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  • 1.
    Heine, Matthew
    et al.
    Boston Coll, MA 02467 USA.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Broido, David
    Boston Coll, MA 02467 USA.
    Effect of thermal lattice and magnetic disorder on phonons in bcc Fe: A first-principles study2019Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, nr 10, artikel-id 104304Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a first-principles theoretical approach to calculate temperature dependent phonon dispersions in bcc Fe, which captures finite temperature spin-lattice coupling by treating thermal disorder in both the spin and lattice systems simultaneously. With increasing temperature, thermal atomic displacements are found to induce increasingly large fluctuations in local magnetic moment magnitudes. The calculated phonon dispersions of bcc Fe show excellent agreement with measured data over a wide range of temperatures both above and below the magnetic and structural transition temperatures, suggesting the applicability of the developed approach to other magnetic materials.

  • 2.
    Xu, Bin
    et al.
    Soochow Univ, Peoples R China; Univ Arkansas, AR 72701 USA; Univ Arkansas, AR 72701 USA.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Bellaiche, L.
    Univ Arkansas, AR 72701 USA.
    Order-disorder transition in the prototypical antiferroelectric PbZrO32019Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, nr 2, artikel-id 020102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The prototypical antiferroelectric PbZrO3 has several unsettled questions, such as the nature of the antiferroelectric transition, a possible intermediate phase, and the microscopic origin of the Pbam ground state. Using first-principles calculations, we show that no phonon becomes truly soft at the cubic-to-Pbam transition temperature, and the order-disorder character of this transition is clearly demonstrated based on molecular dynamics simulations and potential energy surfaces. The out-of-phase octahedral tilting is an important degree of freedom, which can collaborate with other phonon distortions and form a complex energy landscape with multiple minima Candidates of the possible intermediate phase are suggested based on the calculated kinetic barriers between energy minima, and the development of a first-principles-based effective Hamiltonian. The use of this latter scheme further reveals that specific bilinear interactions between local dipoles and octahedral tiltings play a major role in the formation of the Pbam ground state, which contrasts with most of the previous explanations.

  • 3.
    Szymanski, N. J.
    et al.
    Univ Toledo, OH 43606 USA.
    Walters, L. N.
    Northwestern Univ, IL 60208 USA.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. CALTECH, CA 91125 USA.
    Gall, D.
    Rensselaer Polytech Inst, NY 12180 USA.
    Khare, S. V.
    Univ Toledo, OH 43606 USA.
    Dynamical stabilization in delafossite nitrides for solar energy conversion2018Ingår i: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 6, nr 42, s. 20852-20860Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Delafossite structured ternary nitrides, ABN(2), have been of recent experimental investigation for applications such as tandem solar and photoelectrochemical cells. We present a thorough first principles computational investigation of their stability, electronic structure, and optical properties. Nine compounds, where A = Cu, Ag, Au and B = V, Nb, Ta, were studied. For three of these compounds, CuTaN2, CuNbN2, and AgTaN2, our computations agree well with experimental results. Optimized lattice parameters, formation energies, and mechanical properties have been computed using the generalized gradient approximation (GGA). Phonon density of states computed at zero-temperature shows that all compounds are dynamically unstable at low temperatures. Including finite-temperature anharmonic effects stabilizes all compounds at 300 K, with the exception of AgVN2. Analysis of Crystal Orbital Hamiltonian Populations (COHP) provides insight into the bonding and antibonding characters of A-N and B-N pairs. Instability at low temperatures can be attributed to strong A-N antibonding character near the Fermi energy. B-N bonding is found to be crucial in maintaining stability of the structure. AgVN2 is the only compound to display significant B-N antibonding below the Fermi energy, as well as the strongest degree of A-N antibonding, both of which provide explanation for the sustained instability of this compound up to 900 K. Hybrid functional calculations of electronic and optical properties show that real static dielectric constants in the semiconductors are related to corresponding band gaps through the Moss relation. CuTaN2, CuNbN2, AgTaN2, AgNbN2, AgVN2, AuTaN2, and AuNbN2 exhibit indirect electronic band gaps while CuVN2 and AuVN2 are metallic. Imaginary parts of the dielectric function are characterized by d-d interband transitions in the semiconductors and d-d intraband transitions in the metals. Four compounds, CuTaN2, CuNbN2, AgTaN2, and AgNbN2, are predicted to exhibit large light absorption in the range of 1.0 to 1.7 eV, therefore making these materials good candidates for solar-energy conversion applications. Two compounds, AuTaN2 and AuNbN2, have band gaps and absorption onsets near the ideal range for obtaining high solar-cell conversion efficiency, suggesting that these compounds could become potential candidates as absorber materials in tandem solar cells or for band-gap engineering by alloying.

  • 4.
    Bykova, E.
    et al.
    DESY, Germany; Univ Bayreuth, Germany.
    Bykov, M.
    Univ Bayreuth, Germany; Natl Univ Sci and Technol MISIS, Russia.
    Cernok, A.
    Univ Bayreuth, Germany; Open Univ, England.
    Tidholm, Johan
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Simak, Sergey
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. CALTECH, CA 91125 USA.
    Belov, M. P.
    Natl Univ Sci and Technol MISIS, Russia.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Liermann, H. -P.
    DESY, Germany.
    Hanfland, M.
    European Synchrotron Radiat Facil, France.
    Prakapenka, V. B.
    Univ Chicago, IL 60637 USA.
    Prescher, C.
    Univ Chicago, IL 60637 USA; Univ Cologne, Germany.
    Dubrovinskaia, N.
    Univ Bayreuth, Germany.
    Dubrovinsky, L.
    Univ Bayreuth, Germany.
    Metastable silica high pressure polymorphs as structural proxies of deep Earth silicate melts2018Ingår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, artikel-id 4789Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Modelling of processes involving deep Earth liquids requires information on their structures and compression mechanisms. However, knowledge of the local structures of silicates and silica (SiO2) melts at deep mantle conditions and of their densification mechanisms is still limited. Here we report the synthesis and characterization of metastable high-pressure silica phases, coesite-IV and coesite-V, using in situ single-crystal X-ray diffraction and ab initio simulations. Their crystal structures are drastically different from any previously considered models, but explain well features of pair-distribution functions of highly densified silica glass and molten basalt at high pressure. Built of four, five-, and six-coordinated silicon, coesite-IV and coesite-V contain SiO6 octahedra, which, at odds with 3rd Paulings rule, are connected through common faces. Our results suggest that possible silicate liquids in Earths lower mantle may have complex structures making them more compressible than previously supposed.

  • 5.
    Herriman, Jane E.
    et al.
    CALTECH, CA 91125 USA.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. CALTECH, CA 91125 USA.
    Fultz, Brent
    CALTECH, CA 91125 USA.
    Phonon thermodynamics and elastic behavior of GaN at high temperatures and pressures2018Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, nr 21, artikel-id 214105Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effects of temperature and pressure on the phonons of GaN were calculated for both the wurtzite and zinc-blende structures. The quasiharmonic approximation (QHA) gave reasonable results for the temperature dependence of the phonon DOS at zero pressure but unreliably predicted the combined effects of temperature and pressure. Pressure was found to change the explicit anharmonicity, altering the thermal shifts of phonons and more notably qualitatively changing the evolution of phonon lifetimes with increasing temperature. These effects were largest for the optical modes, and phonon frequencies below approximately 5 THz were adequately predicted with the QHA. The elastic anisotropies of GaN in both wurtzite and zinc-blende structures were calculated from the elastic constants as a function of pressure at 0 K. The elastic anisotropy increased with pressure until reaching elastic instabilities at 40 GPa (zinc blende) and 65 GPa (wurtzite). The calculated instabilities are consistent with proposed transformation pathways to rocksalt GaN and place upper bounds on the pressures at which wurtzite and zinc-blende GaN can be metastable.

  • 6.
    Shulumba, Nina
    et al.
    CALTECH, CA 91125 USA.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. CALTECH, CA 91125 USA.
    Minnich, Austin J.
    CALTECH, CA 91125 USA.
    Intrinsic localized mode and low thermal conductivity of PbSe2017Ingår i: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 95, nr 1, artikel-id 014302Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Lead chalcogenides such as PbS, PbSe, and PbTe are of interest for their exceptional thermoelectric properties and strongly anharmonic lattice dynamics. Although PbTe has received the most attention, PbSe has a lower thermal conductivity and a nonlinear temperature dependence of thermal resistivity despite being stiffer, trends that prior first-principles calculations have not fully reproduced. Here, we use ab initio calculations that explicitly account for strong anharmonicity and a computationally efficient stochastic phase-space sampling scheme to identify the origin of this low thermal conductivity as an anomalously large anharmonic interaction, exceeding in strength that in PbTe, between the transverse optic and longitudinal acoustic branches. The strong anharmonicity is reflected in the striking observation of an intrinsic localized mode that forms in the acoustic frequencies. Our work shows the deep insights into thermal phonons that can be obtained from ab initio calculations that do not rely on perturbations from the ground-state phonon dispersion.

  • 7.
    Kerdsongpanya, Sit
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten. Rensselaer Polytech Institute, NY 12180 USA.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. CALTECH, CA 91125 USA.
    Sun, Bo
    National University of Singapore, Singapore.
    Kan Koh, Yee
    National University of Singapore, Singapore.
    Lu, Jun
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Van Nong, Ngo
    Department of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, Roskilde, Denmark.
    Simak, Sergei I.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Alling, Björn
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. Max Planck Institute Eisenforsch GmbH, Germany.
    Eklund, Per
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Phonon thermal conductivity of scandium nitride for thermoelectrics from first-principles calculations and thin-film growth2017Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, nr 19, artikel-id 195417Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The knowledge of lattice thermal conductivity of materials under realistic conditions is vitally important since many modern technologies require either high or low thermal conductivity. Here, we propose a theoretical model for determining lattice thermal conductivity, which takes into account the effect of microstructure. It is based on ab initio description that includes the temperature dependence of the interatomic force constants and treats anharmonic lattice vibrations. We choose ScN as a model system, comparing the computational predictions to the experimental data by time-domain thermoreflectance. Our experimental results show a trend of reduction in lattice thermal conductivity with decreasing domain size predicted by the theoretical model. These results suggest a possibility to control thermal conductivity by microstructural tailoring and provide a predictive tool for the effect of the microstructure on the lattice thermal conductivity of materials based on ab initio calculations.

  • 8.
    Sangiovanni, Davide
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Alling, Björn
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten. Max Planck Institute Eisenforsch GmbH, Germany.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. National University of Science and Technology MISIS, Russia; Tomsk State University, Russia.
    Efficient and accurate determination of lattice-vacancy diffusion coefficients via non equilibrium ab initio molecular dynamics2016Ingår i: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, nr 9, s. 094305-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We revisit the color-diffusion algorithm [Aeberhard et al., Phys. Rev. Lett. 108, 095901 (2012)] in non equilibrium ab initio molecular dynamics (NE-AIMD) and propose a simple efficient approach for the estimation of monovacancy jump rates in crystalline solids at temperatures well below melting. Color-diffusion applied to monovacancy migration entails that one lattice atom (colored atom) is accelerated toward the neighboring defect site by an external constant force F. Considering bcc molybdenum between 1000 and 2800 K as a model system, NE-AIMD results show that the colored-atom jump rate k(NE) increases exponentially with the force intensity F, up to F values far beyond the linear-fitting regime employed previously. Using a simple model, we derive an analytical expression which reproduces the observed k(NE)(F) dependence on F. Equilibrium rates extrapolated by NE-AIMD results are in excellent agreement with those of unconstrained dynamics. The gain in computational efficiency achieved with our approach increases rapidly with decreasing temperatures and reaches a factor of 4 orders of magnitude at the lowest temperature considered in the present study.

  • 9.
    Mosyagin, Igor
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Olovsson, Weine
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten.
    Simak, Sergey
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. NUST MISIS, Russia.
    Highly Efficient Free Energy Calculations of the Fe Equation of State Using Temperature-Dependent Effective Potential Method2016Ingår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 120, nr 43, s. 8761-8768Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Free energy calculations at finite temperature based on ab initio molecular dynamics (AIMD) simulations have become possible, but they are still highly computationally demanding. Besides, achieving simultaneously high accuracy of the calculated results and efficiency of the computational algorithm is still a challenge. In this work we describe an efficient algorithm to determine accurate free energies of solids in simulations using the recently proposed temperature-dependent effective potential method (TDEP). We provide a detailed analysis of numerical approximations employed in the TDEP algorithm. We show that for a model system considered in this work, hcp Fe, the obtained thermal equation of state at 2000 K is in excellent agreement with the results of standard calculations within the quasiharmonic approximation.

  • 10.
    Shulumba, Nina
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten. University of Saarland, Germany.
    Raza, Zamaan
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA.
    Janzén, Erik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. Materials Modeling and Development Laboratory, NUST “MISIS”, Moscow, Russia; LACOMAS Laboratory, Tomsk State University, Tomsk, Russia.
    Odén, Magnus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten.
    Impact of anharmonic effects on the phase stability, thermal transport, and electronic properties of AlN2016Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, nr 10, artikel-id 104305Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Wurtzite aluminium nitride is a technologically important wide band gap semiconductor with an unusually high thermal conductivity, used in optical applications and as a heatsink substrate. Many of its properties depend on an accurate description of its lattice dynamics, which have thus far only been captured in the quasiharmonic approximation. In this work, we demonstrate that anharmonicity has a considerable impact on its phase stability and transport properties, since anharmonicity is much stronger in the rocksalt phase. We compute a pressure-temperature phase diagram of AlN, demonstrating that the rocksalt phase is stabilised by increasing temperature, with respect to the wurtzite phase. We demonstrate that including anharmonicity, we can recover the thermal conductivity of the wurtzite phase (320 Wm−1K−1 under ambient conditions), and compute the hitherto unknown thermal conductivity of the rocksalt phase (96 Wm−1K−1). We also show that the electronic band gap decreases with temperature. These findings provide further evidence that anharmonic effects cannot be ignored in high temperature applications.

  • 11.
    Nilsson, Johan O.
    et al.
    KTH Royal Institute Technology, Sweden.
    Yu Vekilova, Olga
    KTH Royal Institute Technology, Sweden.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Klarbring, Johan
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Simak, Sergey
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Skorodumova, Natalia V.
    KTH Royal Institute Technology, Sweden; Uppsala University, Sweden.
    Ionic conductivity in Gd-doped CeO2: Ab initio color-diffusion nonequilibrium molecular dynamics study2016Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 93, nr 2, s. 024102-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A first-principles nonequilibrium molecular dynamics (NEMD) study employing the color-diffusion algorithm has been conducted to obtain the bulk ionic conductivity and the diffusion constant of gadolinium-doped cerium oxide (GDC) in the 850-1150 K temperature range. Being a slow process, ionic diffusion in solids usually requires simulation times that are prohibitively long for ab initio equilibrium molecular dynamics. The use of the color-diffusion algorithm allowed us to substantially speed up the oxygen-ion diffusion. The key parameters of the method, such as field direction and strength as well as color-charge distribution, have been investigated and their optimized values for the considered system have been determined. The calculated ionic conductivity and diffusion constants are in good agreement with available experimental data.

  • 12.
    Shulumba, Nina
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten. University of Saarland, Germany.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. CALTECH, CA 91125 USA.
    Raza, Zamaan
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten.
    Alling, Björn
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten. Max Planck Institute Eisenforsch GmbH, Germany.
    Barrirero, Jenifer
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten. University of Saarland, Germany.
    Muecklich, Frank
    University of Saarland, Germany.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. NUST MISIS, Russia.
    Odén, Magnus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten.
    Lattice Vibrations Change the Solid Solubility of an Alloy at High Temperatures2016Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 117, nr 20, artikel-id 205502Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We develop a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti1-xAlxN alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy corresponding to the true equilibrium state of the system. We demonstrate that the vibrational contribution including anharmonicity and temperature dependence of the mixing enthalpy have a decisive impact on the calculated phase diagram of a Ti1-xAlxN alloy, lowering the maximum temperature for the miscibility gap from 6560 to 2860 K. Our local chemical composition measurements on thermally aged Ti0.5Al0.5N alloys agree with the calculated phase diagram.

  • 13.
    Dewandre, Antoine
    et al.
    University of Liege, Belgium.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. CALTECH, CA 91125 USA.
    Bhattacharya, Sandip
    Ruhr University of Bochum, Germany.
    Romero, Aldo H.
    West Virginia University, WV 26506 USA; Benemerita University of Autonoma Puebla, Mexico.
    Madsen, Georg K. H.
    TU Wien, Austria.
    Verstraete, Matthieu J.
    University of Liege, Belgium.
    Two-Step Phase Transition in SnSe and the Origins of its High Power Factor from First Principles2016Ingår i: PHYSICAL REVIEW LETTERS, ISSN 0031-9007, Vol. 117, nr 27, artikel-id 276601Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The interest in improving the thermoelectric response of bulk materials has received a boost after it has been recognized that layered materials, in particular SnSe, show a very large thermoelectric figure of merit. This result has received great attention while it is now possible to conceive other similar materials or experimental methods to improve this value. Before we can now think of engineering this material it is important we understand the basic mechanism that explains this unusual behavior, where very low thermal conductivity and a high thermopower result from a delicate balance between the crystal and electronic structure. In this Letter, we present a complete temperature evolution of the Seebeck coefficient as the material undergoes a soft crystal transformation and its consequences on other properties within SnSe by means of first-principles calculations. Our results are able to explain the full range of considered experimental temperatures.

  • 14.
    Shulumba, Nina
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten. University of Saarland, Germany.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA.
    Raza, Zamaan
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Alling, Björn
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten. Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany.
    Barrirero, Jennifer
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten. Functional Materials, Saarland University, Campus D3 3, Saarbrücken, Germany.
    Mücklich, Frank
    Functional Materials, Saarland University, Campus D3 3, Saarbrücken, Germany.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. Materials Modeling and Development Laboratory, NUST “MISIS”, Moscow, Russia / LACOMAS Laboratory, Tomsk State University, Tomsk, Russia.
    Odén, Magnus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten.
    Anharmonicity changes the solid solubility of an alloy at high temperatures2015Manuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We have developed a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti1−xAlxN alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy, corresponding to the true equilibrium state of the system. We demonstrate that the anharmonic contribution and temperature dependence of the mixing enthalpy have a decisive impact on the calculated phase diagram of a Ti1−xAlxN alloy, lowering the maximum temperature for the miscibility gap from 6560 K to 2860 K. Our local chemical composition measurements on thermally aged Ti0.5Al0.5N alloys agree with the calculated phase diagram.

  • 15.
    Mei, A. B.
    et al.
    University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan. CALTECH, CA 91125 USA.
    Wireklint, N.
    Chalmers, Sweden.
    Schlepuetz, C. M.
    Argonne National Lab, IL 60439 USA.
    Sangiovanni, Davide
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Alling, Björn
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Rockett, A.
    University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Hultman, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Petrov, Ivan
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan. University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan. University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Dynamic and structural stability of cubic vanadium nitride2015Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, nr 5, s. 054101-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below T-c = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior I proportional to (T-c - T)(1/2). Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function alpha F-2(tr)(h omega), the product of the phonon density of states F(h omega) and the transport electron/phonon coupling strength alpha(2)(tr)(h omega), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T greater than T-c. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.

  • 16.
    Isaeva, L.
    et al.
    Uppsala University, Sweden.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. CALTECH, CA 91125 USA.
    Lashley, J. C.
    Los Alamos National Lab, NM 87545 USA.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Eriksson, O.
    Uppsala University, Sweden.
    Dynamic stabilization of cubic AuZn2015Ingår i: MATERIALS TODAY-PROCEEDINGS, ELSEVIER SCIENCE BV , 2015, Vol. 2, s. 569-572Konferensbidrag (Refereegranskat)
    Abstract [en]

    A recently developed temperature-dependent effective potential method is employed to study the martensitic phase transformation in AuZn. This method is based on ab initio molecular dynamics and allows to obtain finite-temperature lattice vibrational properties. We show that the transversal acoustic TA(2)[110] mode associated with the phase transition is stabilized at 300 K. Temperature evolution of single-phonon dynamic structure factor at the wave vector q= 1/3[1,1,0], associated with phonon softening and Fermi surface nesting, was also studied. (C) 2015 The Authors. Published by Elsevier Ltd.

  • 17.
    Shulumba, Nina
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten. University of Saarland, Germany.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA.
    Raza, Zamaan
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. Materials Modeling and Development Laboratory, NUST “MISIS”, Moscow, Russia / LACOMAS Laboratory, Tomsk State University, Tomsk, Russia.
    Odén, Magnus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten.
    Influence of vibrational free energy on the phase stability of alloys from first principles2015Manuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We have developed a method to accurately and efficiently determine the vibrational free energy as a function of temperature and pressure for substitutional alloys from first principles. Taking the example of the technologically important hard coating alloy Ti1-xAlxN as an example, we investigate the effect on the vibrational free energy of substituting Ti for other group IV elements. By constructing the phase diagrams for these three alloys, we show why Zr1-xAlxN and Hf1-xAlxN are so difficult to experimentally synthesise in a metastable solid solution: both have solubility regions that span only a small low-AlN concentration range at temperatures above 1500 K. Moreover, Hf1-xAlxN is dynamically unstable at low temperatures and across most of the concentration range. We also show the chemical and thermal expansion effects dominate mass disorder in the Gibbs free energy of mixing.

  • 18.
    Shulumba, Nina
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten. University of Saarland, Germany.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA.
    Rogström, Lina
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten.
    Raza, Zamaan
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Tasnádi, Ferenc
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. Materials Modeling and Development Laboratory, NUST “MISIS”, Moscow, Russia / LACOMAS Laboratory, Tomsk State University, Tomsk, Russia.
    Odén, Magnus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten.
    Temperature-dependent elastic properties of Ti1−xAlxN alloys2015Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 107, nr 23Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ti1−xAlxN is a technologically important alloy that undergoes a process of high temperature age-hardening that is strongly influenced by its elastic properties. We have performed first principles calculations of the elastic constants and anisotropy using the newly developed symmetry imposed force constant temperature dependent effective potential method, that include lattice vibrations and therefore the effects of temperature, including thermal expansion and intrinsic anharmonicity. These are compared with in situ high temperature x-ray diffraction measurements of the lattice parameter. We show that anharmonic effects are crucial to the recovery of finite temperature elasticity. The effects of thermal expansion and intrinsic anharmonicity on the elastic constants are of the same order, and cannot be considered separately. Furthermore, the effect of thermal expansion on elastic constants is such that the volume change induced by zero point motion has a significant effect. For TiAlN, the elastic constants soften non-uniformly with temperature: C11 decreases substantially when the temperature increases for all compositions, resulting in an increased anisotropy. These findings suggest that an increased Al content and annealing at higher temperatures will result in a harder alloy.

  • 19.
    Abrikosov, Igor
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan. NUST MISIS, Russia.
    Ponomareva, A. V.
    NUST MISIS, Russia.
    Nikonov, A. Yu.
    National Research Tomsk State University, Russia; SB RAS, Russia.
    Zharmukhambetova, A. M.
    National Research Tomsk State University, Russia.
    Mosyagin, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan. NUST MISIS, Russia.
    Lugovskoy, A. V.
    NUST MISIS, Russia.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Lind, Hans
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Dmitriev, A. I.
    National Research Tomsk State University, Russia; SB RAS, Russia.
    Barannikova, S. A.
    National Research Tomsk State University, Russia; SB RAS, Russia.
    Theoretical description of pressure-induced phase transitions: a case study of Ti-V alloys2015Ingår i: High Pressure Research, ISSN 0895-7959, E-ISSN 1477-2299, Vol. 35, nr 1, s. 42-48Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We discuss theoretical description of pressure-induced phase transitions by means of first-principles calculations in the framework of density functional theory. We illustrate applications of theoretical tools that allow one to take into account configurational and vibrational disorders, considering Ti-V alloys as a model system. The universality of the first-principles theory allows us to apply it in studies of different phenomena that occur in the Ti-V system upon compression. Besides the transitions between different crystal structures, we discuss isostructural transitions in bcc Ti-V alloys. Moreover, we present arguments for possible electronic transitions in this system, which may explain peculiar behaviour of elastic properties of V upon compression.

  • 20.
    Romero, A. H.
    et al.
    W Virginia University, WV 26506 USA; Max Planck Institute Mikrostrukturphys, Germany.
    Gross, E. K. U.
    Max Planck Institute Mikrostrukturphys, Germany.
    Verstraete, M. J.
    University of Liege, Belgium; University of Liege, Belgium.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. CALTECH, CA 91125 USA.
    Thermal conductivity in PbTe from first principles2015Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, nr 21, s. 214310-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the harmonic and anharmonic contributions to the phonon spectrum of lead telluride and perform a complete characterization of how thermal properties of PbTe evolve as temperature increases. We analyze the thermal resistivitys variationwith temperature and clarify misconceptions about existing experimental literature. The resistivity initially increases sublinearly because of phase space effects and ultra strong anharmonic renormalizations of specific bands. This effect is the strongest factor in the favorable thermoelectric properties of PbTe, and it explains its limitations at higher T. This quantitative prediction opens the prospect of phonon phase space engineering to tailor the lifetimes of crucial heat carrying phonons by considering different structure or nanostructure geometries. We analyze the available scattering volume between TO and LA phonons as a function of temperature and correlate its changes to features in the thermal conductivity.

  • 21.
    Miranda, Alonso L.
    et al.
    CINVESTAV, Mexico; University of Liege, Belgium.
    Xu, Bin
    University of Liege, Belgium; University of Arkansas, AR 72701 USA; University of Arkansas, AR 72701 USA.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Romero, Aldo H.
    CINVESTAV, Mexico; W Virginia University, WV 26506 USA.
    Verstraete, Matthieu J.
    University of Liege, Belgium.
    Ab initio calculation of the thermal conductivity of indium antimonide2014Ingår i: Semiconductor Science and Technology, ISSN 0268-1242, E-ISSN 1361-6641, Vol. 29, nr 12, s. 124002-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A theoretical study based on the density functional theory and the temperature-dependent effective potential method is performed to analyze the changes in the phonon band structure as a function of temperature for indium antimonide. In particular, we show changes in the thermal expansion coefficient and the thermal resistivity that agree rather well with experimental measurements. From the theoretical side, we show a weak dependence with respect to the chosen thermostat used to obtain the inter-atomic force constants, which strengthens our conclusions.

  • 22.
    Li, C.W.
    et al.
    Oak Ridge National Lab, TN 37831 USA .
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Ma, J.
    Oak Ridge National Lab, TN 37831 USA .
    May, A.F.
    Oak Ridge National Lab, TN 37831 USA .
    Cao, H.B.
    Oak Ridge National Lab, TN 37831 USA .
    Chen, X.
    Oak Ridge National Lab, TN 37831 USA .
    Christianson, A.D.
    Oak Ridge National Lab, TN 37831 USA .
    Ehlers, G.
    Oak Ridge National Lab, TN 37831 USA .
    Singh, D.J.
    Oak Ridge National Lab, TN 37831 USA .
    Sales, B.C.
    Oak Ridge National Lab, TN 37831 USA .
    Delaire, O.
    Oak Ridge National Lab, TN 37831 USA .
    Phonon Self-Energy and Origin of Anomalous Neutron Scattering Spectra in SnTe and PbTe Thermoelectrics2014Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, nr 17, s. 175501-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase space for three-phonon scattering processes, combined with the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optic ferroelectric mode.

  • 23.
    Hellman, Olle
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Broido, David A.
    Boston Coll, MA 02467 USA.
    Phonon thermal transport in Bi2Te3 from first principles2014Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, nr 13, s. 134309-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present first-principles calculations of the thermal and thermal transport properties of Bi2Te3 that combine an ab initio molecular dynamics (AIMD) approach to calculate interatomic force constants (IFCs) along with a full iterative solution of the Peierls-Boltzmann transport equation for phonons. The newly developed AIMD approach allows determination of harmonic and anharmonic interatomic forces at each temperature, which is particularly appropriate for highly anharmonic materials such as Bi2Te3. The calculated phonon dispersions, heat capacity, and thermal expansion coefficient are found to be in good agreement with measured data. The lattice thermal conductivity, kappa(l), calculated using the AIMD approach nicely matches measured values, showing better agreement than the kappa(l) obtained using temperature-independent IFCs. A significant contribution to kappa(l) from optic phonon modes is found. Already at room temperature, the phonon line shapes show a notable broadening and onset of satellite peaks reflecting the underlying strong anharmonicity.

  • 24.
    Palumbo, M.
    et al.
    Ruhr University of Bochum, Germany .
    Burton, B.
    NIST, MD 20899 USA .
    Costa e Silva, A.
    EEIMVR UFF, Brazil .
    Fultz, B.
    CALTECH, CA 91125 USA .
    Grabowski, B.
    Max Planck Institute Eisenforsch GmbH, Germany .
    Grimvall, G.
    KTH Royal Institute Technology, Sweden .
    Hallstedt, B.
    Rhein Westfal TH Aachen, Germany .
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Lindahl, B.
    KTH Royal Institute Technology, Sweden .
    Schneider, A.
    Vallourec Deutschland GmbH, Germany .
    Turchi, P. E. A.
    Lawrence Livermore National Lab, CA USA .
    Xiong, W.
    University of Wisconsin, WI 53706 USA .
    Thermodynamic modelling of crystalline unary phases2014Ingår i: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 251, nr 1, s. 14-32Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Progress in materials science through thermodynamic modelling may rest crucially on access to a database, such as that developed by Scientific Group Thermodata Europe (SGTE) around 1990. It gives the Gibbs energy G(T) of the elements in the form of series as a function of temperature, i.e. essentially a curve fitting to experimental data. In the light of progress in theoretical understanding and first-principles calculation methods, the possibility for an improved database description of the thermodynamics of the elements has become evident. It is the purpose of this paper to provide a framework for such work. Lattice vibrations, which usually give the major contribution to G(T), are treated in some detail with a discussion of neutron scattering studies of anharmonicity in aluminium, first-principles calculations including ab initio molecular dynamics (AIMD), and the strength and weakness of analytic model representations of data. Similarly, electronic contributions to G(T) are treated on the basis of the density of states N(E) for metals, with emphasis on effects at high T. Further, we consider G(T) below 300K, which is not covered by SGTE. Other parts in the paper discuss metastable and dynamically unstable lattices, G(T) in the region of superheated solids and the requirement on a database in the calculation of phase diagrams. (C) 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim

  • 25.
    Shulumba, Nina
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska högskolan.
    Alling, Björn
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Mozafari, Elham
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Steneteg, Peter
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Odén, Magnus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska högskolan.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Vibrational free energy and phase stability of paramagnetic and antiferromagnetic CrN from ab initio molecular dynamics2014Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, nr 17, s. 174108-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a theoretical first-principles method to calculate the free energy of a magnetic system in its high-temperature paramagnetic phase, including vibrational, electronic, and magnetic contributions. The method for calculating free energies is based on ab initio molecular dynamics and combines a treatment of disordered magnetism using disordered local moments molecular dynamics with the temperature-dependent effective potential method to obtain the vibrational contribution to the free energy. We illustrate the applicability of the method by obtaining the anharmonic free energy for the paramagnetic cubic and the antiferromagnetic orthorhombic phases of chromium nitride. The influence of lattice dynamics on the transition between the two phases is demonstrated by constructing the temperature-pressure phase diagram.

  • 26.
    Abrikosov, Igor A.
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Alling, Björn
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Steneteg, Peter
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Medie- och Informationsteknik. Linköpings universitet, Tekniska fakulteten.
    Hultberg, Lasse
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Yu Mosyagin, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten. Department of Theoretical Physics and Quantum Technologies, National Research, Technological University MISiS, Moscow, Russia.
    Lugovskoy, Andrey V.
    Department of Theoretical Physics and Quantum Technologies, National Research, Technological University MISiS, Russia.
    Barannikova, Svetlana A.
    Institute of Strength Physics and Materials Science, Siberian Branch of Russian Academy of Science, Tomsk, Russia; Department of Physics and Engineering, Tomsk State University, Tomsk, Russia.
    Finite Temperature, Magnetic, and Many-Body Effects in Ab Initio Simulations of Alloy Thermodynamics2013Ingår i: TMS2013 Supplemental Proceedings, John Wiley & Sons, 2013, s. 617-626Kapitel i bok, del av antologi (Refereegranskat)
    Abstract [en]

    Ab initio electronic structure theory is known as a useful tool for prediction of materials properties. However, majority of simulations still deal with calculations in the framework of density functional theory with local or semi-local functionals carried out at zero temperature. We present new methodological solution.s, which go beyond this approach and explicitly take finite temperature, magnetic, and many-body effects into account. Considering Ti-based alloys, we discuss !imitations of the quasiharmonic approximation for the treatment of lattice vibrations, and present an accurate and easily extendable method to calculate free ,energies of strongly anharmonic solids. We underline the necessity to going beyond the state-of-the-art techniques for the determination of effective cluster interactions in systems exhibiting mctal-to-insulator transition, and describe a unified cluster expansion approach developed for this class of materials. Finally, we outline a first-principles method, disordered local moments molecular dynamics, for calculations of thermodynamic properties of magnetic alloys, like Cr1-x,.AlxN, in their high-temperature paramagnetic state. Our results unambiguously demonstrate importance of finite temperature effects in theoretical calculations ofthermodynamic properties ofmaterials.

  • 27.
    Steneteg, Peter
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Vekilova, Olga
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Shulumba, Nina
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska högskolan.
    Tasnádi, Ferenc
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations2013Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, nr 9Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Elastic properties of cubic TiN are studied theoretically in a wide temperature interval. First-principles simulations are based on ab initio molecular dynamics (AIMD). Computational efficiency of the method is greatly enhanced by a careful preparation of the initial state of the simulation cell that minimizes or completely removes a need for equilibration and therefore allows for parallel AIMD calculations. Elastic constants C11, C12, and C44 are calculated. A strong dependence on the temperature is predicted, with C11 decreasing by more than 29% at 1800 K as compared to its value obtained at T=0 K. Furthermore, we analyze the effect of temperature on the elastic properties of polycrystalline TiN in terms of the bulk and shear moduli, the Young's modulus and Poisson ratio. We construct sound velocity anisotropy maps, investigate the temperature dependence of elastic anisotropy of TiN, and observe that the material becomes substantially more isotropic at high temperatures. Our results unambiguously demonstrate the importance of taking into account finite temperature effects in theoretical calculations of elastic properties of materials intended for high-temperature applications.

  • 28.
    Hellman, Olle
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Steneteg, Peter
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Simak, Sergey
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Temperature dependent effective potential method for accurate free energy calculations of solids2013Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, nr 10Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have developed a thorough and accurate method of determining anharmonic free energies, the temperature dependent effective potential technique (TDEP). It is based on ab initio molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The formalism and the numerical aspects of the technique are described in detail. A number of practical examples are given, and results are presented, which confirm the usefulness of TDEP within ab initio and classical molecular dynamics frameworks. In particular, we examine from first principles the behavior of force constants upon the dynamical stabilization of the body centered phase of Zr, and show that they become more localized. We also calculate the phase diagram for 4He modeled with the Aziz et al. potential and obtain results which are in favorable agreement both with respect to experiment and established techniques.

  • 29.
    Hellman, Olle
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Temperature-dependent effective third-order interatomic force constants from first principles2013Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, nr 14Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The temperature-dependent effective potential (TDEP) method is generalized beyond pair interactions. The second- and third-order force constants are determined consistently from ab initio molecular dynamics simulations at finite temperature. The reliability of the approach is demonstrated by calculations of the mode Grüneisen parameters for Si. We show that the extension of TDEP to a higher order allows for an efficient calculation of the phonon life time, in Si as well as in ε-FeSi; a system that exhibits anomalous softening with temperature.

  • 30.
    Hellman, Olle
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Skorodumova, N V
    Royal Institute Technology.
    Simak, Sergey
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Charge Redistribution Mechanisms of Ceria Reduction2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, nr 13, s. 135504-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Charge redistribution at low oxygen vacancy concentrations in ceria have been studied in the framework of the density functional theory. We propose a model to approach the dilute limit using the results of supercell calculations. It allows one to reproduce the characteristic experimentally observed behavior of composition versus oxygen pressure dependency. We show that in the dilute limit the charge redistribution is likely to be driven by a mechanism different from the one involving electron localization on cerium atoms. We demonstrate that it can involve charge localization on light element impurities.

  • 31.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Thermal properties of materials from first principles2012Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In the search of clean and efficient energy sources intermediate temperature solid oxide fuel cells are among the prime candidates. What sets the limit of their efficiency is the solid electrolyte. A promising material for the electrolyte is ceria. This thesis aims to improve the characteristics of these electrolytes and help provide thorough physical understanding of the processes involved. This is realised using first principles calculations.

    The class of methods based on density functional theory generally ignores temperature effects. To accurately describe the intermediate temperature characteristics I have made adjustments to existing frameworks and developed a qualitatively new method. The new technique, the high temperature effective potential method, is a general theory. The validity is proven on a number of model systems.

    Other subprojects include low-dimensional segregation effects, adjustments to defect concentration formalism and optimisations of ionic conductivity.

    Delarbeten
    1. Lattice dynamics of anharmonic solids from first principles
    Öppna denna publikation i ny flik eller fönster >>Lattice dynamics of anharmonic solids from first principles
    2011 (Engelska)Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, nr 18, s. 180301-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic-or higher order-potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated.

    Ort, förlag, år, upplaga, sidor
    American Physical Society, 2011
    Nationell ämneskategori
    Teknik och teknologier
    Identifikatorer
    urn:nbn:se:liu:diva-72810 (URN)10.1103/PhysRevB.84.180301 (DOI)000297099800001 ()
    Anmärkning
    Funding Agencies|Swedish Research Council (VR)||Tillgänglig från: 2011-12-09 Skapad: 2011-12-08 Senast uppdaterad: 2017-12-08
    2. Double-segregation effect in AgxPd1−x /Ru(0001) thin film nanostructures
    Öppna denna publikation i ny flik eller fönster >>Double-segregation effect in AgxPd1−x /Ru(0001) thin film nanostructures
    Visa övriga...
    2008 (Engelska)Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, s. 125406-1-125406-7Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    We study the structural properties of ultrathin AgxPd1−x films on top of a Ru(0001) substrate. Effective interatomic interactions, obtained from first-principles calculations, have been used in Monte Carlo simulations to derive the distribution of the alloy components in a four-monolayer (4-ML) Ag-Pd film. Though Ag-Pd alloys show complete solubility in the bulk, the thin film geometry leads to a pronounced segregation between Ag and Pd atoms with a strong preference of Ag atoms toward the surface and Pd atoms toward the interface. The theoretical prediction of this double-segregation effect is strongly supported by photoelectron spectroscopy experiments carried out for 4-ML thin films. We also show, in an additional experiment, that even in the case where initially 1 ML Ag is buried under 6 ML Pd, the whole Ag ML segregates to the surface.

    Ort, förlag, år, upplaga, sidor
    APS, 2008
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-44362 (URN)10.1103/PhysRevB.77.125406 (DOI)76396 (Lokalt ID)76396 (Arkivnummer)76396 (OAI)
    Tillgänglig från: 2009-10-10 Skapad: 2009-10-10 Senast uppdaterad: 2017-12-13Bibliografiskt granskad
    3. Charge Redistribution Mechanisms of Ceria Reduction
    Öppna denna publikation i ny flik eller fönster >>Charge Redistribution Mechanisms of Ceria Reduction
    2012 (Engelska)Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, nr 13, s. 135504-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    Charge redistribution at low oxygen vacancy concentrations in ceria have been studied in the framework of the density functional theory. We propose a model to approach the dilute limit using the results of supercell calculations. It allows one to reproduce the characteristic experimentally observed behavior of composition versus oxygen pressure dependency. We show that in the dilute limit the charge redistribution is likely to be driven by a mechanism different from the one involving electron localization on cerium atoms. We demonstrate that it can involve charge localization on light element impurities.

    Ort, förlag, år, upplaga, sidor
    American Physical Society, 2012
    Nationell ämneskategori
    Teknik och teknologier
    Identifikatorer
    urn:nbn:se:liu:diva-76952 (URN)10.1103/PhysRevLett.108.135504 (DOI)000302173000017 ()
    Anmärkning
    Funding Agencies|Swedish Research Council, (VR)||Swedish Government||Tillgänglig från: 2012-05-02 Skapad: 2012-04-27 Senast uppdaterad: 2017-12-07
    4. Tuning ionic conductivity in ceria by volume optimization
    Öppna denna publikation i ny flik eller fönster >>Tuning ionic conductivity in ceria by volume optimization
    (Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Effect of volume change on the ionic conductivity in ceria has been studied in the framework of the density functional theory. We show that properly controlling external conditions one can treat the lattice constant of ceria as an adjustable parameter and change the topology of the energy landscape for the oxygen ion diffusion. We reveal the existence of the narrow range of lattice parameters, which optimize the ionic conductivity in ceria.

    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-78752 (URN)
    Tillgänglig från: 2012-06-20 Skapad: 2012-06-20 Senast uppdaterad: 2012-06-20Bibliografiskt granskad
    5. Temperature dependent effective potential method for accurate free energy calculation of solids
    Öppna denna publikation i ny flik eller fönster >>Temperature dependent effective potential method for accurate free energy calculation of solids
    (Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We have developed a thorough and accurate method of determining anharmonic free energies. The technique is based in ab initio molecular dynamics and map a model Hamiltonian to the fully anharmonic system. We can accurately deal with low-symmetry systems, such as random alloys. The formalism and the numerics are described in great detail. A number of practical examples are given, and favourable results are presented, both with respect to experiment and established techniques.

    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-78753 (URN)
    Tillgänglig från: 2012-06-20 Skapad: 2012-06-20 Senast uppdaterad: 2012-06-20Bibliografiskt granskad
    6. Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations
    Öppna denna publikation i ny flik eller fönster >>Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations
    Visa övriga...
    2013 (Engelska)Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, nr 9Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    Elastic properties of cubic TiN are studied theoretically in a wide temperature interval. First-principles simulations are based on ab initio molecular dynamics (AIMD). Computational efficiency of the method is greatly enhanced by a careful preparation of the initial state of the simulation cell that minimizes or completely removes a need for equilibration and therefore allows for parallel AIMD calculations. Elastic constants C11, C12, and C44 are calculated. A strong dependence on the temperature is predicted, with C11 decreasing by more than 29% at 1800 K as compared to its value obtained at T=0 K. Furthermore, we analyze the effect of temperature on the elastic properties of polycrystalline TiN in terms of the bulk and shear moduli, the Young's modulus and Poisson ratio. We construct sound velocity anisotropy maps, investigate the temperature dependence of elastic anisotropy of TiN, and observe that the material becomes substantially more isotropic at high temperatures. Our results unambiguously demonstrate the importance of taking into account finite temperature effects in theoretical calculations of elastic properties of materials intended for high-temperature applications.

    Ort, förlag, år, upplaga, sidor
    American Physical Society, 2013
    Nationell ämneskategori
    Naturvetenskap Teknik och teknologier
    Identifikatorer
    urn:nbn:se:liu:diva-78754 (URN)10.1103/PhysRevB.87.094114 (DOI)000316791600001 ()
    Anmärkning

    Funding Agencies|Swedish Research Council|621-2008-5535621-2011-4426|Swedish Foundation for Strategic Research (SSF) programs|10-0026|project Designed Multicomponent Coatings (MultiFilms)||Erasmus Mundus doctoral program DocMase||Ministry of Education and Science of the Russian Federation within the framework of Program Research and Pedagogical Personnel for Innovative Russia|14.B37.21.089010.09.2012|

    Tillgänglig från: 2012-06-20 Skapad: 2012-06-20 Senast uppdaterad: 2017-12-07Bibliografiskt granskad
  • 32.
    Hellman, Olle
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Simak, Sergey
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Lattice dynamics of anharmonic solids from first principles2011Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, nr 18, s. 180301-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic-or higher order-potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated.

  • 33.
    Belonoshko, A B
    et al.
    Royal Institute of Technology.
    Derlet, P M
    Paul Scherrer Institute.
    Mikhaylushkin, Arkady
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Simak, Sergey
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Burakovsky, L
    Los Alamos National Laboratory.
    Swift, D C
    Los Alamos National Laboratory.
    Johansson, B
    Royal Institute of Technology.
    Quenching of bcc-Fe from high to room temperature at high-pressure conditions: a molecular dynamics simulation2009Ingår i: NEW JOURNAL OF PHYSICS, ISSN 1367-2630, Vol. 11, s. 093039-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The new high-temperature (T), high-pressure (P), body-centered cubic (bcc) phase of iron has probably already been synthesized in recent diamond anvil cell (DAC) experiments (Mikhaylushkin et al 2007 Phys. Rev. Lett. 99 165505). These DAC experiments on iron revealed that the high-PT phase on quenching transforms into a mixture of close-packed phases. Our molecular dynamics simulation and structural analysis allow us to provide a probable interpretation of the experiments. We show that quenching of the high-PT bcc phase simulated with the embedded-atom model also leads to the formation of the mixture of close-packed phases. Therefore, the assumption of the stability of the high-PT bcc iron phase is consistent with experimental observation.

  • 34.
    Marten, Tobias
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Ruban, Andrei V.
    Department of Materials Science and Engineering Royal Institute of Technology, Stockholm.
    Olovsson, Weine
    Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan.
    Kramer, Charlotte
    Department of Physics and Nanotechnology Aalborg University, Aalborg, Denmark.
    Godowski, Jan P.
    Institute of Experimental Physics, University of Wroclaw, 50-204 Wroclaw, Poland.
    Bech, Lone
    Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg East, Denmark.
    Li, Zheshen
    Institute of Storage Ring Facilities, University of Aarhus, DK-8000 Aarhus, Denmark.
    Onsgaard, Jens
    Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg East, Denmark.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Double-segregation effect in AgxPd1−x /Ru(0001) thin film nanostructures2008Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, s. 125406-1-125406-7Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the structural properties of ultrathin AgxPd1−x films on top of a Ru(0001) substrate. Effective interatomic interactions, obtained from first-principles calculations, have been used in Monte Carlo simulations to derive the distribution of the alloy components in a four-monolayer (4-ML) Ag-Pd film. Though Ag-Pd alloys show complete solubility in the bulk, the thin film geometry leads to a pronounced segregation between Ag and Pd atoms with a strong preference of Ag atoms toward the surface and Pd atoms toward the interface. The theoretical prediction of this double-segregation effect is strongly supported by photoelectron spectroscopy experiments carried out for 4-ML thin films. We also show, in an additional experiment, that even in the case where initially 1 ML Ag is buried under 6 ML Pd, the whole Ag ML segregates to the surface.

  • 35.
    Kerdsongpanya, Sit
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Hellman, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan. Department of Applied Physics and Materials Science, California Institute of Technology, Pasadena, USA.
    Sun, Bo
    Department of Mechanical Engineering, National University of Singapore, Block EA, Singapore..
    Koh, Yee Kan
    Department of Mechanical Engineering, National University of Singapore, Block EA, Singapore..
    Van Nong, Ngo
    Dept. of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, Denmark.
    Lu, Jun
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Simak, Sergei I.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Alling, Björn
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Eklund, Per
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Phonon Thermal Conductivity of Scandium Nitride for Thermoelectric Applications from First-Principles CalculationsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    The knowledge of lattice thermal conductivity of materials under realistic conditions is vitally important since most technologies either require either high or low thermal conductivity. Here, we propose a theoretical model for determining lattice thermal conductivity with the effect of microstructure. This is based on ab initio description that includes the temperature dependence of the interatomic force constants, and treats anharmonic lattice vibrations. We choose ScN as a model system, comparing the computational predictions with the experimental data by Time Domain Thermoreflectance (TDTR). Our results show a trend of reduction in lattice thermal conductivity with decreasing grain size, with good agreement between the theoretical model and experimental data. There results suggest a possibility to control thermal conductivity by tailoring the microstructure of ScN. More importantly, we provide a predictive tool for the effect of the microstructure on the lattice thermal conductivity of materials based on first-principles calculations.

  • 36.
    Hellman, Olle
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Steneteg, Peter
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Simak, Sergey
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Temperature dependent effective potential method for accurate free energy calculation of solidsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We have developed a thorough and accurate method of determining anharmonic free energies. The technique is based in ab initio molecular dynamics and map a model Hamiltonian to the fully anharmonic system. We can accurately deal with low-symmetry systems, such as random alloys. The formalism and the numerics are described in great detail. A number of practical examples are given, and favourable results are presented, both with respect to experiment and established techniques.

  • 37.
    Hellman, Olle
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Skorodumova, N. V.
    Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden.
    Simak, Sergey
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Tuning ionic conductivity in ceria by volume optimizationManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Effect of volume change on the ionic conductivity in ceria has been studied in the framework of the density functional theory. We show that properly controlling external conditions one can treat the lattice constant of ceria as an adjustable parameter and change the topology of the energy landscape for the oxygen ion diffusion. We reveal the existence of the narrow range of lattice parameters, which optimize the ionic conductivity in ceria.

1 - 37 av 37
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