipartite or encoded using noncoupled different degrees of freedom of a single photon. Our protocol should find applications in a broad class of modern quantum information experiments addressing the generation of high-dimensional quantum systems, where quantum tomography may become intractable.

  • 3.
    Andersson, Conny
    Linköping University, Department of Electrical Engineering, Vehicular Systems. Linköping University, Faculty of Science & Engineering.
    Design of the Modelica Library VehProLib with Non-ideal Gas Models in Engines2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This thesis covers the reconstruction and the redesign of the modeling library VehProLib,which is constructed in the modeling language Modelica with help of the modeling toolWolfram SystemModeler. The design choices are discussed and implemented. This thesisalso includes the implementation of a turbocharger package and an initial study of the justificationof the ideal gas law in vehicle modeling. The study is made with help of Van derWaals equation of states as a reference of non-ideal gas model. It will be shown that for themean-value-engine-model, the usage of ideal gas law is justified.

  • 4.
    Andersson, Mike
    Linköping University, Department of Physics, Chemistry and Biology, Applied Physics . Linköping University, The Institute of Technology.
    SiC based field effect sensors and sensor systems for combustion control applications2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Increasing oil prices and concerns about global warming have reinforced the interest in biofuels for domestic and district heating, most commonly through combustion of solid biomass like wood logs, hog fuel and pellets. Combustion at non-optimal conditions can, however, lead to substantial emissions of noxious compounds like unburned hydrocarbons, carbon monoxide, and nitrogen oxides as well as the generation of soot.

    Depending on the rate of combustion more or less air is needed per unit time to completely oxidize the fuel; deficiency of air leading to emissions of unburned matter and too much of excess air to slow combustion kinetics and emissions of mainly carbon monoxide. The rate of combustion is influenced by parameters like fuel quality – moisture and ash content etc. – and in what phase the combustion takes place (in the gas phase through combustion of evaporated substances or on the surface of char coal particles), none of which is constant over time.

    The key to boiler operation, both from an environmental as well as a power to fuel economy point of view, is thus the careful adjustment of the air supply throughout the combustion process. So far, no control schemes have been applied to small-scale combustors, though, mainly due to the lack of cheap and simple means to measure basic flue gas parameters like oxygen, total hydrocarbon, and carbon monoxide concentrations.

    This thesis reports about investigations on and characterization of silicon carbide (SiC) based Metal Insulator Semiconductor (MIS) field effect gas sensors regarding their utility in emissions monitoring and combustion control applications as well as the final development of a sensor based control system for wood fired domestic heating systems.

    From the main sensitivity profiles of such sensor devices, with platinum (Pt) and iridium (Ir) as the catalytic metal contacts (providing the gas sensing ability), towards some typical flue gas constituents as well as ammonia (NH3), a system comprising four individual sensors operated at different temperatures was developed, which through the application of Partial Least Squares (PLS) regression, showed good performance regarding simultaneous monitoring of propene (a model hydrocarbon) and ammonia concentrations in synthetic flue gases of varying content. The sensitivity to CO was, however, negligible. The sensor system also performed well regarding ammonia slip monitoring when tested in real flue gases in a 5.6 MW boiler running SNCR (Selective Non-Catalytic reduction of nitrogen oxides with ammonia).

    When applied to a 200 kW wood pellet fuelled boiler a similar sensor system was, however, not able to follow the flue gas hydrocarbon concentration in all encountered situations. A PCA (Principal Components Analysis) based scheme for the manipulation of sensor and flue gas temperature data, enabling monitoring of the state of combustion (deficiency or too much of excess air), was however possible to develop. The discrepancy between laboratory and field test results was suspected and later on shown to depend on the larger variation in CO and oxygen concentrations in the flue gases as compared to the laboratory tests.

    Detailed studies of the CO response characteristics for Pt gate MISiC sensors revealed a highly non-linear sensitivity towards CO, a large response only encountered at high CO/O2 ratios or low temperatures. The response exhibits a sharp switch between a small and a large value when crossing a certain CO/O2 ratio at constant operating temperature, correlated to the transition from an oxygen dominated to an almost fully CO covered Pt surface, originating from the difference in adsorption kinetics between CO and O2. Indications were also given pointing towards an increased sensitivity to background hydrogen as being the mediator of at least part of the CO response. Some general characteristics regarding the response mechanism of field effect sensors with differently structured metal contacts were also indicated.

    The CO response mechanism of Pt metal MISiC sensors could also be utilized in developing a combustion control system based on two sensors and a thermocouple, which when tested in a 40 kW wood fired boiler exhibited a good performance for fuels with extremely low to normal moisture content, substantially decreasing emissions of unburned matter.

    List of papers
    1. Investigations on the possibilities of a MISiCFET sensor system for OBD and combustion control utilizing different catalytic gate materials
    Open this publication in new window or tab >>Investigations on the possibilities of a MISiCFET sensor system for OBD and combustion control utilizing different catalytic gate materials
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    2004 (English)In: Topics in catalysis, ISSN 1022-5528, E-ISSN 1572-9028, Vol. 30-31, no 1, p. 365-368Article in journal (Refereed) Published
    Abstract [en]

    Different catalytic materials, like Pt and Ir, applied as gate contacts on metal insulator silicon carbide field effect transistors — MISiCFET—facilitate the manufacture of gas sensor devices with differences in selectivity, devices which due to the chemical stability and wide band gap of SiC are suitable for high temperature applications. The combination of such devices in a sensor system, utilizing multivariate analysis/modeling, have been tested and some promising results in respect of monitoring a few typical exhaust and flue gas constituents, in the future aiming at on board diagnostics (OBD) and combustion control, have been obtained.

    Keywords
    platinum, iridium, sensor system, field effect sensors, SiC, OBD, combustion control, car exhaust, flue gases
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-13092 (URN)10.1023/B:TOCA.0000029776.18603.74 (DOI)
    Available from: 2008-04-01 Created: 2008-04-01 Last updated: 2017-12-13Bibliographically approved
    2. On the applicability of MISiCFET gas sensors regarding ammonia slip monitoring in small-scale boilers running SNCR
    Open this publication in new window or tab >>On the applicability of MISiCFET gas sensors regarding ammonia slip monitoring in small-scale boilers running SNCR
    2007 (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    Gas sensitive Metal Insulator Silicon Carbide Field Effect Transistor – MISiCFET – devices have shown good possibilities of realizing sensors for high temperature applications. One such application could be the monitoring of ammonia slip from boilers running SNCR – Selective Non-Catalytic Reduction of nitrogen oxides (NOx) with ammonia (NH3). In this study a number of MISiCFET gas sensors operated at different temperatures and with both platinum (Pt) and iridium (Ir) as the gate contact have been tested for their ability to detect and quantify ammonia slip in flue gases from a 5.6 MW wood fired boiler during a test of a new SNCR-system. The individual sensors have been evaluated and compared to each other for the sensitivity towards NH3 and possible cross-sensitivities to other flue gas species through the comparison of the sensor signals with the signals from analytical instruments like FTIR – Fourier Transform Infrared spectroscopy – for nitrogen oxides (NO + NO2), NH3, and carbon monoxide (CO) and an FID – Flame Ionization Detector – for the Total Hydrocarbon Concentration (THC). The ability of a combination of sensors to provide extra or more accurate information about the NH3 concentration was also evaluated through the construction and validation of a Partial Least Squares – PLS – regression model based on all the sensor signals. Under the assumption that the sensors’ responses follow a logarithmic dependence on NH3 concentration the results regarding  ammonia slip quantification were promising both for a single Ir sensor and for the system based on all sensors. There is still a question mark for the long-term stability of the devices in real flue gases, however.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-13093 (URN)
    Available from: 2008-04-01 Created: 2008-04-01 Last updated: 2016-09-01Bibliographically approved
    3. SiC-FE gas sensor based combustion control in domestic heating systems, characteristics and utility
    Open this publication in new window or tab >>SiC-FE gas sensor based combustion control in domestic heating systems, characteristics and utility
    Show others...
    2007 (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    SiC based field effect gas sensors have been evaluated for future possible use in combustion control schemes for domestic heating systems. Emphasis has been on the possibility to monitor the state of combustion and follow the development of the combustion process from an emissions point of view and to determine its cause. The sensor signals have been compared to true emissions data – CO and total hydrocarbon concentration – as obtained by an IR spectrometer and a flame ionization detector (FID) as well as flue gas concentration of oxygen as obtained by a paramagnetic cell. The sensor characteristics have been evaluated using multivariate statistics and the results suggest that, by using the signals from one or more SiC-based field effect sensors and a thermocouple, it seems possible to provide a rough picture of the state of combustion applicable to a control scheme in order to reduce emissions and increase the power to fuel economy.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-13094 (URN)
    Available from: 2008-04-01 Created: 2008-04-01 Last updated: 2016-09-01Bibliographically approved
    4. On the CO response mechanism of SiC based field effect gas sensors
    Open this publication in new window or tab >>On the CO response mechanism of SiC based field effect gas sensors
    (English)Manuscript (Other academic)
    Abstract [en]

    The response characteristics of Metal Insulator Silicon Carbide (MISiC) field effect sensor devices, with platinum (Pt) as the metal contact, towards carbon monoxide (CO) at varying oxygen (O2) concentrations and over a wide range of temperatures have been investigated in detail at atmospheric pressure. The influence of hydrogen (H2) on the CO response was also studied. Sensor devices with thin, porous as well as dense, homogeneous Pt films on top of both silicon dioxide (SiO2) and magnesium oxide (MgO) as insulator materials were investigated in this study. The reaction products generated on the sensor surfaces were also monitored with a mass spectrometer connected to the gas flow just downstream of the sensor location and the results compared to CO oxidation characteristics over Pt/SiO2 and to some extent Pt/MgO catalysts as reported in literature. By correlating the response characteristics of these devices with CO2 formation and hydrogen consumption on the sensor surfaces, strong indications for a CO response mechanism involving a CO induced increased sensitivity to background hydrogen have been obtained, this mechanism being hypothesized to be the only one behind the CO sensitivity of devices with dense Pt metal contacts. The results also give further support to the idea that also other processes than an increased sensitivity to background hydrogen contribute to the CO response of sensor devices with a porous platinum film as the metal contact, one candidate being the removal of oxygen anions from the surface of exposed oxide areas through the oxidation reaction with CO.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:liu:diva-13095 (URN)
    Note

    This manuscript was never submitted to a journal and will not be published.

    Available from: 2008-04-01 Created: 2008-04-01 Last updated: 2016-06-14Bibliographically approved
    5. On the response mechanism of SiC based field effect gas sensors towards non-hydrogen containing species and specifically NO
    Open this publication in new window or tab >>On the response mechanism of SiC based field effect gas sensors towards non-hydrogen containing species and specifically NO
    (English)Manuscript (Other academic)
    Abstract [en]

    The response characteristics of Metal Insulator Silicon Carbide (MISiC) field effect sensor devices, with platinum (Pt) as the metal contact, towards nitrogen oxide (NO) for both low as well as relatively high background oxygen (O2) concentrations and different temperatures have been investigated at atmospheric pressure. Devices with both porous and dense Pt metal gate contacts have been investigated and the results seem to confirm the theories and results from earlier measurements regarding the requirement of porous metal films for the existence of a response to NO for this kind of sensor device. The results also suggest that no NO induced increased sensitivity to background hydrogen exists, at least it does not play any role in the observed NO sensitivity, as opposed to what has been suggested in the case of CO. The obtained results are also discussed in relation to some of the proposed sensing mechanisms for non-hydrogen containing substances and in comparison to NO reduction characteristics on Pt/SiO2 catalysts, as reported in literature. The results further give some indications about also some other process/ processes being important for the response of SiC based field effect sensors towards NO than just adsorption/desorption.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:liu:diva-13096 (URN)
    Note

    This manuscript was never submitted to a journal and will not be published.

    Available from: 2008-04-01 Created: 2008-04-01 Last updated: 2016-06-14Bibliographically approved
    6. A novel sensor system based on gas sensitive MISiCFET devices for combustion control in small-scale wood fired boilers
    Open this publication in new window or tab >>A novel sensor system based on gas sensitive MISiCFET devices for combustion control in small-scale wood fired boilers
    Show others...
    2007 (English)Article in journal (Refereed) Submitted
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-13097 (URN)
    Available from: 2008-04-01 Created: 2008-04-01 Last updated: 2014-01-09
  • 5.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Study of novel electronic materials by mid-infrared and terahertz optical Hall effect2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Development of silicon based electronics have revolutionized our every day life during the last three decades. Nowadays Si based devices operate close to their theoretical limits that is becoming a bottleneck for further progress. In particular, for the growing field of high frequency and high power electronics, Si cannot offer the required properties. Development of materials capable of providing high current densities, carrier mobilities and high breakdown fields is crucial for a progress in state of the art electronics.

    Epitaxial graphene grown on semi-insulating silicon carbide substrates has a high potential to be integrated in the current planar device technologies. High electron mobilities and sheet carrier densities make graphene extremely attractive for high frequency analog applications. One of the remaining challenges is the interaction of epitaxial graphene with the substrate. Typically, much lower free charge carrier mobilities, compared to free standing graphene, and doping, due to charge transfer from the substrate, is reported. Thus, a good understanding of the intrinsic free charge carriers properties and the factors affecting them is very important for further development of epitaxial graphene.

    III-group nitrides have been extensively studied and already have proven their high efficiency as light sources for short wavelengths. High carrier mobilities and breakdown electric fields were demonstrated for III-group nitrides, making them attractive for high frequency and high power applications. Currently, In-rich InGaN alloys and AlGaN/GaN high electron mobility structures are of high interest for the research community due to open fundamental questions.

    Electrical characterization techniques, commonly used for the determination of free charge carrier properties, require good ohmic and Schottky contacts, which in certain cases can be difficult to achieve. Access to electrical properties of buried conductive channels in multilayered structures requires modification of samples and good knowledge of the electrical properties of all electrical contact within the structure. Moreover, the use of electrical contacts to electrically characterize two-dimensional electronic materials, such as graphene, can alter their intrinsic properties. Furthermore, the determination of effective mass parameters commonly employs cyclotron resonance and Shubnikov-de Haas oscillations measurements, which require long scattering times of free charge carriers, high magnetic fields and low temperatures.

    The optical Hall effect is an external magnetic field induced optical anisotropy in  conductive layers due to the motion of the free charge carriers under the influence of the Lorentz force, and is equivalent to the electrical Hall effect at optical frequencies. The optical Hall effect can be measured by generalized ellipsometry and provides a powerful method for the determination of free charge carrier properties in a non-destructive and contactless manner. In principle, a single optical Hall effect measurement can provide quantitative information about free charge carrier types, concentrations, mobilities and effective mass parameters at temperatures ranging from few kelvins to room temperature and above. Further, it was demonstrated that for transparent samples, a backside cavity can be employed to enhance the optical Hall effect.

    Measurement of the optical Hall effect by generalized ellipsometry is an indirect technique requiring subsequent data analysis. Parameterized optical models are fitted to match experimentally measured ellipsometric data by varying physically significant parameters. Analysis of the optical response of samples, containing free charge carriers, employing optical models based on the classical Drude model, which is augmented with an external magnetic field contribution, provide access to the free charge carrier properties.

    The main research results of the graduate studies presented in this licentiate thesis are summarized in the five scientific papers.

    Paper I. Description of the custom-built terahertz frequency-domain spectroscopic ellipsometer at Linköping University. The terahertz ellipsometer capabilities are demonstrated by an accurate determination of the isotropic and anisotropic refractive indices of silicon and m-plane sapphire, respectively. Further, terahertz optical Hall effect measurements of an AlGaN/GaN high electron mobility structures were employed to extract the two-dimensional electron gas sheet density, mobility and effective mass parameters. Last, in-situ optical Hall effect measurement on epitaxial graphene in a gas cell with controllable environment, were used to study the effects of environmental doping on the mobility and carrier concentration.

    Paper II. Presents terahertz cavity-enhanced optical Hall measurements of the monolayer and multilayer epitaxial graphene on semi-insulating 4H-SiC (0001) substrates. The data analysis revealed p-type doping for monolayer graphene with a carrier density in the low 1012 cm−2 range and a carrier mobility of 1550 cm2/V·s. For the multilayer epitaxial graphene, n-type doping with a carrier density in the low 1013 cm−2 range, a mobility of 470 cm2/V·s and an effective mass of (0.14 ± 0.03) m0 were extracted. The measurements demonstrate that cavity-enhanced optical Hall effect measurements can be applied to study electronic properties of two-dimensional materials.

    Paper III. Terahertz cavity-enhanced optical Hall effect measurements are employed to study anisotropic transport in as-grown monolayer, quasi free-standing monolayer and quasi free-standing bilayer epitaxial graphene on semi-insulating 4H-SiC (0001) substrates. The data analysis revealed a strong anisotropy in the carrier mobilities of the quasi freestanding bilayer graphene. The anisotropy is demonstrated to be induced by carriers scattering at the step edges of the SiC, by showing that the mobility is higher along the step than across them. The scattering mechanism is discussed based on the results of the optical Hall effect, low-energy electron microscopy, low-energy electron diffraction and Raman measurements.

    Paper IV. Mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect measurements are employed to determine the electron effective mass in an In0.33Ga0.67N epitaxial layer. The data analysis reveals slightly anisotropic effective mass and carrier mobility parameters together with the optical phonon frequencies and broadenings.

    Paper V. Terahertz cavity-enhanced optical Hall measurements are employed to study the free charge carrier properties in a set of AlGaN/GaN high electron mobility structures with modified interfaces. The results show that the interface structure has a significant effect on the free charge carrier mobility and that the sample with a sharp interface between an AlGaN barrier and a GaN buffer layers exhibits a record mobility of 2332±73 cm2/V·s. The determined effective mass parameters showed an increase compared to the GaN value, that is attributed the the penetration of the electron wavefunction into the AlGaN barrier layer.

    List of papers
    1. Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies
    Open this publication in new window or tab >>Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies
    Show others...
    2017 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 421, p. 357-360Article in journal (Refereed) Published
    Abstract [en]

    Cavity-enhanced optical Hall effect at terahertz (THz) frequencies is employed to determine the free charge carrier properties in epitaxial graphene (EG) with different number of layers grown by high-temperature sublimation on 4H-SiC(0001). We find that one monolayer (ML) EG possesses p-type conductivity with a free hole concentration in the low 1012 cmᅵᅵᅵ2 range and a free hole mobility parameter as high as 1550 cm2/Vs. We also find that 6 ML EG shows n-type doping behavior with a much lower free electron mobility parameter of 470 cm2/Vs and an order of magnitude higher free electron density in the low 1013 cmᅵᅵᅵ2 range. The observed differences are discussed. The cavity-enhanced THz optical Hall effect is demonstrated to be an excellent tool for contactless access to the type of free charge carriers and their properties in two-dimensional materials such as EG.

    Place, publisher, year, edition, pages
    Elsevier, 2017
    Keywords
    THz optical Hall effect, Epitaxial graphene, Free charge carrier properties
    National Category
    Physical Sciences Condensed Matter Physics Atom and Molecular Physics and Optics Ceramics
    Identifiers
    urn:nbn:se:liu:diva-132407 (URN)10.1016/j.apsusc.2016.10.023 (DOI)000408756700015 ()
    Note

    Funding agencies: Swedish Research Council (VR) [2013-5580]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program [2011-03486, 2014-04712]; Swedish foundation for strategic research (SSF) [FFL12-0181, RIF14-055]

    Available from: 2016-11-09 Created: 2016-11-09 Last updated: 2019-03-05Bibliographically approved
    2. Properties of two-dimensional electron gas in AlGaN/GaN HEMT structures determined by cavity-enhanced THz optical Hall effect
    Open this publication in new window or tab >>Properties of two-dimensional electron gas in AlGaN/GaN HEMT structures determined by cavity-enhanced THz optical Hall effect
    Show others...
    2016 (English)In: Physica Status Solidi C-Current Topics in Solid State Physics, Vol 13 No 5-6, Wiley-VCH Verlagsgesellschaft, 2016, Vol. 13, no 5-6, p. 369-373Conference paper, Published paper (Refereed)
    Abstract [en]

    In this work we employ terahertz (THz) ellipsometry to determine two-dimensional electron gas (2DEG) density, mobility and effective mass in AlGaN/GaN high electron mobility transistor structures grown on 4H-SiC substrates. The effect of the GaN interface exposure to low-flow-rate trimethylaluminum (TMA) on the 2DEG properties is studied. The 2DEG effective mass and sheet density are determined tobe in the range of 0.30-0.32m0 and 4.3-5.5×1012 cm–2, respectively. The 2DEG effective mass parameters are found to be higher than the bulk effective mass of GaN, which is discussed in view of 2DEG confinement. It is shown that exposure to TMA flow improves the 2DEG mobility from 2000 cm2/Vs to values above 2200 cm2/Vs. A record mobility of 2332±61 cm2/Vs is determined for the sample with GaN interface exposed to TMA for 30 s. This improvement in mobility is suggested to be due to AlGaN/GaN interface sharpening causing the reduction of interface roughness scattering of electrons in the 2DEG.

    Place, publisher, year, edition, pages
    Wiley-VCH Verlagsgesellschaft, 2016
    Series
    Physica Status Solidi C-Current Topics in Solid State Physics, ISSN 1862-6351
    Keywords
    AlGaN/GaN HEMTs, THz ellipsometry, 2DEG properties, THz optical Hall effect
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-133135 (URN)10.1002/pssc.201510214 (DOI)000387957200045 ()
    Conference
    11th International Conference on Nitride Semiconductors (ICNS), Beijing, China, August 30-September 4. 2015
    Available from: 2016-12-12 Created: 2016-12-09 Last updated: 2019-03-05Bibliographically approved
  • 6.
    Arnlind, Joakim
    et al.
    Linköping University, Department of Mathematics, Mathematics and Applied Mathematics. Linköping University, Faculty of Science & Engineering.
    Al-Shujary, Ahmed
    Linköping University, Department of Mathematics, Mathematics and Applied Mathematics. Linköping University, Faculty of Science & Engineering.
    Kahler-Poisson algebras2019In: Journal of Geometry and Physics, ISSN 0393-0440, E-ISSN 1879-1662, Vol. 136, p. 156-172Article in journal (Refereed)
    Abstract [en]

    We introduce Kahler-Poisson algebras as analogues of algebras of smooth functions on Kahler manifolds, and prove that they share several properties with their classical counterparts on an algebraic level. For instance, the module of inner derivations of a Kahler-Poisson algebra is a finitely generated projective module, and allows for a unique metric and torsion-free connection whose curvature enjoys all the classical symmetries. Moreover, starting from a large class of Poisson algebras, we show that every algebra has an associated Kahler-Poisson algebra constructed as a localization. At the end, detailed examples are provided in order to illustrate the novel concepts. (C) 2018 Elsevier B.V. All rights reserved.

  • 7.
    Arnlind, Joakim
    et al.
    Linköping University, Department of Mathematics, Mathematics and Applied Mathematics. Linköping University, Faculty of Science & Engineering.
    Wilson, Mitsuru
    University of Western Ontario, Canada.
    On the Chern-Gauss-Bonnet theorem for the noncommutative 4-sphere2017In: JOURNAL OF GEOMETRY AND PHYSICS, ISSN 0393-0440, Vol. 111, p. 126-141Article in journal (Refereed)
    Abstract [en]

    We construct a differential calculus over the noncommutative 4-sphere in the framework of pseudo-Riemannian calculi, and show that for every metric in a conformal class of perturbations of the round metric, there exists a unique metric and torsion-free connection. Furthermore, we find a localization of the projective module corresponding to the space of vector fields, which allows us to formulate a Chern-Gauss-Bonnet type theorem for the noncommutative 4-sphere. (C) 2016 Elsevier B.V. All rights reserved.

  • 8.
    Arwin, Hans
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Fernandez Del Rio, Lia
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Åkerlind, Christina
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Swedish Def Research Agency FOI, Div Command and Control Syst, SE-58111 Linkoping, Sweden.
    Valyukh, Sergiy
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Mendoza-Galvan, A.
    CINVESTAV IPN, Mexico.
    Magnusson, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Landin, Jan
    Linköping University, Department of Physics, Chemistry and Biology, Ecology. Linköping University, Faculty of Science & Engineering.
    Järrendahl, Kenneth
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    On the polarization of light reflected from beetle cuticle2017In: MATERIALS TODAY-PROCEEDINGS, ELSEVIER SCIENCE BV , 2017, Vol. 4, no 4, p. 4933-4941Conference paper (Refereed)
    Abstract [en]

    The use of Mueller matrices for studies of polarizing properties and cuticle structure of scarab beetles are partly reviewed. Specifically we show how the polarization of the reflected light can be quantified in terms of degree of polarization and ellipticity. It is also shown that sum decomposition of Mueller matrices reveals cuticle reflection characteristics in different spectral regions, e.g. in terms of mirrors and circular polarizers. With a differential decomposition of cuticle transmission Mueller matrices, we determine the spectral variation in the fundamental optical properties circular birefringence and dichroism. (C) 2017 Elsevier Ltd. All rights reserved.

  • 9.
    Arwin, Hans
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Applied Optics . Linköping University, Faculty of Science & Engineering.
    Magnusson, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Applied Optics . Linköping University, Faculty of Science & Engineering.
    Garcia-Caurel, Enric
    CNRS 91128 Palaiseau, France.
    de Martino, Antonello
    CNRS 91128 Palaiseau, FranceCNRS 91128 Palaiseau, France.
    Järrendahl, Kenneth
    Linköping University, Department of Physics, Chemistry and Biology, Applied Optics . Linköping University, Faculty of Science & Engineering.
    Ossikovski, Razvigor
    CNRS 91128 Palaiseau, France.
    Sum decomposition of Mueller matrices from beetle cuticles2015Conference paper (Other academic)
    Abstract [en]

    Spectral Mueller matrices are very rich in information about physical properties of a sample. We have recently shown that polarizing properties like ellipticity and degree of polarization can be extracted from a Mueller matrix measured on a beetle cuticle (exoskeleton). Mueller matrices can also be used in regression analysis to model nanostructures in cuticles. Here we present the use of sum decomposition of Mueller matrices from these depolarizing biological reflectors to explore the fundamental character of these reflectors. The objective is to decompose a Mueller matrix into well- defined ideal non-depolarizing matrices corresponding to mirrors, circular polarizers, halfwave retarders etc.Generally it is possible to decompose a measured depolarizing Mueller matrix M into four (or fewer) non-depolarizing matrices according to M=λ1M1+λ2M2+λ3M3+λ4M4, where λ1, λ2, λ3 and λ4 are eigenvalues of the covariance matrix of M. Two strategies for decomposition will be discussed. A Cloude decomposition will provide the eigenvalues and also the Mi’s although the latter will contain severe noise in some spectral regions. However, a major advantage with the Cloude decomposition is that the number of nonzero eigenvalues is directly obtained, i.e. the number of contributing Mi matrices. In an alternative decomposition, the Mi’s are assumed and the eigenvalues are found by regression analysis based on M. In the case with two non-zero eigenvalues we define a model Mueller matrix MD=αRM1+βRM2 with αR+βR=1. With αR as adjustable parameter, the Frobenius norm ||M-MD|| is minimized for each wavelength in the spectral range of M. For more complex structures, the regression can be extended by adding more matrices up to a total of four. Advantages with a regression approach are its simplicity and stability compared to a Cloude decomposition.Mueller-matrix spectra of beetle cuticles are recorded with a dual rotating compensator ellipsometer in the spectral range 400 – 900 nm at angles of incidence in the range 20 - 75°. The application of decomposition on biological reflectors is demonstrated on M measured on the beetle Cetonia aurata, which represents a narrow-band chiral Bragg reflector with two non-zero eigenvalues. A decomposition in an ideal mirror and a circular polarizer is feasible. In another example, the broad-band and gold-colored beetle Chrysina argenteola, we show that more than two eigenvalues can be nonzero, especially at oblique incidence, and additional matrices are involved.

  • 10.
    Aschebrock, Thilo
    et al.
    University of Bayreuth, Germany.
    Armiento, Rickard
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Kuemmel, Stephan
    University of Bayreuth, Germany.
    Challenges for semilocal density functionals with asymptotically nonvanishing potentials2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 7, article id 075140Article in journal (Refereed)
    Abstract [en]

    The Becke-Johnson model potential [A. D. Becke and E. R. Johnson, J. Chem. Phys. 124, 221101 ( 2006)] and the potential of the AK13 functional [R. Armiento and S. Kummel, Phys. Rev. Lett. 111, 036402 ( 2013)] have been shown to mimic features of the exact Kohn-Sham exchange potential, such as step structures that are associated with shell closings and particle-number changes. A key element in the construction of these functionals is that the potential has a limiting value far outside a finite system that is a system-dependent constant rather than zero. We discuss a set of anomalous features in these functionals that are closely connected to the nonvanishing asymptotic potential. The findings constitute a formidable challenge for the future development of semilocal functionals based on the concept of a nonvanishing asymptotic constant.

  • 11.
    Asker, Christian
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Effects of disorder in metallic systems from First-Principles calculations2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis, quantum-mechanical calculations within density-functional theory on metallic systems are presented. The overarching goal has been to investigate effects of disorder. In particular, one of the properties investigated is the bindingenergy shifts for core electrons in binary alloys using different theoretical methods. These methods are compared with each other and with experimental results. One such method, the so-called Slater-Janak transition state method relies on the assumption that the single-particle eigenvalues within density-functional theory are linear functions of their respective occupation number. This assumption is investigated and it is found that while the eigenvalues to a first approximation show linear behavior, there are also nonlinearities which can influence the core-level binding energy shifts.

    Another area of investigation has been iron based alloys at pressures corresponding to those in the Earth’s inner core. This has been done for the hexagonal close packed and face entered cubic structures. The effects of alloying iron with magnesium and nickel on the equation of state as well on the elastic properties have been investigated. The calculations have shown that the hexagonal close packed structure in FeNi is more isotropic than the face-centered cubic structure, and that adding Mg to Fe has a large impact on the elastic properties.

    Finally, the effects of disorder due to thermal motion of the atoms have been investigated through ab-initio molecular dynamics simulations. Within the limits of this method and the setup, it is found that the face-centered cubic structure of molybdenum can be dynamically stabilized at high temperature, leading to a metastable structure, on the average. The dynamical stabilization of face-centered cubic molybdenum also rendered it possible to accurately calculate the lattice stability relative to the body-centered cubic phase. Inclusion of temperature effects for the lattice stability using ab-initio molecular dynamics simulations resolves the disagreement between ab-initio calculations and thermochemical methods.

    List of papers
    1. Core-level shifts in fcc random alloys: A first-principles approach
    Open this publication in new window or tab >>Core-level shifts in fcc random alloys: A first-principles approach
    Show others...
    2005 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 6, p. 064203-Article in journal (Refereed) Published
    Abstract [en]

    First-principles theoretical calculations of the core-level binding-energy shift (CLS) for eight binary face-centered-cubic (fcc) disordered alloys, CuPd, AgPd, CuNi, NiPd, CuAu, PdAu, CuPt, and NiPt, are carried out within density-functional theory (DFT) using the coherent potential approximation. The shifts of the Cu and Ni 2p3∕2, Ag and Pd 3d5∕2, and Pt and Au 4f7∕2 core levels are calculated according to the complete screening picture, which includes both initial-state (core-electron energy eigenvalue) and final-state (core-hole screening) effects in the same scheme. The results are compared with available experimental data, and the agreement is shown to be good. The CLSs are analyzed in terms of initial- and final-state effects. We also compare the complete screening picture with the CLS obtained by the transition-state method, and find very good agreement between these two alternative approaches for the calculations within the DFT. In addition the sensitivity of the CLS to relativistic and magnetic effects is studied.

    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-12565 (URN)10.1103/PhysRevB.72.064203 (DOI)
    Note

    Original publication: W. Olovsson, C. Göransson, L. V. Pourovskii, B. Johansson and I. A. Abrikosov, Core-level shifts in fcc random alloys: A first-principles approach, 2005, Physical Review B, (72), 064203. Copyright: The America Physical Society, http://prb.aps.org/

    Available from: 2008-09-15 Created: 2008-09-15 Last updated: 2017-12-14
    2. Numerical investigation of the validity of the Slater-Janak transition-state model in metallic systems
    Open this publication in new window or tab >>Numerical investigation of the validity of the Slater-Janak transition-state model in metallic systems
    2005 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 13Article in journal (Refereed) Published
    Abstract [en]

    According to the so-called Janak’s theorem, the eigenstates of the Kohn-Sham Hamiltonian are given by the derivative of the total energy with respect to the occupation numbers of the corresponding one-electron states. The linear dependence of the Kohn-Sham eigenvalues on the occupation numbers is often assumed in order to use the Janak’s theorem in applications, for instance, in calculations of the core-level shifts in materials by means of the Slater-Janak transition state model. In this work first-principles density-functional theory calculations using noninteger occupation numbers for different core states in 24 different random alloy systems were carried out in order to verify the assumptions of linearity. It is found that, to a first approximation, the Kohn-Sham eigenvalues show a linear behavior as a function of the occupation numbers. However, it is also found that deviations from linearity have observable effects on the core-level shifts for some systems. A way to reduce the error with minimal increase of computational efforts is suggested.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12559 (URN)10.1103/PhysRevB.72.134203 (DOI)
    Note

    Original publication: C. Göransson, W. Olovsson and I. A. Abrikosov, Numerical investigation of the validity of the Slater-Janak transition-state model in metallic systems, 2005, Physical Review B, (72), 134203. Copyright: The America Physical Society, http://prb.aps.org/

    Available from: 2008-09-15 Created: 2008-09-15 Last updated: 2017-12-14
    3. Core-level shifts in complex metallic systems from first principle
    Open this publication in new window or tab >>Core-level shifts in complex metallic systems from first principle
    2006 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 243, no 11, p. 2447-2464Article in journal (Refereed) Published
    Abstract [en]

    We show that core-level binding energy shifts (CLS) can be reliably calculated within density functional theory. The scheme includes both the initial (electron energy eigenvalue) as well as final state (relaxation due to core-hole screening) effects in the same framework. The results include CLS as a function of composition in substitutional random bulk and surface alloys. Sensitivity of the CLS to the local chemical environment in the bulk and at the surface is demonstrated. A possibility to use the CLS for structural determination is discussed. Finally, an extension of the model is made for Auger kinetic energy shift calculations.

    Place, publisher, year, edition, pages
    Wiley, 2006
    Keywords
    71.15.-m, 71.23.-k, 79.20.Fv, 79.60.Dp, 79.60.Ht, 79.60.Jv
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-37239 (URN)10.1002/pssb.200642165 (DOI)34061 (Local ID)34061 (Archive number)34061 (OAI)
    Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2017-12-13
    4. Elastic properties of Fe–Mn random alloys studied by ab initio calculations
    Open this publication in new window or tab >>Elastic properties of Fe–Mn random alloys studied by ab initio calculations
    Show others...
    2007 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 91, no 19, p. 191904-Article in journal (Refereed) Published
    Abstract [en]

    We have studied the influence of the Mn content on the elastic properties of Fe–Mn random alloys (space group of Fmm) using ab initio calculations. The magnetic effects in Fe–Mn alloys have a strong influence on the elastic properties, even above the Néel temperature. As the Mn content is increased from 5  to  40  at.  %, the C44 elastic constant is unaffected, while C11 and C12 decrease. This behavior can be understood based on the magnetovolume effect which softens the lattice. Since the amplitude of local magnetic moments is less sensitive to volume conserving distortions, the softening is not present during shearing.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12567 (URN)10.1063/1.2807677 (DOI)
    Note
    Original publication: Denis Music, Tetsuya Takahashi, Levente Vitos, Christian Asker, Igor A. Abrikosov and Jochen M. Schneider, Elastic properties of Fe–Mn random alloys studied by ab initio calculations, 2007, Applied Physics Letters, (91), 191904. Copyright: The America Institute of Physics, http://www.aip.org/ Available from: 2008-09-15 Created: 2008-09-15 Last updated: 2017-12-14
    5. First-principles solution to the problem of Mo lattice stability
    Open this publication in new window or tab >>First-principles solution to the problem of Mo lattice stability
    2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 220102(R)Article in journal (Refereed) Published
    Abstract [en]

    The energy differences between the ground state body-centered structure and closed-packed face-centered structure for transition metals in the middle of the series show unusually large disagreements when they are obtained by the thermochemical approach based on the analysis of experimental data or by first-principles electronic structure calculations. Considering a typical example, the lattice stability of Mo, we present a solution to this long-standing problem. We carry out ab initio molecular dynamics simulations for the two phases at high temperature and show that the configurational energy difference approaches the value derived by means of the thermochemical approach. The main contribution to the effect comes from the modification of the canonical band structure due to anharmonic thermal motion at high temperature.

     

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12560 (URN)10.1103/PhysRevB.77.220102 (DOI)
    Note
    Original publication: C. Asker, A. B. Belonoshko, A. S. Mikhaylushkin and I. A. Abrikosov, First-principles solution to the problem of Mo lattice stability, 2008, Physical Review B, (77), 220102(R). Copyright: The America Physical Society, http://prb.aps.org/Available from: 2008-09-15 Created: 2008-09-15 Last updated: 2017-12-14
    6. Elastic constants and anisotropy in FeNi alloys at high pressures from first-principles calculations
    Open this publication in new window or tab >>Elastic constants and anisotropy in FeNi alloys at high pressures from first-principles calculations
    2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 21, p. 214112-Article in journal (Refereed) Published
    Abstract [en]

    The single-crystal and polycrystalline elastic constants and the elastic anisotropy in face-centered cubic and hexagonal close-packed FeNi alloys have been investigated at ultrahigh pressures by means of first-principles calculations using the exact muffin-tin orbitals method and the coherent-potential approximation. Comparisons with earlier calculations for pure Fe and experimental results are presented and discussed. We show that Ni alloying into Fe increases slightly the density and has very little effect on bulk moduli. Moreover, the relative decrease in c(44) elastic constant is much stronger in the hcp phase than in the fcc one. It is found that the elastic anisotropy is higher for face-centered cubic than for the hexagonal close-packed structure of FeNi, even though the face-centered cubic phase has a higher degree of symmetry. The anisotropy in face-centered cubic structure decreases with increasing nickel concentration while a very weak increase is observed for the hexagonal close-packed structure.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-20159 (URN)10.1103/PhysRevB.79.214112 (DOI)
    Available from: 2009-09-01 Created: 2009-08-31 Last updated: 2017-12-13
    7. Equation of state and elastic properties of face-centered cubic FeMg alloy at ultrahigh pressures from first-principles
    Open this publication in new window or tab >>Equation of state and elastic properties of face-centered cubic FeMg alloy at ultrahigh pressures from first-principles
    2010 (English)In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 293, no 1-2, p. 130-134Article in journal (Refereed) Published
    Abstract [en]

    We have ca1culated the equation of state and elastic properties of face-centered cubic Fe and Fe-rich FeMg alloy at ultrahigh pressures from first principles using the Exact Muffin-Tin Orbitals method. The results show that adding Mg into Fe influences strongly the equation of state, and cause a large degree of softening of the elastic constants, even at concentrations as small as 1-2 at. %. Moreover, the e1astic anisotropy increases, and the effect is higher at higher pressures.

    Keywords
    Ab initio, Elasticity, equation of state, iron, magnesium, Earths inner core, Pressure
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-53906 (URN)10.1016/j.epsl.2010.02.032 (DOI)000277217100013 ()
    Note
    Original Publication: Christian Asker, U. Kargén, L. Dubrovinsky and Igor Abrikosov, Equation of state and elastic properties of face-centered cubic FeMg alloy at ultrahigh pressures from first-principles, 2010, Earth and Planetary Science Letters, (293), 1-2, 130-134. http://dx.doi.org/10.1016/j.epsl.2010.02.032 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com/ Available from: 2010-02-11 Created: 2010-02-11 Last updated: 2017-12-12
    8. Electronic and atomic structure of Mo from high-temperature molecular dynamics simulations
    Open this publication in new window or tab >>Electronic and atomic structure of Mo from high-temperature molecular dynamics simulations
    (English)Manuscript (preprint) (Other (popular science, discussion, etc.))
    Abstract [en]

    By means of ab initio molecular dynamics (AIMD) simulations we carry out a detailed stdly of the electronic and atomic structure of Mo upon the thermal stabilization of its dynamically unstable face-centered cubic (fcc) phase, Wc calculate how the atomic positions, radial distribution function, and the ei<xtronic density of states of fcc Mo evolve with temperature. The results are compared with those for dynamically stable body-centered cubic (bcc) phase of Mo, as well as with bcc Zr, which is dynamically unstable at T = OK, but (in contrast to fcc Mo) becomes thermodynamically stable at high temperature, In particular, wc emphasize the difference between the local positions of atoms in the simulation boxes at a particular step of AIMD simulation and the average positions, around which the atoms vibrate, and show that the former are solcly responsible for the electronic properties of the material. WE observe that while the average atomic positions in fcc Mo correspond perfectly to the ideal structure at high temperature, the electronic structure of the metal calculated from AIMD differs substantially from the canonical shape of the density of states for the ideal fcc crystaL From a comparison of our results obtained for fcc Mo arid bcc Zr, we advocate the use of the electronic structure calculations, complemented with studies of radial distribution functions, as a sensitive test of a degree of the temperature induced stabilization of phases, which are dynamically unstable at T = OK.

    Identifiers
    urn:nbn:se:liu:diva-53779 (URN)
    Available from: 2010-02-03 Created: 2010-02-03 Last updated: 2010-02-11
  • 12.
    Backman, Björn
    Linköping University, The Department of Physics, Chemistry and Biology.
    Future Upgrades of the LHC Beam Screen Cooling System2006Independent thesis Advanced level (degree of Magister), 20 points / 30 hpStudent thesis
    Abstract [en]

    The topic of this thesis concerns the LHC, the next large particle accelerator at CERN which will start operating in 2007. Being based on superconductivity, the LHC needs to operate at very low temperatures, which makes great demands on the cryogenic system of the accelerator. To cope with the heat loads induced by the particle beam, a beam screen cooled with forced flow of supercritical helium is used.

    There is an interest in upgrading the energy and luminosity of the LHC in the future and this would require a higher heat load to be extracted by the beam screen cooling system. The objective of this thesis is to quantify different ways to upgrade this system by mainly studying the effects of different pressure and temperatures levels as well as a different cooling medium, neon.

    For this a numerical program which simulates one-dimensional pipe flow was constructed. The frictional forces were accounted for by the empirical concept of friction factor. For the fluid properties, software using empirically made correlations was used. To validate the numerical program, a comparison with previous experimental work was done. The agreement with experimental data was good for certain flow configurations, worse for others. From this it was concluded that further comparisons with experimental data must be made in order to tell the accuracy of the mathematical model and the correlations for fluid properties used.

    When using supercritical helium, thermo-hydraulic instabilities may arise in the cooling loop. It was of special interest to see how well a numerical program could simulate and predict this phenomenon. It was found that the numerical program did not function for such unstable conditions; in fact it was much more sensitive than what reality is.

    For the beam screen cooling system we conclude that to cope with the increased heat loads of future upgrades, an increase in pressure level is needed regardless if the coolant remains helium, or is changed to neon. Increasing the pressure level also makes that the problems with thermo-hydraulic instabilities can be avoided. Of the two coolants, helium gave the best heat extraction capacity. Unlike neon, it is also possible to keep the present temperature level when using helium.

  • 13.
    Beaulieu, Luc
    et al.
    Centre hospitalier universitaire de Québec, Canada and Université Laval, Québec, Canada .
    Carlsson Tedgren, Åsa
    Linköping University, Department of Medical and Health Sciences, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Carrier, Jean-François
    Centre hospitalier de l’Université de Montréal, Québec, Canada and Département de physique, Université de Montréal, Québec Canada .
    Davis, Stephen D.
    University of Wisconsin-Madison, USA and McGill University Health Centre, Montréal, Québec, Canada .
    Mourtada, Firas
    Helen F. Graham Cancer Center, Christiana Care Health System, Newark, Delaware, USA.
    Rivard, Mark J.
    Tufts University School of Medicine, Boston, Massachusetts, USA.
    Thomson, Rowan M.
    Carleton University, Ottawa, Ontario, Canada.
    Verhaegen, Frank
    Maastricht University Medical Center, the Netherlands and McGill University Health Centre, Montréal, Québec, Canada .
    Wareing, Todd A.
    Transpire Inc., Gig Harbor, Washington, USA.
    Williamson, Jeffrey F.
    Virginia Commonwealth University, Richmond, USA.
    Report of the Task Group 186 on model-based dose calculation methods in brachytherapy beyond the TG-43 formalism: Current status and recommendations for clinical implementation2012In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 39, no 10, p. 6208-6236Article in journal (Refereed)
    Abstract [en]

    The charge of Task Group 186 (TG-186) is to provide guidance for early adopters of model-based dose calculation algorithms (MBDCAs) for brachytherapy (BT) dose calculations to ensure practice uniformity. Contrary to external beam radiotherapy, heterogeneity correction algorithms have only recently been made available to the BT community. Yet, BT dose calculation accuracy is highly dependent on scatter conditions and photoelectric effect cross-sections relative to water. In specific situations, differences between the current water-based BT dose calculation formalism (TG-43) and MBDCAs can lead to differences in calculated doses exceeding a factor of 10. MBDCAs raise three major issues that are not addressed by current guidance documents: (1) MBDCA calculated doses are sensitive to the dose specification medium, resulting in energy-dependent differences between dose calculated to water in a homogeneous water geometry (TG-43), dose calculated to the local medium in the heterogeneous medium, and the intermediate scenario of dose calculated to a small volume of water in the heterogeneous medium. (2) MBDCA doses are sensitive to voxel-by-voxel interaction cross sections. Neither conventional single-energy CT nor ICRU∕ICRP tissue composition compilations provide useful guidance for the task of assigning interaction cross sections to each voxel. (3) Since each patient-source-applicator combination is unique, having reference data for each possible combination to benchmark MBDCAs is an impractical strategy. Hence, a new commissioning process is required. TG-186 addresses in detail the above issues through the literature review and provides explicit recommendations based on the current state of knowledge. TG-43-based dose prescription and dose calculation remain in effect, with MBDCA dose reporting performed in parallel when available. In using MBDCAs, it is recommended that the radiation transport should be performed in the heterogeneous medium and, at minimum, the dose to the local medium be reported along with the TG-43 calculated doses. Assignments of voxel-by-voxel cross sections represent a particular challenge. Electron density information is readily extracted from CT imaging, but cannot be used to distinguish between different materials having the same density. Therefore, a recommendation is made to use a number of standardized materials to maintain uniformity across institutions. Sensitivity analysis shows that this recommendation offers increased accuracy over TG-43. MBDCA commissioning will share commonalities with current TG-43-based systems, but in addition there will be algorithm-specific tasks. Two levels of commissioning are recommended: reproducing TG-43 dose parameters and testing the advanced capabilities of MBDCAs. For validation of heterogeneity and scatter conditions, MBDCAs should mimic the 3D dose distributions from reference virtual geometries. Potential changes in BT dose prescriptions and MBDCA limitations are discussed. When data required for full MBDCA implementation are insufficient, interim recommendations are made and potential areas of research are identified. Application of TG-186 guidance should retain practice uniformity in transitioning from the TG-43 to the MBDCA approach.

  • 14.
    Belicev, P. P.
    et al.
    University of Belgrade, Serbia.
    Gligoric, G.
    University of Belgrade, Serbia.
    Maluckov, A.
    University of Belgrade, Serbia.
    Stepic, M.
    University of Belgrade, Serbia.
    Johansson, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Localized gap modes in nonlinear dimerized Lieb lattices2017In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 96, no 6, article id 063838Article in journal (Refereed)
    Abstract [en]

    Compact localized modes of ring type exist in many two-dimensional lattices with a flat linear band, such as the Lieb lattice. The uniform Lieb lattice is gapless, but gaps surrounding the flat band can be induced by various types of bond alternations (dimerizations) without destroying the compact linear eigenmodes. Here, we investigate the conditions under which such diffractionless modes can be formed and propagated also in the presence of a cubic on-site (Kerr) nonlinearity. For the simplest type of dimerization with a three-site unit cell, nonlinearity destroys the exact compactness, but strongly localized modes with frequencies inside the gap are still found to propagate stably for certain regimes of system parameters. By contrast, introducing a dimerization with a 12-site unit cell, compact (diffractionless) gap modes are found to exist as exact nonlinear solutions in continuation of flat band linear eigenmodes. These modes appear to be generally weakly unstable, but dynamical simulations show parameter regimes where localization would persist for propagation lengths much larger than the size of typical experimental waveguide array configurations. Our findings represent an attempt to realize conditions for full control of light propagation in photonic environments.

  • 15.
    Berggren, Karl
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    A high sensitivity imaging detector for the study of the formation of (anti)hydrogen2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    AEGIS (Antimatter Experiment, Gravity, Interferometry and Spectroscopy) isan experiment under development at CERN which will measure earth's gravitationalforce on antimatter. This will be done by creating a horizontal pulsedbeam of low energy antihydrogen, an atom consisting of an antiproton anda positron. The experiment will measure the vertical de ection of the beamthrough which it is possible to calculate the gravitational constant for antimatter.To characterise the production process in the current state of the experimentit is necessary to develop an imaging detector for single excited hydrogenatoms. This thesis covers the design phase of that detector and includes studiesand tests of detector components. Following literature studies, tests and havingdiscarded several potential designs, a baseline design was chosen. The suggesteddetector will contain a set of ionising rings followed by an electron multiplyingmicrochannel plate, a light emitting phosphor screen, a lens system and nallya CCD camera for readout. The detector will be able to detect single hydrogenatoms, measure their time of ight as well as being able to image electronplasmas and measure the time of ight of the initial particles in such a plasma.Tests were made to determine the behaviour of microchannel plates at the lowtemperatures used in the experiment. Especially, the resistance and multiplicationfactor of the microchannel plates have been measured at temperaturesdown to 14 K.

  • 16.
    Berggren, Karl-Fredrik
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Tellander, Felix
    Lund Univ, Sweden.
    Yakymenko, Iryna
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Basic modelling of transport in 2D wave-mechanical nanodots and billiards with balanced gain and loss mediated by complex potentials2018In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, no 20, article id 204003Article in journal (Refereed)
    Abstract [en]

    Non-Hermitian quantum mechanics with parity-time (PT) symmetry is presently gaining great interest, especially within the fields of photonics and optics. Here, we give a brief overview of low-dimensional semiconductor nanodevices using the example of a quantum dot with input and output leads, which are mimicked by imaginary potentials for gain and loss, and how wave functions, particle flow, coalescence of levels and associated breaking of PT symmetry may be analysed within such a framework. Special attention is given to the presence of exceptional points and symmetry breaking. Related features for musical string instruments and wolf-notes are outlined briefly with suggestions for further experiments.

  • 17.
    Bergstedt, Mikael
    Linköping University, The Department of Physics, Chemistry and Biology.
    Theoretical investigation of the first-order hyperpolarizability in the two-photon resonant region2007Independent thesis Advanced level (degree of Magister), 10 points / 15 hpStudent thesis
    Abstract [en]

    Time-dependent density functional theory calculations have been carried out to determine the complex first-order hyperpolarizability in the two-photon resonance region of the molecule IDS-Cab. Calculations show that three strongly absorbing states, in the ultraviolet region, are separated to the extent that no significant interference of the imaginary parts of the tensor elements of the first-order hyper-polarizability occurs. Consequently, and in contrast to experimental findings [27], no reduced imaginary parts of the first-order hyperpolarizability in the two-photon resonant region can be seen.

  • 18.
    Bouhafs, Chamseddine
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Structural and Electronic Properties of Graphene on 4H- and 3C-SiC2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Graphene is a one-atom-tick carbon layer arranged in a honeycomb lattice. Graphene was first experimentally demonstrated by Andre Geim and Konstantin Novoselov in 2004 using mechanical exfoliation of highly oriented pyrolytic graphite (exfoliated graphene flakes), for which they received the Nobel Prize in Physics in 2010. Exfoliated graphene flakes show outstanding electronic properties, e.g., very high free charge carrier mobility parameters and ballistic transport at room temperature. This makes graphene a suitable material for next generation radio-frequency and terahertz electronic devices. Such applications require fabrication methods of large-area graphene compatible with electronic industry. Graphene grown by sublimation on silicon carbide (SiC) offers a viable route towards production of large-area, electronic-grade material on semi-insulating substrate without the need of transfer. Despite the intense investigations in the field, uniform wafer-scale graphene with very high-quality that matches the properties of exfoliated graphene has not been achieved yet. The key point is to identify and control how the substrate affects graphene uniformity, thickness, layer stacking, structural and electronic properties. Of particular interest is to understand the effects of SiC surface polarity and polytype on graphene properties in order to achieve large-area material with tailored properties for electronic applications. The main objectives of this thesis are to address these issues by investigating the structural and electronic properties of epitaxial graphene grown on 4HSiC and 3C-SiC substrates with different surface polarities. The first part of the thesis includes a general description of the properties of graphene, bilayer graphene and graphite. Then, the properties of epitaxial graphene on SiC by sublimation are detailed. The experimental techniques used to characterize graphene are described. A summary of all papers and contribution to the field is presented at the end of Part I. Part II consists of seven papers.

    List of papers
    1. Structural properties and dielectric function of graphene grown by high-temperature sublimation on 4H-SiC(000-1)
    Open this publication in new window or tab >>Structural properties and dielectric function of graphene grown by high-temperature sublimation on 4H-SiC(000-1)
    Show others...
    2015 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 8, p. 085701-Article in journal (Refereed) Published
    Abstract [en]

    Understanding and controlling growth of graphene on the carbon face (C-face) of SiC presents a significant challenge. In this work, we study the structural, vibrational, and dielectric function properties of graphene grown on the C-face of 4H-SiC by high-temperature sublimation in an argon atmosphere. The effect of growth temperature on the graphene number of layers and crystallite size is investigated and discussed in relation to graphene coverage and thickness homogeneity. An amorphous carbon layer at the interface between SiC and the graphene is identified, and its evolution with growth temperature is established. Atomic force microscopy, micro-Raman scattering spectroscopy, spectroscopic ellipsometry, and high-resolution cross-sectional transmission electron microscopy are combined to determine and correlate thickness, stacking order, dielectric function, and interface properties of graphene. The role of surface defects and growth temperature on the graphene growth mechanism and stacking is discussed, and a conclusion about the critical factors to achieve decoupled graphene layers is drawn. (C) 2015 AIP Publishing LLC.

    Place, publisher, year, edition, pages
    American Institute of Physics (AIP), 2015
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-117253 (URN)10.1063/1.4908216 (DOI)000351132500070 ()
    Note

    Funding Agencies|Marie Curie actions [264613-NetFISiC]; Swedish Research Council (VR) [2011-4447, 2013-5580]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program [2011-03486]; Swedish foundation for strategic research (SSF) [FFL12-0181]; FP7 EU project Nano-Rf [FP7-ICT-2011-8]; French ANR under the Grafonics Project [ANR-10-NANO-0004]; European Union Seventh Framework Programme under Graphene Flagship [604391]; Knut and Alice Wallenbergs foundation

    Available from: 2015-04-22 Created: 2015-04-21 Last updated: 2017-12-04
    2. Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies
    Open this publication in new window or tab >>Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies
    Show others...
    2017 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 421, p. 357-360Article in journal (Refereed) Published
    Abstract [en]

    Cavity-enhanced optical Hall effect at terahertz (THz) frequencies is employed to determine the free charge carrier properties in epitaxial graphene (EG) with different number of layers grown by high-temperature sublimation on 4H-SiC(0001). We find that one monolayer (ML) EG possesses p-type conductivity with a free hole concentration in the low 1012 cmᅵᅵᅵ2 range and a free hole mobility parameter as high as 1550 cm2/Vs. We also find that 6 ML EG shows n-type doping behavior with a much lower free electron mobility parameter of 470 cm2/Vs and an order of magnitude higher free electron density in the low 1013 cmᅵᅵᅵ2 range. The observed differences are discussed. The cavity-enhanced THz optical Hall effect is demonstrated to be an excellent tool for contactless access to the type of free charge carriers and their properties in two-dimensional materials such as EG.

    Place, publisher, year, edition, pages
    Elsevier, 2017
    Keywords
    THz optical Hall effect, Epitaxial graphene, Free charge carrier properties
    National Category
    Physical Sciences Condensed Matter Physics Atom and Molecular Physics and Optics Ceramics
    Identifiers
    urn:nbn:se:liu:diva-132407 (URN)10.1016/j.apsusc.2016.10.023 (DOI)000408756700015 ()
    Note

    Funding agencies: Swedish Research Council (VR) [2013-5580]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program [2011-03486, 2014-04712]; Swedish foundation for strategic research (SSF) [FFL12-0181, RIF14-055]

    Available from: 2016-11-09 Created: 2016-11-09 Last updated: 2019-03-05Bibliographically approved
  • 19.
    Brolin, Gustav
    et al.
    Department of Medical Radiation Physics, Lund University, Sweden.
    Granerus, Göran
    Linköping University, Department of Medical and Health Sciences, Clinical Physiology. Linköping University, Department of Medical and Health Sciences, Cardiology. Linköping University, Faculty of Health Sciences. Östergötlands Läns Landsting, Heart and Medicine Center, Department of Clinical Physiology in Linköping.
    Olsson, Anna
    Linköping University, Department of Medical and Health Sciences, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Edenbrandt, Lars
    Department of Clinical Sciences, Malmö, Lund University, Sweden.
    Ljungberg, Michael
    Department of Medical Radiation Physics, Lund University, Sweden.
    A new Method for Monte Carlo Simulations of Dynamic Scintillation Camera Imaging: 99mTc MAG3 Renography Studies2012Conference paper (Other academic)
  • 20.
    Bugaev, K. A.
    et al.
    Bogolyubov Inst Theoret Phys, Ukraine.
    Sagun, V. V.
    Bogolyubov Inst Theoret Phys, Ukraine; Univ Lisbon, Portugal.
    Ivanytskyi, A. I.
    Bogolyubov Inst Theoret Phys, Ukraine.
    Yakimenko, Iryna P.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Nikonov, E. G.
    JINR, Russia.
    Taranenko, A. V.
    Natl Res Nucl Univ MEPhI, Russia.
    Zinovjev, G. M.
    Bogolyubov Inst Theoret Phys, Ukraine.
    Going beyond the second virial coefficient in the hadron resonance gas model2018In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 970, p. 133-155Article in journal (Refereed)
    Abstract [en]

    We develop a novel formulation of the hadron resonance gas model which, besides a hard-core repulsion, explicitly accounts for the surface tension induced by the interaction between the particles. Such an equation of state allows us to go beyond the Van der Waals approximation for any number of different hard-core radii. A comparison with the Carnahan Starling equation of state shows that the new model is valid for packing fractions 0.2-0.22, while the usual Van der Waals model is inapplicable at packing fractions above 0.1-0.11. Moreover, it is shown that the equation of state with induced surface tension is softer than the one of hard spheres and remains causal at higher particle densities. The great advantage of our model is that there are only two equations to be solved and neither their number nor their form depend on the values of the hard-core radii used for different hadronic resonances. Such an advantage leads to a significant mathematical simplification compared to other versions of truly multi-component hadron resonance gas models. Using this equation of state we obtain a high-quality fit of the ALICE hadron multiplicities measured at the center-of-mass energy 2.76 TeV per nucleon and we find that the dependence of chi(2)/ndf on the temperature has a single global minimum in the traditional hadron resonance gas model with the multi-component hard-core repulsion. Also we find two local minima of chi(2)/ndf in the model in which the proper volume of each hadron is proportional to its mass. However, it is shown that in the latter model a second local minimum located at higher temperatures always appears far above the limit of its applicability. (C) 2017 Elsevier B.V. All rights reserved.

  • 21.
    Buyanova, Irina
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering. Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials.
    Chen, Weimin
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering. Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials.
    Ishikawa, F.
    Ehime University, Japan.
    Tu, C. W.
    University of Calif San Diego, USA.
    Novel GaNAs and GaNP-based Nanowires - Promising Materials for Optoelectronics and Photonics2016In: 2016 IEEE 16TH INTERNATIONAL CONFERENCE ON NANOTECHNOLOGY (IEEE-NANO), IEEE , 2016, p. 38-41Conference paper (Refereed)
    Abstract [en]

    In this paper we review our recent results on optical properties of coaxial nanowires (NWs) based on dilute nitride alloys, such as GaAsN and GaNP. We show that these structures have a high structural and optical quality, and can potentially be used as polarized nano-scale light sources that emit linearly polarized light with the polarization direction perpendicular to the wire axis even in zincblende NWs of various diameters. We also demonstrate that, though the GaNxP1-x alloys have rather wide bandgap energies of 1.9 - 2.3 eV, the coaxial GaNP NWs absorb infrared light via two-step two-photon absorption.

  • 22.
    Bäckdahl, Thomas
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Multipole moments of axisymmetric spacetimes2006Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis we study multipole moments of axisymmetric spacetimes. Using the recursive definition of the multipole moments of Geroch and Hansen we develop a method for computing all multipole moments of a stationary axisymmetric spacetime without the use of a recursion. This is a generalisation of a method developed by Herberthson for the static case.

    Using Herberthson’s method we also develop a method for finding a static axisymmetric spacetime with arbitrary prescribed multipole moments, subject to a specified convergence criteria. This method has, in general, a step where one has to find an explicit expression for an implicitly defined function. However, if the number of multipole moments are finite we give an explicit expression in terms of power series.

    List of papers
    1. Static axisymmetric spacetimes with prescribed multipole moments
    Open this publication in new window or tab >>Static axisymmetric spacetimes with prescribed multipole moments
    2005 (English)In: Classical and Quantum Gravity, ISSN 0264-9381, Vol. 22, no 9, p. 1607-1621Article in journal (Refereed) Published
    Abstract [en]

    In this paper we develop a method of finding the static axisymmetric spacetime corresponding to any given set ofmultipolemoments. In addition to an implicit algebraic form for the general solution, we also give a power series expression for all finite sets of multipole moments. As conjectured by Geroch we prove in the special case of axisymmetry, that there is a static spacetime for any given set of multipole moments subject to a (specified) convergence criterion. We also use this method to confirm a conjecture of Hernández-Pastora and Martín concerning the monopole–quadrupole solution.

    Keywords
    static, axisymmetric, exact solution, multipole moments, spacetime, algebraic equation
    National Category
    Mathematics
    Identifiers
    urn:nbn:se:liu:diva-13749 (URN)10.1088/0264-9381/22/9/009 (DOI)
    Available from: 2006-01-12 Created: 2006-01-12 Last updated: 2009-05-07
    2. Explicit multipole moments of stationary axisymmetric spacetimes
    Open this publication in new window or tab >>Explicit multipole moments of stationary axisymmetric spacetimes
    2005 (English)In: Classical and Quantum Gravity, ISSN 0264-9381, Vol. 22, no 17, p. 3585-3594Article in journal (Refereed) Published
    Abstract [en]

    In this paper, we study multipole moments of axisymmetric stationary asymptotically flat spacetimes. We show how the tensorial recursion of Geroch and Hansen can be reduced to a recursion of scalar functions. We also demonstrate how a careful choice of conformal factor collects all moments into one complex-valued function on , where the moments appear as the derivatives at 0. As an application, we calculate the moments of the Kerr solution. We also discuss the freedom in choosing the potential for the moments.

    National Category
    Mathematics
    Identifiers
    urn:nbn:se:liu:diva-13750 (URN)10.1088/0264-9381/22/17/017 (DOI)
    Available from: 2006-01-12 Created: 2006-01-12 Last updated: 2009-02-16
  • 23.
    Bäckdahl, Thomas
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Multipole Moments of Stationary Spacetimes2008Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis we study the relativistic multipole moments for stationary asymptotically flat spacetimes as introduced by Geroch and Hansen. These multipole moments give an asymptotic description of the gravitational field in a coordinate independent way.

    Due to this good description of the spacetimes, it is natural to try to construct a spacetime from only the set of multipole moments. Here we present a simple method to do this for the static axisymmetric case. We also give explicit solutions for the cases where the number of non-zero multipole moments are finite. In addition, for the general stationary axisymmetric case, we present methods to generate solutions.

    It has been a long standing conjecture that the multipole moments give a complete characterization of the stationary spacetimes. Much progress toward a proof has been made over the years. However, there is one remaining difficult task: to prove that a spacetime exists with an a-priori given arbitrary set of multipole moments subject to some given condition.

    Here we present such a condition for the axisymmetric case, and prove that it is both necessary and sufficient. We also extend this condition to the general case without axisymmetry, but in this case we only prove the necessity of our condition.

    List of papers
    1. Static axisymmetric spacetimes with prescribed multipole moments
    Open this publication in new window or tab >>Static axisymmetric spacetimes with prescribed multipole moments
    2005 (English)In: Classical and Quantum Gravity, ISSN 0264-9381, Vol. 22, no 9, p. 1607-1621Article in journal (Refereed) Published
    Abstract [en]

    In this paper we develop a method of finding the static axisymmetric spacetime corresponding to any given set ofmultipolemoments. In addition to an implicit algebraic form for the general solution, we also give a power series expression for all finite sets of multipole moments. As conjectured by Geroch we prove in the special case of axisymmetry, that there is a static spacetime for any given set of multipole moments subject to a (specified) convergence criterion. We also use this method to confirm a conjecture of Hernández-Pastora and Martín concerning the monopole–quadrupole solution.

    Keywords
    static, axisymmetric, exact solution, multipole moments, spacetime, algebraic equation
    National Category
    Mathematics
    Identifiers
    urn:nbn:se:liu:diva-13749 (URN)10.1088/0264-9381/22/9/009 (DOI)
    Available from: 2006-01-12 Created: 2006-01-12 Last updated: 2009-05-07
    2. Explicit multipole moments of stationary axisymmetric spacetimes
    Open this publication in new window or tab >>Explicit multipole moments of stationary axisymmetric spacetimes
    2005 (English)In: Classical and Quantum Gravity, ISSN 0264-9381, Vol. 22, no 17, p. 3585-3594Article in journal (Refereed) Published
    Abstract [en]

    In this paper, we study multipole moments of axisymmetric stationary asymptotically flat spacetimes. We show how the tensorial recursion of Geroch and Hansen can be reduced to a recursion of scalar functions. We also demonstrate how a careful choice of conformal factor collects all moments into one complex-valued function on , where the moments appear as the derivatives at 0. As an application, we calculate the moments of the Kerr solution. We also discuss the freedom in choosing the potential for the moments.

    National Category
    Mathematics
    Identifiers
    urn:nbn:se:liu:diva-13750 (URN)10.1088/0264-9381/22/17/017 (DOI)
    Available from: 2006-01-12 Created: 2006-01-12 Last updated: 2009-02-16
    3. Calculation of, and bounds for, the multipole moments of stationary spacetimes
    Open this publication in new window or tab >>Calculation of, and bounds for, the multipole moments of stationary spacetimes
    2006 (English)In: Classical and Quantum Gravity, ISSN 0264-9381, Vol. 23, no 20, p. 5997-6006Article in journal (Refereed) Published
    Abstract [en]

    In this paper, the multipole moments of stationary asymptotically flat spacetimes are considered. We show how the tensorial recursion of Geroch and Hansen can be replaced by a scalar recursion on . We also give a bound on the multipole moments. This gives a proof of the 'necessary part' of a long-standing conjecture due to Geroch.

    Keywords
    multipole moments, stationary, spacetimes
    National Category
    Mathematics
    Identifiers
    urn:nbn:se:liu:diva-13158 (URN)10.1088/0264-9381/23/20/019 (DOI)
    Available from: 2008-05-21 Created: 2008-05-21 Last updated: 2009-05-07
    4. Axisymmetric stationary solutions with arbitrary multipole moments
    Open this publication in new window or tab >>Axisymmetric stationary solutions with arbitrary multipole moments
    2007 (English)In: Classical and Quantum Gravity, ISSN 0264-9381, Vol. 24, no 9, p. 2205-2215Article in journal (Refereed) Published
    Abstract [en]

    In this paper, the problem of finding an axisymmetric stationary spacetime from a specified set of multipole moments is studied. The condition on the multipole moments, for the existence of a solution, is formulated as a convergence condition on a power series formed from the multipole moments. The methods in this paper can also be used to give approximate solutions to any order, as well as estimates on each term of the resulting power series.

    Keywords
    multipole moments, statinary, axisymmetric, existence, spacetime
    National Category
    Mathematics
    Identifiers
    urn:nbn:se:liu:diva-13159 (URN)10.1088/0264-9381/24/9/004 (DOI)
    Available from: 2008-05-21 Created: 2008-05-21 Last updated: 2009-04-24
  • 24.
    Bäckström, Louise
    Linköping University, Department of Physics, Chemistry and Biology.
    In Situ Heating During XRD Measurements as a Method to Study Recrystallisation of Aluminium Alloy AA30032018Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Recrystallisation is an important topic in the metal industry since the process may drastically alter the properties of the materials subjected to it. By controlling the recrystallisation process, the material properties can be adjusted as desired, which could lead to stronger materials and hence lighter constructions, decreasing our material consumption. This is currently regulated using softening curves complied from tensile tests, a method which does not show the degree of recrystallisation of a metal. This thesis work therefore explores the possibility to characterise the recrystallisation process using in situ X-ray diffraction, XRD, during heating.The method proposed is using in situ heat treatments of aluminium samples combined with XRD measurements. The results show that it is possible to follow the recrystallisation process of rolled aluminium alloy AA3003 by using in situ XRD during heating, a discovery that could facilitate development and understanding of new materials. Nevertheless, further investigations of the subject is required before the method will be profitable.

  • 25.
    Cabello, Adan
    et al.
    University of Seville, Spain.
    Gu, Mile
    Nanyang Technology University, Singapore; National University of Singapore, Singapore.
    Guehne, Otfried
    University of Siegen, Germany.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Wiesner, Karoline
    University of Bristol, England.
    Thermodynamical cost of some interpretations of quantum theory2016In: PHYSICAL REVIEW A, ISSN 2469-9926, Vol. 94, no 5, article id 052127Article in journal (Refereed)
    Abstract [en]

    The interpretation of quantum theory is one of the longest-standing debates in physics. Type I interpretations see quantum probabilities as determined by intrinsic properties of the observed system. Type II see them as relational experiences between an observer and the system. It is usually believed that a decision between these two options cannot be made simply on purely physical grounds but requires an act of metaphysical judgment. Here we show that, under some assumptions, the problem is decidable using thermodynamics. We prove that type I interpretations are incompatible with the following assumptions: (i) The choice of which measurement is performed can be made randomly and independently of the system under observation, (ii) the system has limited memory, and (iii) Landauers erasure principle holds.

  • 26.
    Charrier, Dimitri S. H.
    et al.
    Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    Smalbrugge, Barry E.
    Eindhoven University of Technology, Netherlands.
    de Vries, Tjibbe
    Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    Real versus measured surface potentials in scanning Kelvin probe microscopy2008In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 2, no 4, p. 622-626Article in journal (Refereed)
    Abstract [en]

    Noncontact potentiometry or scanning Kelvin probe microscopy (SKPM) is a widely used technique to study charge injection and transport in (in)organic devices by measuring a laterally resolved local potential. This technique suffers from the significant drawback that experimentally obtained curves do not generally reflect the true potential profile in the device due to nonlocal coupling between the probing tip and the device. In this work, we quantitatively explain the experimental SKPM response and by doing so directly link theoretical device models to real observables. In particular, the model quantitatively explains the effects of the tip-sample distance and the dependence on the orientation of the probing tip with respect to the device.

  • 27.
    Cheng, Li-Peng
    et al.
    Soochow Univ, Peoples R China.
    Huang, Jing-Sheng
    Soochow Univ, Peoples R China.
    Shen, Yang
    Soochow Univ, Peoples R China.
    Li, Guo-Peng
    Hefei Univ Technol HFUT, Peoples R China.
    Liu, Xiaoke
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Li, Wei
    Soochow Univ, Peoples R China.
    Wang, Yu-Han
    Soochow Univ, Peoples R China.
    Li, Yan-Qing
    Soochow Univ, Peoples R China.
    Jiang, Yang
    Hefei Univ Technol HFUT, Peoples R China.
    Gao, Feng
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Lee, Chun-Sing
    City Univ Hong Kong, Peoples R China.
    Tang, Jian-Xin
    Soochow Univ, Peoples R China.
    Efficient CsPbBr3 Perovskite Light-Emitting Diodes Enabled by Synergetic Morphology Control2019In: Advanced Optical Materials, ISSN 2162-7568, E-ISSN 2195-1071, Vol. 7, no 4, article id 1801534Article in journal (Refereed)
    Abstract [en]

    The development of solution-processed inorganic metal halide perovskite light-emitting diodes (PeLEDs) is currently hindered by low emission efficiency due to morphological defects and severe non-radiative recombination in all-inorganic perovskite emitters. Herein, bright PeLEDs are demonstrated by synergetic morphology control over cesium lead bromide (CsPbBr3) perovskite films with the combination of two additives. The phenethylammonium bromide additive enables the formation of mixed-dimensional CsPbBr3 perovskites featuring the reduced grain size (amp;lt;15 nm) and efficient energy funneling, while the dielectric polyethyleneglycol additive promotes the formation of highly compact and pinhole-free perovskite films with defect passivation at grain boundaries. Consequently, green PeLEDs achieve a current efficiency of 37.14 cd A(-1) and an external quantum efficiency of 13.14% with the maximum brightness up to 45 990 cd m(-2) and high color purity. Furthermore, this method can be effectively extended to realize flexible PeLEDs on plastic substrates with a high efficiency of 31.0 cd A(-1).

  • 28.
    Chr Germs, W.
    et al.
    Eindhoven University of Technology, Netherlands.
    Roeling, E. M.
    Eindhoven University of Technology, Netherlands.
    van IJzendoorn, L. J.
    Eindhoven University of Technology, Netherlands.
    Janssen, R. A. J.
    Eindhoven University of Technology, Netherlands.
    Kemerink, M.
    Eindhoven University of Technology, Netherlands.
    Diffusion enhancement in on/off ratchets2013In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 102, no 7, article id 073104Article in journal (Refereed)
    Abstract [en]

    We show a diffusion enhancement of suspended polystyrene particles in an electrical on/off ratchet. The enhancement can be described by a simple master equation model. Furthermore, we find that the diffusion enhancement can be described by a general curve whose shape is only determined by the asymmetry of the ratchet repeat unit. The scaling of this curve can be explained from an analytical expression valid for small off-times. Finally, we demonstrate how the master equation model can be used to find the driving parameters for optimal particle separation. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793198]

  • 29.
    Chr Germs, Wijnand
    et al.
    Eindhoven University of Technology, Netherlands.
    Roeling, Erik M.
    [Germs, Netherlands.
    van IJzendoorn, Leo J.
    [Germs, Netherlands.
    Smalbrugge, Barry
    Eindhoven University of Technology, Netherlands.
    de Vries, Tjibbe
    Eindhoven University of Technology, Netherlands.
    Jan Geluk, Erik
    Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    [Germs, Netherlands.
    Kemerink, Martijn
    [Germs, Netherlands.
    High-efficiency dielectrophoretic ratchet2012In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 86, no 4, article id 041106Article in journal (Refereed)
    Abstract [en]

    Brownian ratchets enable the use of thermal motion in performing useful work. They typically employ spatial asymmetry to rectify nondirected external forces that drive the system out of equilibrium (cf. running marbles on a shaking washboard). The major application foreseen for Brownian ratchets is high-selectivity fractionation of particle or molecule distributions. Here, we investigate the functioning of an important model system, the on/off ratchet forwater-suspended particles, in which interdigitated finger electrodes can be switched on and off to create a time-dependent, spatially periodic but asymmetric potential. Surprisingly, we find that mainly dielectrophoretic rather than electrophoretic forces are responsible for the ratchet effect. This has major implications for the (a) symmetry of the ratchet potential and the settings needed for optimal performance. We demonstrate that by applying a potential offset the ratchet can be optimized such that its particle displacement efficiency reaches the theoretical upper limit corresponding to the electrode geometry and particle size. Efficient fractionation based on size selectivity is therefore not only possible for charged species, but also for uncharged ones, which greatly expands the applicability range of this type of Brownian ratchet.

  • 30.
    Christova, C. G.
    et al.
    Eindhoven University of Technology, Netherlands.
    Stouwdam, J. W.
    Eindhoven University of Technology, Netherlands.
    Eijkemans, T. J.
    Eindhoven University of Technology, Netherlands.
    Silov, A. Yu.
    Eindhoven University of Technology, Netherlands.
    van der Heijden, R. W.
    Eindhoven University of Technology, Netherlands.
    Kemerink, M.
    Eindhoven University of Technology, Netherlands.
    Janssen, R. A. J.
    Eindhoven University of Technology, Netherlands; Eindhoven University of Technology, Netherlands.
    Salemink, H. W. M.
    Delft University of Technology, Netherlands.
    Photoluminescence enhancement in thin films of PbSe nanocrystals2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 93, no 12, article id 121906Article in journal (Refereed)
    Abstract [en]

    Remarkable photoluminescence enhancement (PLE) in submonolayer films of PbSe nanocrystals (NCs) upon continuous illumination was observed. The intensity increase from films on InP substrates was highest in vacuum, while for films on Si/SiO(2) substrates the PLE was stronger in air. The magnitude of the PLE was found to depend on the excitation intensity, being higher for a weaker irradiation power. The possible mechanisms behind the phenomenon of the PLE are discussed and it is suggested to originate mainly from charge trapping outside the NCs core. (C) 2008 American Institute of Physics.

  • 31.
    Cornelissen, Tim
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Biler, Michal
    KTH Royal Inst Technol, Sweden.
    Urbanaviciute, Indre
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Norman, Patrick
    KTH Royal Inst Technol, Sweden.
    Linares, Mathieu
    KTH Royal Inst Technol, Sweden.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Kinetic Monte Carlo simulations of organic ferroelectrics2019In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, no 3, p. 1375-1383Article in journal (Refereed)
    Abstract [en]

    Ferroelectrics find broad applications, e.g. in non-volatile memories, but the switching kinetics in real, disordered, materials is still incompletely understood. Here, we develop an electrostatic model to study ferroelectric switching using 3D Monte Carlo simulations. We apply this model to the prototypical small molecular ferroelectric trialkylbenzene-1,3,5-tricarboxamide (BTA) and find good agreement between the Monte Carlo simulations, experiments, and molecular dynamics studies. Since the model lacks any explicit steric effects, we conclude that these are of minor importance. While the material is shown to have a frustrated antiferroelectric ground state, it behaves as a normal ferroelectric under practical conditions due to the large energy barrier for switching that prevents the material from reaching its ground state after poling. We find that field-driven polarization reversal and spontaneous depolarization have orders of magnitude different switching kinetics. For the former, which determines the coercive field and is relevant for data writing, nucleation occurs at the electrodes, whereas for the latter, which governs data retention, nucleation occurs at disorder-induced defects. As a result, by reducing the disorder in the system, the polarization retention time can be increased dramatically while the coercive field remains unchanged.

  • 32.
    Cukras, Janusz
    et al.
    University of Trieste, Italy; University of Warsaw, Poland.
    Kauczor, Joanna
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Norman, Patrick
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Chemistry. Linköping University, Faculty of Science & Engineering.
    Rizzo, Antonio
    IPCF CNR, Italy.
    Rikken, Geert L. J. A.
    UPS, France; UPS, France.
    Coriani, Sonia
    University of Trieste, Italy; Aarhus University, Denmark.
    A complex-polarization-propagator protocol for magneto-chiral axial dichroism and birefringence dispersion2016In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 19, p. 13267-13279Article in journal (Refereed)
    Abstract [en]

    A computational protocol for magneto-chiral dichroism and magneto-chiral birefringence dispersion is presented within the framework of damped response theory, also known as complex polarization propagator theory, at the level of time-dependent Hartree-Fock and time-dependent density functional theory. Magneto-chiral dichroism and magneto-chiral birefringence spectra in the (resonant) frequency region below the first ionization threshold of R-methyloxirane and L-alanine are presented and compared with the corresponding results obtained for both the electronic circular dichroism and the magnetic circular dichroism. The additional information content yielded by the magneto-chiral phenomena, as well as their potential experimental detectability for the selected species, is discussed.

  • 33.
    Danielsson, Örjan
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Simulations of Silicon Carbide Chemical Vapor Deposition2002Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Most of the modern electronics technology is based on the semiconducting material silicon. The increasing demands for smaller electronic devices with improved performance at lower costs drive the conventional silicon technology to its limits. To meet the requirements from the industry and to explore new application areas, other materials and fabrication methods must be used. For devices operating at high powers, high temperatures and high frequencies, the so-called wide bandgap semiconductors can be used with great success. Silicon carbide (SiC) and III-nitrides are wide bandgap materials that have gained increased interest in recent years. One important technique in manufacturing of electronic devices is chemical vapor deposition (CVD), by which thin layers can be deposited. These layers may have different electrical properties, depending on the choice of material and doping. Generally in CVD, a reactive gas mixture flows through a heated reactor chamber, where the substrates are placed. Complex chemical reactions take place in the gas and on the substrate surface, leading to many intermediate species and by-products, and eventually to the desired deposition. For the growth of device quality material it is important to be able to control the properties of the grown layers. These properties generally depend on the growth conditions in the reaction chamber, and on the chemistry of the deposition process. So far, empirical trial-and-error methods have been employed in the development of growth processes. Due to the lack of basic understanding of the governing physical processes, progress is costly and time consuming. Improving and optimizing the CVD process, as well as improving the fundamental understanding of the whole process is of great importance when good quality material should be produced. For this, computer simulations of the relevant physical and chemical phenomena can provide the necessary tools. This thesis focuses on computer simulations of the CVD process, in particular CVD of SiC. Simulations can be used not only as a tool for optimizing growth processes and reactor designs, they can also give information about physical phenomena that are difficult to measure, such as the gas-phase composition or the flow paths inside the reactor.

    Heating of the CVD susceptor is a central part of the process. For the growth of high quality SiC a relatively high temperature must be used. A convenient method for heating to high temperatures is by induction. A low resistive material, such as graphite, is placed inside a coil, which is given an alternating current. The graphite is then heated by the induced currents due to ohmic resistance. In this thesis the temperature distribution inside a CVD reactor, and how it is influenced by changes in coil frequency, power input to the coil and graphite thickness, is investigated. It is shown that by changing the placement and shape of the coil and by using insulation material correctly, a more uniform temperature distribution can be obtained.

    A model for the growth of SiC is used to predict growth rates at various process parameters. A number of possible factors influencing the growth rate are investigated using this model. The importance of including thermal diffusion and the effect of etching by hydrogen is shown, and the effect of parasitic growth investigated. Simulations show a mass transport limited growth, as seen from experiments.

    An improved susceptor design with an up-lifted substrate holder plate is investigated and compared to a conventional hot-wall reactor and to a cold-wall reactor. It is shown that stress induced by thermal gradients through the substrate is significantly reduced in the hot-wall reactor, and that stress due to backside growth can be diminished using the new design. Positive side effects are that slightly higher growth rates can be achieved, and that the growth temperature can be slightly lowered in the new susceptor.

    The doping incorporation behavior is thoroughly investigated experimentally for intentional doping with nitrogen and aluminum. The doping incorporation on both faces of SiC, as well as on two different polytypes is investigated. Equilibrium calculations are preformed, giving possible candidates for species responsible for the doping incorporation. To predict nitrogen doping concentrations, a simplified quantitative model is developed and applied to a large number of process parameters. It is seen that the same species as predicted by equilibrium calculations are produced, but the reactions producing these species are relatively slow, so that the highest concentrations are at the outlet of the reactor. It is thus concluded that N2 must be the major specie responsible for the nitrogen incorporation in SiC.

    For the growth of III-nitrides, ammonia is often used to give the nitrogen needed. It is well known that ammonia forms a solid adduct with the metalorganic gas, which is used as the source for the group III elements. It would thus be beneficial to use some other gas instead of ammonia. Since purity is of great importance, N2 gas would be the preferred choice. However, N2 is a very stable molecule and difficult to crack, even at high temperatures. It is shown that hydrogen can help in cracking nitrogen, and that growth of III-nitrides can be performed using N2 as the nitrogen-bearing gas, by only small changes to a conventional hot-wall CVD reactor.

    List of papers
    1. Investigation of the temperature profile in a hot-wall SiC chemical vapour deposition reactor
    Open this publication in new window or tab >>Investigation of the temperature profile in a hot-wall SiC chemical vapour deposition reactor
    2002 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 235, no 1-4, p. 352-364Article in journal (Refereed) Published
    Abstract [en]

    The chemical vapor deposition (CVD) technique is widely used to grow epitaxial layers of silicon carbide. To meet the demands for high quality epitaxial layers, which have good morphology and a minimum variation of the doping and thickness, a good knowledge of the CVD process is essential. The present work uses a simulation tool to investigate several parameters influencing the heating of <!--[if !vml]--><!--[endif]-->a hot-wall CVD reactor. The simulations are set up as 2D axisymmetric problems and validation is made in a 2D horizontal hot-wall CVD reactor. By applying the knowledge achieved from the simulations, the temperature profile is optimized to give as large area as possible with homogeneous temperature. New susceptor and coil designs are tested. A very good agreement between the simulated and the measured results is obtained. The new design has a temperature variation of less than 0.5% over more than 70% of the total susceptor length at an operating temperature of 1650°C. In addition, the power input needed to reach the operating temperature is decreased by 15% compared to the original design. 3D simulations are performed to show that the changes made in the 2D case give similar results for the real 3D case.

    Place, publisher, year, edition, pages
    ScienceDirect, 2002
    Keywords
    A1. Computer simulation, A1. Heat transfer, A3. Chemical, vapor deposition, A3. Hot-wall epitaxy, B2. Semiconducting silicon carbide
    National Category
    Other Engineering and Technologies not elsewhere specified
    Identifiers
    urn:nbn:se:liu:diva-15064 (URN)10.1016/S0022-0248(01)01831-0 (DOI)
    Available from: 2008-10-13 Created: 2008-10-13 Last updated: 2017-12-11Bibliographically approved
    2.
    The record could not be found. The reason may be that the record is no longer available or you may have typed in a wrong id in the address field.
    3. Predicted nitrogen doping concentrations in silicon carbide epitaxial layers grown by hot-wall chemical vapor deposition
    Open this publication in new window or tab >>Predicted nitrogen doping concentrations in silicon carbide epitaxial layers grown by hot-wall chemical vapor deposition
    2003 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 250, no 3-4, p. 471-478Article in journal (Refereed) Published
    Abstract [en]

    A simple quantitative model for the surface adsorption of nitrogen has been developed to simulate the doping incorporation in intentionally doped 4H-SiC samples during epitaxial growth. Different reaction schemes are necessary for the two faces of SiC. The differences are discussed, and implications to the necessary model adjustments are stressed. The simulations are validated by experimental values for a large number of different process parameters with good agreement.

    Keywords
    A1. Doping, A1. Growth models, A3. Chemical vapor deposition, A3. Hot wall epitaxy, B2. Semiconducting silicon carbide
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-46680 (URN)10.1016/S0022-0248(02)02513-7 (DOI)
    Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-13
    4. Reducing stress in silicon carbide epitaxial layers
    Open this publication in new window or tab >>Reducing stress in silicon carbide epitaxial layers
    2003 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 252, no 1-3, p. 289-296Article in journal (Refereed) Published
    Abstract [en]

    A susceptor for the epitaxial growth of silicon carbide, with an up-lifted substrate holder, is investigated and compared to other susceptor designs both experimentally and by the use of computational fluid dynamics simulations. It is shown that the wafer bending due to temperature gradients is diminished in a hot-wall reactor compared to growth in a cold-wall reactor. The substrate backside growth is diminished using the up-lifted substrate holder, limiting the substrate bending due to the backside growth. Thereby the stress built into the epitaxial layers during growth is significantly reduced. Simulations indicate a lower effective C/Si ratio over the wafer, and a lower preferable growth temperature, as compared to the original susceptor design. In addition a slightly higher growth rate is achieved

    Place, publisher, year, edition, pages
    Elsevier, 2003
    Keywords
    A1. Computer simulation; A3. Chemical vapor deposition; A3. Hot wall epitaxy; B2. Semiconducting silicon carbide
    National Category
    Condensed Matter Physics Other Physics Topics
    Identifiers
    urn:nbn:se:liu:diva-104590 (URN)10.1016/S0022-0248(03)00938-2 (DOI)
    Available from: 2014-02-19 Created: 2014-02-19 Last updated: 2017-12-06Bibliographically approved
    5. Using N2 as precursor gas in III-nitride CVD growth
    Open this publication in new window or tab >>Using N2 as precursor gas in III-nitride CVD growth
    2003 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 253, no 1-4, p. 26-37Article in journal (Refereed) Published
    Abstract [en]

    Computational fluid dynamics simulations have been performed to explore the possibility of using nitrogen gas as a precursor to III-nitride growth. A chemical model for the gas-phase decomposition of N2 has been used to show that large enough amounts of reactive species can be formed under conditions not far from those used in normal metalorganic chemical vapor deposition. Simulations were performed in 2D for various concentrations of N2, and comparisons with the use of NH3 were made. A modified reactor design needed to achieve high enough concentrations of reactive species is suggested. The possibility to increase the growth rate and material quality in III-nitride growth is discussed.

    Place, publisher, year, edition, pages
    Elsevier, 2003
    Keywords
    A1. Computer simulation; A1. Growth models; A3. Hot wall epitaxy; A3. Metalorganic chemical vapor deposition; B2. Semiconducting III–V materials
    National Category
    Materials Chemistry
    Identifiers
    urn:nbn:se:liu:diva-104589 (URN)10.1016/S0022-0248(03)00971-0 (DOI)
    Available from: 2014-02-19 Created: 2014-02-19 Last updated: 2017-12-06Bibliographically approved
    6. Nitrogen doping of epitaxial Silicon Carbide
    Open this publication in new window or tab >>Nitrogen doping of epitaxial Silicon Carbide
    Show others...
    2002 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 236, no 1-3, p. 101-112Article in journal (Refereed) Published
    Abstract [en]

    Intentional doping with nitrogen of 4H- and 6H-SiC has been performed using a hot-wall CVD reactor. The nitrogen doping dependence on the temperature, pressure, C/Si ratio, growth rate and nitrogen flow has been investigated. The nitrogen incorporation for C-face material showed to be C/Si ratio independent, whereas the doping decreased with increasing C/Si ratio for the Si-face material in accordance with the “site-competition” model. The nitrogen incorporation was constant in a temperature “window” of 75°C on Si-face material indicating a mass transport limited incorporation. Increasing the growth rate resulted in a decrease of nitrogen incorporation on Si-face but an increase on C-face material. Finally, a comparison between previously published results on cold-wall CVD-grown material and the present hot-wall-grown material is presented.

    Place, publisher, year, edition, pages
    ScienceDirect, 2002
    Keywords
    A1. Doping, A3. Hot wall epitaxy, B2. Superconducting materials
    National Category
    Other Engineering and Technologies not elsewhere specified
    Identifiers
    urn:nbn:se:liu:diva-15068 (URN)10.1016/S0022-0248(01)02198-4 (DOI)
    Note
    The status of this article on the day of the defence was: Submitted and the title of the article was "Nitrogen doping of Silicon Carbide: Effect of Process Parameters"Available from: 2008-10-13 Created: 2008-10-13 Last updated: 2017-12-11Bibliographically approved
    7. Aluminum doping of epitaxial Silicon Carbide
    Open this publication in new window or tab >>Aluminum doping of epitaxial Silicon Carbide
    Show others...
    2003 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 253, no 1-4, p. 340-350Article in journal (Refereed) Published
    Abstract [en]

    Intentional doping of aluminum in 4H and 6H SiC has been performed using a hot-wall CVD reactor. The dependence of aluminum incorporation on temperature, pressure, C/Si ratio, growth rate, and TMA flow has been investigated. The aluminum incorporation showed to be polarity dependent. The high aluminum incorporation on the Si-face is closely related to the carbon coverage on the SiC surface. Changes in process parameters changes the effective C/Si ratio close to the SiC surface. Increased growth rate and C/Si ratio increases the aluminum incorporation on the Si-face. Diffusion limited incorporation occurs at high growth rate. Reduced pressure increases the effective C/Si ratio, and at low growth rate, the aluminum incorporation increases initially, levels off at a critical pressure, and continues to decrease below the critical pressure. The aluminum incorporation showed to be constant in a temperature range of 50°C. The highest atomic concentration of aluminum observed in this study was 3×1017 and 8×1018 cm−3 in Si and C-face, respectively.

    Place, publisher, year, edition, pages
    ScienceDirect, 2003
    Keywords
    A1. Doping; A1. Growth models; A3. Chemical vapor deposition processes; A3. Hot wall epitaxy; B2. Semiconducting silicon carbide
    National Category
    Other Engineering and Technologies not elsewhere specified
    Identifiers
    urn:nbn:se:liu:diva-15055 (URN)10.1016/S0022-0248(03)01045-5 (DOI)
    Note
    The status of the article on the defence day was: Submitted and the original title was "Aluminum doping of Silicon Carbide: Effect of Process Parameters".Available from: 2008-10-13 Created: 2008-10-13 Last updated: 2017-12-11Bibliographically approved
  • 34.
    Danielsson, Örjan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Hallin, Christer
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Reducing stress in silicon carbide epitaxial layers2003In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 252, no 1-3, p. 289-296Article in journal (Refereed)
    Abstract [en]

    A susceptor for the epitaxial growth of silicon carbide, with an up-lifted substrate holder, is investigated and compared to other susceptor designs both experimentally and by the use of computational fluid dynamics simulations. It is shown that the wafer bending due to temperature gradients is diminished in a hot-wall reactor compared to growth in a cold-wall reactor. The substrate backside growth is diminished using the up-lifted substrate holder, limiting the substrate bending due to the backside growth. Thereby the stress built into the epitaxial layers during growth is significantly reduced. Simulations indicate a lower effective C/Si ratio over the wafer, and a lower preferable growth temperature, as compared to the original susceptor design. In addition a slightly higher growth rate is achieved

  • 35.
    Darmastuti, Zhafira
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Andersson, Mike
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Lindqvist, Niclas
    Alstom Power AB, Växjö, Sweden.
    Lloyd Spetz, Anita
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    SiC based field effect transistor for H2S detectionManuscript (preprint) (Other academic)
    Abstract [en]

    Pt-gate and Ir-gate SiC-FETs were tested as H2S sensors. Both sensors showed good response towards H2S in dry conditions with oxygen present. Ir-gate sensors showed high sensitivity and low drift, which makes them a suitable candidate for leak detection applications. Further testing was performed with Ir-gate sensors for geothermal applications. This involved humid environments and low oxygen concentrations. The sensitivity of the sensors decreased significantly at these conditions. When propene was added to the gas mixture, crosssensitivity was observed in the sensor signal. Further investigation to reveal the surface chemistry using spectroscopic techniques and modelling is needed to improve the selectivity of Ir-gate sensors in humid conditions and oxygen deficient environments.

  • 36.
    Derigs, Dominik
    et al.
    I. Physikalisches Institut, Universität zu Köln, Köln, Germany.
    Winters, Andrew Ross
    Mathematisches Institut, Universität zu Köln, Köln, Germany.
    Gassner, Gregor J
    Mathematisches Institut, Universität zu Köln, Köln, Germany.
    Walch, Stefanie
    I. Physikalisches Institut, Universität zu Köln, Köln, Germany.
    Bohm, Marvin
    Mathematisches Institut, Universität zu Köln, Köln, Germany.
    Ideal GLM-MHD: About the entropy consistent nine-wave magnetic field divergence diminishing ideal magnetohydrodynamics equations2018In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 364, p. 420-467Article in journal (Refereed)
    Abstract [en]

    The paper presents two contributions in the context of the numerical simulation of magnetized fluid dynamics. First, we show how to extend the ideal magnetohydrodynamics (MHD) equations with an inbuilt magnetic field divergence cleaning mechanism in such a way that the resulting model is consistent with the second law of thermodynamics. As a byproduct of these derivations, we show that not all of the commonly used divergence cleaning extensions of the ideal MHD equations are thermodynamically consistent. Secondly, we present a numerical scheme obtained by constructing a specific finite volume discretization that is consistent with the discrete thermodynamic entropy. It includes a mechanism to control the discrete divergence error of the magnetic field by construction and is Galilean invariant. We implement the new high-order MHD solver in the adaptive mesh refinement code FLASH where we compare the divergence cleaning efficiency to the constrained transport solver available in FLASH (unsplit staggered mesh scheme).

    The full text will be freely available from 2020-07-13 08:00
  • 37.
    Di Nuzzo, Daniele
    et al.
    Technical University of Eindhoven, Netherlands.
    van Reenen, Stephan
    Technical University of Eindhoven, Netherlands.
    Janssen, Rene A. J.
    Technical University of Eindhoven, Netherlands.
    Kemerink, Martijn
    Technical University of Eindhoven, Netherlands.
    Meskers, Stefan C. J.
    Technical University of Eindhoven, Netherlands.
    Evidence for space-charge-limited conduction in organic photovoltaic cells at open-circuit conditions2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 8, article id 085207Article in journal (Refereed)
    Abstract [en]

    The ac admittance of solar cells under illumination is investigated under open-circuit conditions. Open-circuit conditions are imposed by inserting a probe capacitor into the circuit. The capacitance and conductance of the cells are investigated as function of frequency and continuous illumination intensity. Results are compared with numerical and analytical modeling of charge recombination and transport. In bulk heterojunction solar cells with [6,6]-Phenyl-C-61(C-71)-butyric acid methyl ester as acceptor and poly(3-hexylthiophene) or poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] as electron donor, the high-frequency capacitance C and conductance G follow a power-law dependence on intensity of white light I, with G(I) proportional to I-3/4 and C(I) proportional to I-1/4. The modeling shows that these dependencies can be explained in terms of space-charge-limited current in combination with Langevin type recombination of carriers. For poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b]dithiophene-2,6-diyl]] the capacitance shows a weaker dependence on intensity, indicating fast recombination of photogenerated carriers. Results indicate that the fill factor of relatively well performing polymer solar cells can still be limited by space charge effects and can be improved by enhancing the charge carrier mobility or by reducing the bimolecular Langevin recombination. DOI: 10.1103/PhysRevB.87.085207

  • 38.
    Dieckmann, Mark E
    et al.
    Linköping University, Department of Science and Technology, Media and Information Technology. Linköping University, Faculty of Science & Engineering.
    Doria, D.
    Queens University of Belfast, North Ireland.
    Sarri, G.
    Queens University of Belfast, North Ireland.
    Romagnani, L.
    Ecole Polytech, France.
    Ahmed, H.
    Queens University of Belfast, North Ireland.
    Folini, D.
    University of Lyon, France.
    Walder, R.
    University of Lyon, France.
    Bret, A.
    University of Castilla La Mancha, Spain; Institute Invest Energet and Aplicac Ind, Spain.
    Borghesi, M.
    Queens University of Belfast, North Ireland.
    Electrostatic shock waves in the laboratory and astrophysics: similarities and differences2018In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 60, no 1, article id 014014Article in journal (Refereed)
    Abstract [en]

    Contemporary lasers allow us to create shocks in the laboratory that propagate at a speed that matches that of energetic astrophysical shocks like those that ensheath supernova blast shells. The rapid growth time of the shocks and the spatio-temporal resolution, with which they can be sampled, allow us to identify the processes that are involved in their formation and evolution. Some laser-generated unmagnetized shocks are mediated by collective electrostatic forces and effects caused by binary collisions between particles can be neglected. Hydrodynamic models, which are valid for many large-scale astrophysical shocks, assume that collisions enforce a local thermodynamic equilibrium in the medium; laser-generated shocks are thus not always representative for astrophysical shocks. Laboratory studies of shocks can improve the understanding of their astrophysical counterparts if we can identify processes that affect electrostatic shocks and hydrodynamic shocks alike. An example is the nonlinear thin-shell instability (NTSI). We show that the NTSI destabilises collisionless and collisional shocks by the same physical mechanism.

  • 39.
    Dong, Linpeng
    et al.
    Xidian Univ, Peoples R China.
    Yu, Jiangang
    Xidian Univ, Peoples R China.
    Jia, Renxu
    Xidian Univ, Peoples R China.
    Hu, Jichao
    Xian Univ Technol, Peoples R China.
    Zhang, Yuming
    Xidian Univ, Peoples R China.
    Sun, Jianwu
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Self-powered MSM deep-ultraviolet beta-Ga2O3 photodetector realized by an asymmetrical pair of Schottky contacts2019In: Optical Materials Express, ISSN 2159-3930, E-ISSN 2159-3930, Vol. 9, no 3, p. 1191-1199Article in journal (Refereed)
    Abstract [en]

    Self-powered photodetectors working in solar-blind region (below 280 nm) have attracted growing attention due to their wide applicability. Monoclinic Ga2O3 (beta-Ga2O3) with excellent merits and a wide bandgap (4.9 eV) is regarded as a good candidate for solar-blind photodetector application. Self-powered photodetectors generally based on homo/heterojunction suffer from a complex fabrication process and slow photoresponse because of the interface defects and traps. Herein, we demonstrated a fabrication and characterization of a self-powered metal-semiconductor-metal (MSM) deep-ultraviolet (DUV) photodetector based on single crystal beta-Ga2O3 . The self-powered property was realized through a simple one-step deposition of an asymmetrical pair of Schottky interdigital contacts. The photocurrent and responsivity increase with the degenerating symmetrical contact. For the device with the most asymmetric interdigital contacts operated at 0 V bias, the maximum photocurrent reaches 2.7 nA. The responsivity R-lambda. external quantum efficiency EQE, detectivity D*, and linear dynamic range LDR are 1.28 mA/W, 0.63, 1.77 x 10(11) Jones, and 23.5 dB, respectively. The device exhibits excellent repeatability and stability at the same time. Besides, the device presents a fast response speed with a rise time of 0.03 s and a decay time of 0.08 s. All these results indicate a promising and simple method to fabricate a zero-powered DUV photodetector. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

  • 40.
    Edberg, Jesper
    Linköping University, Department of Science and Technology, Physics and Electronics. Linköping University, Faculty of Science & Engineering. Linköping University.
    Flexible and Cellulose-based Organic Electronics2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Organic electronics is the study of organic materials with electronic functionality and the applications of such materials. In the 1970s, the discovery that polymers can be made electrically conductive led to an explosion within this field which has continued to grow year by year. One of the attractive features of organic electronic materials is their inherent mechanical flexibility, which has led to the development of numerous flexible electronics technologies such as organic light emitting diodes and solar cells on flexible substrates. The possibility to produce electronics on flexible substrates like plastic or paper has also had a large impact on the field of printed, electronics where inks with electronic functionality are used for large area fabrication of electronic devices using classical printing methods, such as screen printing, inkjet printing and flexography.

    Recently, there has been a growing interest in the use of cellulose in organic and printed electronics, not only as a paper substrate but also as a component in composite materials where the cellulose provides mechanical strength and favorable 3D-microstructures. Nanofibrillated cellulose is composed of cellulose fibers with high aspect-ratio and diameters in the nanometer range. Due to its remarkable mechanical strength, large area-to-volume ratio, optical transparency and solution processability it has been widely used as a scaffold or binder for electronically active materials in applications such as batteries, supercapacitors and optoelectronics.

    The focus of this thesis is on flexible devices based on conductive polymers and can be divided into two parts: (1) Composite materials of nanofibrillated cellulose and the conductive polymer PEDOT:PSS and (2) patterning of vapor phase polymerized conductive polymers. In the first part, it is demonstrated how the combination of cellulose and conductive polymers can be used to make electronic materials of various form factors and functionality. Thick, freestanding and flexible “papers” are used to realize electrochemical devices such as transistors and supercapacitors while lightweight, porous and elastic aerogels are used for sensor applications. The second focus of the thesis is on a novel method of patterning conductive polymers produced by vapor phase polymerization using UV-light. This method is used to realize flexible electrochromic smart windows with high-resolution images and tunable optical contrast.

    List of papers
    1. An Organic Mixed Ion–Electron Conductor for Power Electronics
    Open this publication in new window or tab >>An Organic Mixed Ion–Electron Conductor for Power Electronics
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    2016 (English)In: Advanced Science, ISSN 2198-3844, article id 1500305Article in journal (Refereed) Published
    Abstract [en]

    A mixed ionic–electronic conductor based on nanofibrillated cellulose composited with poly(3,4-ethylene-dioxythio­phene):­poly(styrene-sulfonate) along with high boiling point solvents is demonstrated in bulky electrochemical devices. The high electronic and ionic conductivities of the resulting nanopaper are exploited in devices which exhibit record values for the charge storage capacitance (1F) in supercapacitors and transconductance (1S) in electrochemical transistors.

    Place, publisher, year, edition, pages
    Wiley-VCH Verlagsgesellschaft, 2016
    National Category
    Electrical Engineering, Electronic Engineering, Information Engineering
    Identifiers
    urn:nbn:se:liu:diva-123225 (URN)10.1002/advs.201500305 (DOI)000370336500011 ()
    Note

    Funding agencies:  Knut and Alice Wallenberg foundation [KAW 2011.0050]; Onnesjo Foundation; Advanced Functional Materials Center at Linkoping University; Stiftelsen for strategisk forskning (SSF); RISE Research Institutes of Sweden; U.S. National Science Foundation [DMR-12

    Available from: 2015-12-08 Created: 2015-12-08 Last updated: 2018-02-15
    2. Thermoelectric Polymers and their Elastic Aerogels
    Open this publication in new window or tab >>Thermoelectric Polymers and their Elastic Aerogels
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    2016 (English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 28, no 22, p. 4556-4562Article in journal (Refereed) Published
    Abstract [en]

    Electronically conducting polymers constitute an emerging class of materials for novel electronics, such as printed electronics and flexible electronics. Their properties have been further diversified to introduce elasticity, which has opened new possibility for "stretchable" electronics. Recent discoveries demonstrate that conducting polymers have thermoelectric properties with a low thermal conductivity, as well as tunable Seebeck coefficients - which is achieved by modulating their electrical conductivity via simple redox reactions. Using these thermoelectric properties, all-organic flexible thermoelectric devices, such as temperature sensors, heat flux sensors, and thermoelectric generators, are being developed. In this article we discuss the combination of the two emerging fields: stretchable electronics and polymer thermoelectrics. The combination of elastic and thermoelectric properties seems to be unique for conducting polymers, and difficult to achieve with inorganic thermoelectric materials. We introduce the basic concepts, and state of the art knowledge, about the thermoelectric properties of conducting polymers, and illustrate the use of elastic thermoelectric conducting polymer aerogels that could be employed as temperature and pressure sensors in an electronic-skin.

    Place, publisher, year, edition, pages
    WILEY-V C H VERLAG GMBH, 2016
    National Category
    Polymer Chemistry
    Identifiers
    urn:nbn:se:liu:diva-129660 (URN)10.1002/adma.201505364 (DOI)000377123500029 ()26836440 (PubMedID)
    Note

    Funding Agencies|European Research Council (ERC) [307596]; Swedish Foundation for Strategic Research; Knut and Alice Wallenberg Foundation; Swedish Energy Agency; Advanced Functional Materials Center at Linkoping University; Research Institute of Sweden (RISE)

    Available from: 2016-06-27 Created: 2016-06-23 Last updated: 2018-09-07
    3. Patterning and Conductivity Modulation of Conductive Polymers by UV Light Exposure
    Open this publication in new window or tab >>Patterning and Conductivity Modulation of Conductive Polymers by UV Light Exposure
    Show others...
    2016 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 26, no 38, p. 6950-6960Article in journal (Refereed) Published
    Abstract [en]

    A novel patterning technique of conductive polymers produced by vapor phase polymerization is demonstrated. The method involves exposing an oxidant film to UV light which changes the local chemical environment of the oxidant and subsequently the polymerization kinetics. This procedure is used to control the conductivity in the conjugated polymer poly(3,4-ethylenedioxythiophene): tosylate by more than six orders of magnitude in addition to producing high-resolution patterns and optical gradients. The mechanism behind the modulation in the polymerization kinetics by UV light irradiation as well as the properties of the resulting polymer are investigated.

    Place, publisher, year, edition, pages
    WILEY-V C H VERLAG GMBH, 2016
    National Category
    Polymer Chemistry
    Identifiers
    urn:nbn:se:liu:diva-133122 (URN)10.1002/adfm.201601794 (DOI)000386159300010 ()
    Note

    Funding Agencies|Knut and Alice Wallenberg Foundation [KAW 2011.0050, KAW 2014.0041, KAW 2012.0302]

    Available from: 2016-12-12 Created: 2016-12-09 Last updated: 2018-02-15
  • 41.
    Ekström, Ulf
    Linköping University, Department of Physics, Chemistry and Biology, Computational Physics . Linköping University, The Institute of Technology.
    Time-dependent molecular properties in the optical and x-ray regions2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Time-dependent molecular properties are important for the experimental characterization of molecular materials. We show how these properties can be calculated, for optical and x-ray frequencies, using novel quantum chemical methods. For xray absorption there are important relativistic effects appearing, due to the high velocity electrons near the atomic nuclei. These effects are treated rigorously within the four-component static exchange approximation. We also show how electron correlation can be taken into account in the calculation of x-ray absorption spectra, in time-dependent density functional theory based on the complex polarization propagator approach. The methods developed have been applied to systems of experimental interest|molecules in the gas phase and adsorbed on metal surfaces. The effects of molecular vibrations have been take into account both within and beyond the harmonic approximation.

    List of papers
    1. Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: Experiment and theory
    Open this publication in new window or tab >>Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: Experiment and theory
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    2005 (English)In: Journal of chemical physics Online, ISSN 1089-7690, Vol. 122, no 12, p. 124305-Article in journal (Refereed) Published
    Abstract [en]

    The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288  eV photon energy, due to absorption to * virtual orbitals, and broader structures at higher energy, involving * virtual orbitals. The sharp absorption structures to the * empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of * symmetry, from the six chemically shifted C 1s core orbitals. ©2005 American Institute of Physics

    Keywords
    organic compounds, photoexcitation, photoelectron spectra, SCF calculations, ab initio calculations, chemical shift, molecular electronic states, molecule-photon collisions
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12685 (URN)10.1063/1.1864852 (DOI)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2013-11-25
    2. Probing the potential energy surface by highresolution x-ray absorption spectroscopy: The umbrella motion of the coreexcited CH3 free radical
    Open this publication in new window or tab >>Probing the potential energy surface by highresolution x-ray absorption spectroscopy: The umbrella motion of the coreexcited CH3 free radical
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    2007 (English)In: Physical Review. A, ISSN 1050-2947, Vol. 76, no 2, p. 124305-Article in journal (Refereed) Published
    Abstract [en]

    A detailed study of the umbrellalike vibration in inner-shell spectroscopy is presented. The high-resolution x-ray absorption spectrum for the lowest lying core excitation of the CH3 free radical was recorded. High quality potential energy surfaces (PES) for the initial and final states of the transition were calculated as a function of the symmetrical stretching and the umbrella deformation coordinates. The strong anharmonicity along the umbrella coordinate in the double-well region of the PES of the core excited state has a strong effect on the bending vibrational progressions. The excellent agreement between the experiment and theory allows an accurate spectroscopic characterization of the vibrational structure of the electronic transition, and the estimation of the umbrella inversion time of 149  fs.

    Keywords
    free radicals, molecular configurations, optical rotation, organic compounds, potential energy surfaces, vibrational states, X-ray absorption spectra
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:liu:diva-12686 (URN)10.1103/PhysRevA.76.022509 (DOI)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2009-04-23Bibliographically approved
    3. The umbrella motion of core-excited CH3 and CD3 methyl radicals
    Open this publication in new window or tab >>The umbrella motion of core-excited CH3 and CD3 methyl radicals
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    2008 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 128, no 4, p. 044302-1-044302-11Article in journal (Refereed) Published
    Abstract [en]

    An accurate experimental and theoretical study of the lowest core excitationof CH3 and CD3 methyl radicals is presented. The complexvibrational structure of the lowest band of the x-ray absorptionspectrum (XAS) is due to the large variation of themolecular geometry, which is planar in the ground state andpyramidal in the core-excited state. The XAS spectra of thetwo radicals were recorded at high resolution and assigned bytheoretical simulations of the spectra, taking into account the couplingof symmetrical stretching and symmetrical bending (umbrellalike) deformations of theradicals. An excellent agreement between experimental and theoretical spectral profilesallowed us to accurately characterize the vibrational structure of theelectronic transition. The similarities, as well as the differences, ofthe peculiar vibrational progression observed for the two radicals areexplained by the strong anharmonicity along the umbrella coordinate andby the isotopic variation, leading to a different probing ofthe double-well potential energy surface of the core excited stateduring the nuclear motion.

    Place, publisher, year, edition, pages
    College Park, MD United States: American Institute of Physics (AIP), 2008
    Keywords
    excited states, free radicals, ground states, molecular configurations, organic compounds, potential energy surfaces, vibrational states, X-ray scattering
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12687 (URN)10.1063/1.2822246 (DOI)000252821200025 ()
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2017-12-14Bibliographically approved
    4. Relativistic four-component static-exchange approximation for core-excitation processes in molecules
    Open this publication in new window or tab >>Relativistic four-component static-exchange approximation for core-excitation processes in molecules
    2006 (English)In: Physical review A, ISSN 1050-2947, Vol. 73, no 2, p. 022501-Article in journal (Refereed) Published
    Abstract [en]

    A generalization of the static-exchange approximation for core-electron spectroscopies to the relativistic four-component realm is presented. The initial state is a Kramers restricted Hartree-Fock state and the final state is formed as the configuration-interaction single excited state, based on the average of configurations for (n–1) electrons in n near-degenerate core orbitals for the reference ionic state. It is demonstrated that the static-exchange Hamiltonian can be made real by considering a set of time-reversal symmetric electron excitation operators. The static-exchange Hamiltonian is constructed at a cost that parallels a single Fock matrix construction in a quaternion framework that fully exploits time-reversal and spatial symmetries for the D2h point group and subgroups. The K- and L-edge absorption spectra of H2S are used to illustrate the methodology. The calculations adopt the Dirac-Coulomb Hamiltonian, but the theory is open ended toward improvements in the electron-electron interaction operator. It is demonstrated that relativistic effects are substantial for the L-edge spectrum of sulfur, and substantial deviations from the statistical 2:1 spin-orbit splitting of the intensity distribution are found. The average ratio in the mixed region is 1.54 at the present level of theory.

    Keywords
    hydrogen compounds, relativistic corrections, configuration interactions, HF calculations, excited states, spin-orbit interactions
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12688 (URN)10.1103/PhysRevA.73.022501 (DOI)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2013-11-25
    5. Resonant LII;III x-ray raman scattering from HCl
    Open this publication in new window or tab >>Resonant LII;III x-ray raman scattering from HCl
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    2006 (English)In: Physical Review A, ISSN 1050-2947, Vol. 74, no 6, p. 062512-Article in journal (Refereed) Published
    Abstract [en]

    We have studied the spectral features of Cl LII,III resonant x-ray Raman scattering of HCl molecules in gas phase both experimentally and theoretically. The theory, formulated in the intermediate-coupling scheme, takes into account the spin-orbital and molecular-field splittings in the Cl 2p shells, as well as the Coulomb interaction of the core hole with unoccupied molecular orbitals. Experiment and theory display nondispersive dissociative peaks formed by decay transitions in both molecular and dissociative regions. The molecular and atomic peaks collapse in a single narrow resonance because the dissociative potentials of core-excited and final states are parallel to each other along the whole pathway of the nuclear wave packet.

    Keywords
    hydrogen compounds, X-ray spectra, Raman spectra, dissociation energies, spin-orbit interactions, excited states
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12689 (URN)10.1103/PhysRevA.74.062512 (DOI)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2009-06-05
    6. Characterization of the chemisorption of methylsilane on a Au(1,1,1) surface from the silicon K- and L-edge spectra: a theoretical study using the four-component static exchange approximation
    Open this publication in new window or tab >>Characterization of the chemisorption of methylsilane on a Au(1,1,1) surface from the silicon K- and L-edge spectra: a theoretical study using the four-component static exchange approximation
    2005 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, p. 13846-13850Article in journal (Refereed) Published
    Abstract [en]

    X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectra (NEXAFS) of methylsilane, isolated and chemisorbed to a Au(1,1,1) surface, are determined in the fully relativistic four-component static exchange approximation¯both the K- and the L-edge of silicon are addressed in this investigation. In the fully chemisorbed structure, three H(Si) atoms have been cleaved off when Si binds in the hollow site of Au forming three Si−Au bonds of normal length. As due to the tri-coordinated chemisorption, the onsets of the K- and L-edge NEXAFS absorption bands occur some 2.0 and 2.5 eV lower in energy, respectively. The spin−orbit splittings in the silicon 2p-shell are not significantly changed due to adsorption. A partly chemisorbed methylsilane with only one H(Si) bond cleaved was also studied, and it is shown that the polarization dependence in the surface spectra contains details that can be used experimentally to identify the surface coordination of silicon. The red-shifts in the XPS silicon 1s (2p) spectra upon surface binding are 0.95 (0.65) and 1.15 (0.83) eV for the mono- and tricoordinated system, respectively.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12690 (URN)10.1021/jp0717084 (DOI)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2017-12-14
    7. X-ray absorption spectra from the resonantconvergent first-order polarization propagator approach
    Open this publication in new window or tab >>X-ray absorption spectra from the resonantconvergent first-order polarization propagator approach
    2006 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, no 4, p. 042722-Article in journal (Refereed) Published
    Abstract [en]

    The one-photon absorption cross sections of molecular systems have been determined in the high-energy region from the imaginary part of the electric dipole polarizability tensor. In contrast to commonly adopted state-specific methodologies, the complex polarization propagator approach does not require explicit consideration of the excited states and it is open-ended towards multiphoton absorption. It is shown that the electronic relaxation in the core-hole state is well accounted for in the present approach with use of standard density-functional based electronic structure methods. Sample calculations are presented of the K-edge x-ray absorption spectra for H2O, CO, C4H4N, and C6H6.

    Keywords
    water, carbon compounds, organic compounds, X-ray absorption spectra, molecular moments, polarisability, electric moments, excited states, multiphoton spectra, density functional theory
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12691 (URN)10.1103/PhysRevA.74.042722 (DOI)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2017-12-14
    8. Polarization propagator for x-ray spectra
    Open this publication in new window or tab >>Polarization propagator for x-ray spectra
    2006 (English)In: Physical Review Letters, ISSN 1050-2947, Vol. 97, no 14, p. 143001-Article in journal (Refereed) Published
    Abstract [en]

    A polarization propagator for x-ray spectra is outlined and implemented in density functional theory. It rests on a formulation of a resonant-convergent first-order polarization propagator approach which makes it possible to directly calculate the x-ray absorption cross section at a particular frequency without explicitly addressing the excited states. The quality of the predicted x-ray spectrum relates only to the type of density functional applied without any separate treatment of dynamical relaxation effects.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12692 (URN)10.1103/PhysRevLett.97.143001 (DOI)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2009-05-11
    9. Self-interaction-corrected time-dependent density functional theory calculations of x-ray absorption spectra
    Open this publication in new window or tab >>Self-interaction-corrected time-dependent density functional theory calculations of x-ray absorption spectra
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    2007 (English)In: Physical Review A, ISSN 1050-2947, Vol. 76, no 2, p. 022506-Article in journal (Refereed) Published
    Abstract [en]

    We outline an approach within time-dependent density functional theory that predicts x-ray spectra on an absolute scale. The approach rests on a recent formulation of the resonant-convergent first-order polarization propagator [P. Norman et al., J. Chem. Phys. 123, 194103 (2005)] and corrects for the self-interaction energy of the core orbital. This polarization propagator approach makes it possible to directly calculate the x-ray absorption cross section at a particular frequency without explicitly addressing the excited-state spectrum. The self-interaction correction for the employed density functional accounts for an energy shift of the spectrum, and fully correlated absolute-scale x-ray spectra are thereby obtained based solely on optimization of the electronic ground state. The procedure is benchmarked against experimental spectra of a set of small organic molecules at the carbon, nitrogen, and oxygen K edges.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12693 (URN)10.1103/PhysRevA.76.022506 (DOI)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2009-05-15
    10. Near-edge X-ray absorption and natural circular dichroism spectra of L-alanine: a theoretical study based on the complex polarization propagator approach
    Open this publication in new window or tab >>Near-edge X-ray absorption and natural circular dichroism spectra of L-alanine: a theoretical study based on the complex polarization propagator approach
    2007 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, no 16, p. 165104-Article in journal (Refereed) Published
    Abstract [en]

    The complex polarization propagator method [J. Chem. Phys. 123, 194103 (2005)] has been employed in conjunction with density functional theory and gauge-including atomic orbitals in order to determine the near-edge x-ray absorption and natural circular dichroism spectra of L-alanine in its neutral and zwitterionic forms. Results are presented for the K-edges of carbon, nitrogen, and oxygen. In contrast to traditional methods, the proposed approach enables a direct determination ofspectra at an arbitrary frequency instead of focusing on the rotatory strengths for individual electronic transitions. The propagator includes a complete set ofand allows for full core-hole relaxation. The theoretical spectrum at the nitrogen K-edge of the zwitterion compares well with the experimental spectrum. the nonredundant electron-transfer operators

    Keywords
    X-ray absorption near edge structure, absorption spectra, dichroism, polarization spectroscopy, density functionals, atomic orbitals
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12724 (URN)10.1063/1.2800024 (DOI)
    Available from: 2007-11-16 Created: 2007-11-16 Last updated: 2017-12-14
    11. Four-component Hartree–Fock calculations of magnetic-field induced circular birefringence—Faraday effect—in noble gases and dihalogens
    Open this publication in new window or tab >>Four-component Hartree–Fock calculations of magnetic-field induced circular birefringence—Faraday effect—in noble gases and dihalogens
    2005 (English)In: The Journal of Chemical Physics, ISSN 1050-2947, Vol. 122, p. 074321-Article in journal (Refereed) Published
    Abstract [en]

    Theeffects of relativity on the magnetic-field induced circular birefringence, orFaraday effect, in He, Ne, Ar, Xe, Rn, F2, Cl2,Br2, and I2 have been determined at the four-component Hartree–Focklevel of theory. A measure of the birefringence is givenby the Verdet constant, which is a third-order molecular propertyand thus relates to quadratic response functions. A fully analyticalnonlinear polarization propagator approach is employed. The results are gaugeinvariant as a consequence of the spatial symmetries in themolecular systems. The calculations include electronic as well as vibrationalcontributions to the property. Comparison with experiment is made forHe, Ne, Ar, Xe, and Cl2, and, apart from neon,the theoretical values of the Verdet constant are within 10%of the experimental ones. The inclusion of nonrelativistically spin-forbidden excitationsin the propagator parametrization has significant effects on the dispersionin general, but such effects are in the general caselargely explained by the use of a resonant-divergent propagator theory.In the present work we do, however, observe noticeable relativisticcorrections to the Verdet constant in the off-resonant regions forsystems with light elements (F2 and Cl2), and nonrelativistic resultsfor the Verdet constant of Br2 are in error by25% in the low-frequency region. ©2005 American Institute of Physics.

    Keywords
    HF calculations, birefringence, helium, neon, argon, xenon, radon, fluorine, chlorine, bromine, iodine, Faraday effect, polarisation, relativistic corrections, vibrational states, excited states, resonant states, magnetic fields
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12695 (URN)10.1063/1.1849167 (DOI)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2013-11-25
    12. The molecular electric quadrupole moment and electric field gradient induced birefringence (Buckingham effect) of Cl2
    Open this publication in new window or tab >>The molecular electric quadrupole moment and electric field gradient induced birefringence (Buckingham effect) of Cl2
    2004 (English)In: Journal of Computational Methods in Sciences and Engineering, ISSN 1472-7978, Vol. 4, no 3, p. 365-380Article in journal (Refereed) Published
    Abstract [en]

    An ab initio investigation of the molecular properties rationalizing the electric-field-gradient induced birefringence (Buckingham effect) for Cl_2 is presented. The quadrupole moment is determined using hierarchies of basis sets and wavefunction models. The electric dipole polarizability, the dipole - dipole - quadrupole and dipole - dipole - magnetic dipole hyperpolarizabilities are determined exploiting a Coupled Cluster Singles and Doubles (CCSD) response approach. The properties are zero-point vibrationally averaged, and the contribution of excited ro-vibrational states accounted for. To this end, the interatomic ^1Σ_g^+ ground state potential has been computed at CCSD plus perturbative triples - CCSD(T) - level employing a large augmented correlation consistent basis set. The effect of relativity is estimated at the Dirac-Hartree-Fock level. Our best value for the quadrupole moment of Cl_2 is (2.327 ± 0.010) au and it is in excellent agreement with experiment which, after revision and dependent on the procedure employed for correcting the original estimate of (2.24 ± 0.04) au of Graham et al., [Mol. Phys., 93, 49, (1998)], ranges from (2.31 ± 0.04) au to (2.36 ± 0.04) au.

    Keywords
    molecular quadrupole moment, birefringence, electric field gradient, electric dipole (hyper)polarizabilities, coupled cluster theory, relativistic effects, molecular vibrations
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-12696 (URN)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2013-11-25
    13. On the evaluation of quadratic response functions at the four-component Hartree-Fock level: Nonlinear polarization and two-photon absorption in bromo- and iodobenzen
    Open this publication in new window or tab >>On the evaluation of quadratic response functions at the four-component Hartree-Fock level: Nonlinear polarization and two-photon absorption in bromo- and iodobenzen
    2006 (English)In: The Journal of Chemical Physics, ISSN 0021-9606, Vol. 124, no 21, p. 214311-Article in journal (Refereed) Published
    Abstract [en]

    The nonlinear polarization and two-photon absorption parameters have been determined for dibromo- and di-iodobenzene in their meta- and ortho-conformations and with relativistic effects accounted for to a varying degree. By exclusion of small component integrals in the calculations of the first-order hyperpolarizability, results within 1% of fully relativistic four-component Hartree-Fock values are obtained at a cost of 8.7 times the corresponding nonrelativistic calculations. It is shown that the nonlinear absorption in bromobenzene (and even more so in iodobenzene) is broad banded due to spin-orbit interactions among the excited states, and nonrelativistic and scalar relativistic calculations are not to be used in this case.

    Keywords
    organic compounds, HF calculations, two-photon processes, relativistic corrections, polarisability, spin-orbit interactions, excited states
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-13999 (URN)10.1063/1.2204604 (DOI)
    Available from: 2006-10-05 Created: 2006-10-05 Last updated: 2009-05-18
  • 42.
    Ektarawong, Annop
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Thermodynamic consideration and ground-state search of icosahedral boron subselenide B-12(B1-xSex)(2) from a first-principles cluster expansion2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 17, article id 174103Article in journal (Refereed)
    Abstract [en]

    The phase stability of icosahedral boron subselenide B-12(B1-xSex)(2), where 0.5 amp;lt;= x amp;lt;= 1, is explored using a first-principles cluster expansion. The results shows that, instead of a continuous solid solution, B-12(B1-xSex)(2) is thermodynamically stable as an individual line compound at the composition of B9.5Se. The ground-state configuration of B9.5Se is represented by a mixture of B-12(Se-Se), B-12(B-Se), and B-12(Se-B) with a ratio of 1: 1: 1, where they form a periodic ABCABC... stacking sequence of B-12(Se-Se), B-12(B-Se), and B-12(Se-B) layers along the c axis of the hexagonal conventional unit cell. The structural and electronic properties of the ground-state B9.5Se are also derived and discussed. By comparing the derived ground-state properties of B9.5Se to the existing experimental data of boron subselenide B similar to 13Se, I proposed that the as-synthesized boron subselenide B similar to 13Se, as reported in the literature, has the actual composition of B9.5Se.

  • 43.
    Ektarawong, Annop
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Max Planck Inst Eisenforsch GmbH, Germany.
    Structural models of increasing complexity for icosahedral boron carbide with compositions throughout the single-phase region from first principles2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 17, article id 174104Article in journal (Refereed)
    Abstract [en]

    We perform first-principles calculations to investigate the phase stability of boron carbide, concentrating on the recently proposed alternative structural models composed not only of the regularly studied B11Cp(CBC) and B-12(CBC), but also of B-12(CBCB) and B-12(B-4). We find that a combination of the four structural motifs can result in low-energy electron precise configurations of boron carbide. Among several considered configurations within the composition range of B10.5C and B4C, we identify in addition to the regularly studied B11Cp(CBC) at the composition of B4C two low-energy configurations, resulting in a new view of the B-C convex hull. Those are [B-12(CBC)](0.67)[B-12(B-4)](0.33) and [B-12(CBC)](0.67)[B-12(CBCB)](0.33), corresponding to compositions of B10.5C and B6.67C, respectively. As a consequence, B-12(CBC) at the composition of B6.5C, previously suggested in the literature as a stable configuration of boron carbide, is no longer part of the B-C convex hull. By inspecting the electronic density of states as well as the elastic moduli, we find that the alternative models of boron carbide can provide a reasonably good description for electronic and elastic properties of the material in comparison with the experiments, highlighting the importance of considering B-12(CBCB) and B-12(B-4), together with the previously proposed B11Cp(CBC) and B-12(CBC), as the crucial ingredients for modeling boron carbide with compositions throughout the single-phase region.

  • 44.
    Elofsson, Viktor