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  • 151.
    Zheng, Qiye
    et al.
    University of Illinois, USA.
    Mei, Antonio B.
    University of Illinois, USA.
    Tuteja, Mohit
    University of Illinois, USA.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr University of Bochum, Germany.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, USA.
    Cahill, David G.
    University of Illinois, IL 61801 USA.
    Phonon and electron contributions to the thermal conductivity of VNx epitaxial layers2017In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 1, no 6, article id 065002Article in journal (Refereed)
    Abstract [en]

    Thermal conductivities of VNx/MgO(001) (0.76 amp;lt;= x amp;lt;= 1.00) epitaxial layers, grown by reactive magnetron sputter deposition, are measured in the temperature range 300 amp;lt; T amp;lt; 1000 K using time-domain thermore-flectance (TDTR). Data for the total thermal conductivity are compared to the electronic contribution to the thermal conductivity calculated from the measured electrical conductivity, the Wiedemann-Franz law, and an estimate of the temperature dependence of the Lorenz number L(T). The total thermal conductivity is dominated by electron contribution and varies between 13 W m(-1) K-1 at x = 0.76 and 20 W m(-1) K-1 at x = 1.00 for T = 300 K and between 25 and 35 W m(-1) K-1 for T = 1000 K. The lattice thermal conductivity vs x ranges from 5 to 7 W m(-1) K-1 at 300 K and decreases by 20% at 500 K. The low magnitude and weak temperature dependence of the lattice thermal conductivity are attributed to strong electron-phonon coupling in VN.

  • 152.
    Kerdsongpanya, Sit
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Rensselaer Polytech Institute, NY 12180 USA.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. CALTECH, CA 91125 USA.
    Sun, Bo
    National University of Singapore, Singapore.
    Kan Koh, Yee
    National University of Singapore, Singapore.
    Lu, Jun
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Van Nong, Ngo
    Department of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, Roskilde, Denmark.
    Simak, Sergei I.
    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 Institute Eisenforsch GmbH, Germany.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Phonon thermal conductivity of scandium nitride for thermoelectrics from first-principles calculations and thin-film growth2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 19, article id 195417Article in journal (Refereed)
    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.

  • 153.
    Ribeiro, Luiz Antonio
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. University of Brasilia, Brazil.
    Stafström, Sven
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Polaron dynamics in anisotropic Holstein-Peierls systems2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 5, p. 4078-4084Article in journal (Refereed)
    Abstract [en]

    Polaron dynamics in anisotropic organic molecular semiconductors is theoretically investigated and simulated in the framework of a semi-classical Holstein-Peierls model. Our computational protocol is presented and applied to studies of a two-dimensional molecular crystal. The intermolecular (Peierls) parameters for a particular crystal direction are systematically changed in order to study the effect of anisotropy in the system. The usefulness of this methodology is highlighted by studying the polaron dynamics on a picosecond timescale, which provides a microscopic insight into the influence of the interplay between different intramolecular parameters on the charge transport mechanism. Our results show that the polaron mobility is substantially reduced in going from an anisotropic to an isotropic relationship between the Peierls parameters for different directions in the crystal. Interestingly, the molecular charge distribution presents three different signatures corresponding to a one-dimensional polaron, a two-dimensional polaron, and an intermediate state for which the polaron localization depends on the degree of anisotropy. Importantly, the two-dimensional polaron, which is present in the essentially isotropic system, is immobile whereas the other two types of polarons are mobile. This, in order for polaron transport to occur in a two-dimensional molecular based system, this system has to be anisotropic.

  • 154.
    Leonov, I.
    et al.
    University of Augsburg, Germany; National University of Science and Technology MISIS, Russia.
    Ponomareva, A. V.
    National University of Science and Technology MISIS, Russia.
    Nazarov, R.
    Lawrence Livermore National Lab, CA 94551 USA.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Pressure-induced spin-state transition of iron in magnesiowustite (Fe,Mg)O2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 7, article id 075136Article in journal (Refereed)
    Abstract [en]

    We present a detailed theoretical study of the electronic, magnetic, and structural properties of magnesiowustite Fe-1 Mg-x(x) O with x in the range between 0 and 0.875 using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory method. In particular, we compute the electronic structure and phase stability of the rocksalt B1-structured (Fe,Mg) O at high pressures relevant for the Earths lower mantle. We find that upon compression paramagnetic (Fe,Mg) O exhibits a spin-state transition of Fe2+ ions from a high-spin to low-spin (HS-LS) state which is accompanied by a collapse of local magnetic moments. The HS-LS transition results in a substantial drop in the lattice volume by about 4%-8%, implying a complex interplay between electronic and lattice degrees of freedom. Our results reveal a strong sensitivity of the calculated transition pressure P-tr. upon addition of Mg. While, for Fe-rich magnesiowustite with Mg x amp;lt; 0.5, Ptr. is about 80 GPa, for Mg x = 0.75 it drops to 52 GPa, i. e., by 35%. This behavior is accompanied by a substantial change in the spin transition range from 50 to 140 GPa in FeO to 30 to 90 GPa for x = 0.75. In addition, the calculated bulk modulus (in the HS state) is found to increase by similar to 12% from 142 GPa in FeO to 159 GPa in (Fe,Mg) O with Mg x = 0.875. We find that the pressure-induced HS-LS transition has different consequences for the electronic properties of the Fe-rich and -poor (Fe,Mg) O. For the Fe-rich (Fe,Mg) O, the transition is found to be accompanied by a Mott insulator to a (semi) metal phase transition. In contrast to that, for x amp;gt; 0.25, (Fe,Mg) O remains insulating up to the highest studied pressures, implying a Mott-insulator to band-insulator phase transition at the HS-LS transformation.

  • 155.
    Burakovsky, L.
    et al.
    Los Alamos National Lab, NM 87545 USA.
    Cawkwell, M. J.
    Los Alamos National Lab, NM 87545 USA.
    Preston, D. L.
    Los Alamos National Lab, NM 87545 USA.
    Errandonea, D.
    University of Valencia, Spain.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Recent ab initio phase diagram studies: Iridium2017In: JOINT AIRAPT-25TH and EHPRG-53RD INTERNATIONAL CONFERENCE ON HIGH PRESSURE SCIENCE AND TECHNOLOGY, 2015, IOP PUBLISHING LTD , 2017, Vol. 950, article id UNSP 042021Conference paper (Refereed)
    Abstract [en]

    The phase diagram of iridium is investigated using the Z methodology in conjunction with the VASP ab initio molecular dynamics package. The Z methodology is a novel technique for phase diagram studies which combines the direct Z method for the computation of melting curves and the inverse Z method for the calculation of solid-solid phase boundaries. We compare our results to the available experimental data on iridium. We offer explanation for the 14-layer hexagonal structure observed in experiments by Cerenius and Dubrovinsky.

  • 156.
    Tellander, Felix
    et al.
    Lund University, Sweden.
    Berggren, Karl-Fredrik
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Spectra, current flow, and wave-function morphology in a model PT-symmetric quantum dot with external interactions2017In: PHYSICAL REVIEW A, ISSN 2469-9926, Vol. 95, no 4, article id 042115Article in journal (Refereed)
    Abstract [en]

    In this paper we use numerical simulations to study a two-dimensional (2D) quantum dot (cavity) with two leads for passing currents (electrons, photons, etc.) through the system. By introducing an imaginary potential in each lead the system is made symmetric under parity-time inversion (PT symmetric). This system is experimentally realizable in the form of, e.g., quantum dots in low-dimensional semiconductors, optical and electromagnetic cavities, and other classical wave analogs. The computational model introduced here for studying spectra, exceptional points (EPs), wave-function symmetries and morphology, and current flow includes thousands of interacting states. This supplements previous analytic studies of few interacting states by providing more detail and higher resolution. The Hamiltonian describing the system is non-Hermitian; thus, the eigenvalues are, in general, complex. The structure of the wave functions and probability current densities are studied in detail at and in between EPs. The statistics for EPs is evaluated, and reasons for a gradual dynamical crossover are identified.

  • 157.
    Mukhamedov, B. O.
    et al.
    National University of Science and Technology MISIS, Russia.
    Ponomareva, A. V.
    National University of Science and Technology MISIS, Russia.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Spinodal decomposition in ternary Fe-Cr-Co system2017In: JOURNAL OF ALLOYS AND COMPOUNDS, ISSN 0925-8388, Vol. 695, p. 250-256Article in journal (Refereed)
    Abstract [en]

    Using the exact muffin-tin orbitals method in conjunction with the coherent potential approximation we have studied the tendency towards spinodal decomposition of solid solution in ternary Fe-Cr-Co system. In addition, we have estimated the Curie temperature of the alloys, and considered the influence of the magnetic state on the decomposition thermodynamics of the ternary alloys. Using the mean field approximation, we have estimated the finite temperature effects on the alloys free energy. We predict that an increase Co and Cr content in the ternary Fe-Cr-Co system increases the tendency of the bcc (alpha)-FexCryCoz alloys towards the spinodal decomposition. Because of this, high magnetic properties and high thermal stability of these properties can be expected in the Fe-Cr-Co alloys with high Co content. (C) 2016 Elsevier B.V. All rights reserved.

  • 158.
    Belonoshko, Anatoly B.
    et al.
    Royal Institute Technology KTH, Sweden.
    Lukinov, Timofei
    Royal Institute Technology KTH, Sweden.
    Fu, Jie
    Royal Institute Technology KTH, Sweden; Dalian University of Technology, Peoples R China.
    Zhao, Jijun
    Dalian University of Technology, Peoples R China.
    Davis, Sergio
    Chilean Comm Nucl Energy CCHEN, Chile.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Stabilization of body-centred cubic iron under inner-core conditions2017In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 10, no 4, p. 312-+Article in journal (Refereed)
    Abstract [en]

    The Earths solid core is mostly composed of iron. However, despite being central to our understanding of core properties, the stable phase of iron under inner-core conditions remains uncertain. The two leading candidates are hexagonal close-packed and body-centred cubic (bcc) crystal structures, but the dynamic and thermodynamic stability of bcc iron under inner-core conditions has been challenged. Here we demonstrate the stability of the bcc phase of iron under conditions consistent with the centre of the core using ab initio molecular dynamics simulations. We find that the bcc phase is stabilized at high temperatures by a diffusion mechanism that arises due to the dynamical instability of the phase at lower temperatures. On the basis of our simulations, we reinterpret experimental data as support for the stability of bcc iron under inner-core conditions. We suggest that the diffusion of iron atoms in solid state may explain both the anisotropy and the low shear modulus of the inner core.

  • 159.
    Landälv, Ludvig
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Sandvik Coromant AB, Sweden.
    Lu, Jun
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Spitz, S.
    Karlsruhe Institute Technology, Germany.
    Leiste, H.
    Karlsruhe Institute Technology, Germany.
    Ulrich, S.
    Karlsruhe Institute Technology, Germany.
    Johansson-Jöesaar, Mats P
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering. SECO Tools AB, Sweden.
    Ahlgren, M.
    Sandvik Coromant AB, Sweden.
    Gothelid, E.
    Sandvik Coromant AB, Sweden.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Max Planck Institute Eisenforsch GmbH, Germany.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Stueber, M.
    Karlsruhe Institute Technology, Germany.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Structural evolution in reactive RF magnetron sputtered (Cr,Zr)2O3 coatings during annealing2017In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 131, p. 543-552Article in journal (Refereed)
    Abstract [en]

    Reactive RF-magnetron sputtering is used to grow Cr0.28Zr0.10O0.61 coatings at 500 degrees C. Coatings are annealed at 750 degrees C, 810 degrees C, and 870 degrees C. The microstructure evolution of the pseudobinary oxide compound is characterized through high resolution state of the art HRSTEM and HREDX-maps, revealing the segregation of Cr and Zr on the nm scale. The as-deposited coating comprises cc-(Cr,Zr)(2)O-3 solid solution with a Zr-rich (Zr,Cr)O-x. amorphous phase. After annealing to 750 degrees C tetragonal ZrO2 nucleates and grows from the amorphous phase. The ZrO2 phase is stabilized in its tetragonal structure at these fairly low annealing temperatures, possibly due to the small grain size (below 30 nm). Correlated with the nucleation and growth of the tetragonal-ZrO2 phase is an increase in hardness, with a maximum hardness after annealing to 750 degrees C, followed by a decrease in hardness upon coarsening, bcc metallic Cr phase formation and loss of oxygen, during annealing to 870 degrees C. The observed phase segregation opens up future design routes for pseudobinary oxides with tunable microstructural and mechanical properties. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • 160.
    Fashandi, Hossein
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Dahlqvist, Martin
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Lu, Jun
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Palisaitis, Justinas
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Rosén, Johanna
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Andersson, Mike
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Lloyd Spetz, Anita
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Synthesis of Ti3AuC2, Ti3Au2C2 and Ti3IrC2 by noble metal substitution reaction in Ti3SiC2 for high-temperature-stable Ohmic contacts to SiC2017In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 16, no 8, p. 814-818Article in journal (Refereed)
    Abstract [en]

    The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of Ti3AuC2 and Ti3Au2C2 phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into Ti3SiC2 single-crystal thin films with simultaneous out-diffusion of Si. Ti3IrC2 is subsequently produced by a substitution reaction of Ir for Au in Ti3Au2C2. These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 degrees C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.

  • 161.
    Wang, Fei
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. University of Saarland, Germany.
    Holec, David
    University of Leoben, Austria.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Muecklich, Frank
    University of Saarland, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. NUST MISIS, Russia.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Systematic ab initio investigation of the elastic modulus in quaternary transition metal nitride alloys and their coherent multilayers2017In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 127, p. 124-132Article in journal (Refereed)
    Abstract [en]

    We give a comprehensive overview of the elastic properties of cubic quaternary transition metal nitride alloys and coherent nitride multilayers for design of wear resistant hard coatings. The elastic stiffness constants of the alloys are calculated using the special quasirandom structure method. For multilayers with sharp interfaces we prove the applicability of a linear-elasticity approximation and show that it can be used with success instead of performing direct computationally demanding ab initio calculations. We explore the trends and the potential of multicomponent alloying in engineering the strength and ductility of both, quaternary alloys and their multilayers. We investigate X(i-x-y)TixAlyN alloys where Xis Zr, Hf, V, Nb or Ta, and present an analysis based on increasing x. We show that with increasing Ti content ductility can increase in each alloy. Elastic isotropy is observed only in (Zr,Hf,V)((i-x-y))TixAlyN alloys in the middle of the compositional triangle, otherwise a high Youngs modulus is observed along [001]. We predict that coherent TiN/X(1-x-y)TixAlyN and ZrN/X(i-x-3)TixAlyN alloy multilayers with the [111] interfacial direction show increasing ductility with increasing x, while the multilayers with the [001] orientation become more brittle. We show that the Youngs moduli variation in the parent bulk quaternary nitride alloy provide a reliable descriptor to screen the Youngs modulus of coherent multilayers in high-throughput calculations. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • 162.
    Lin, Tao
    et al.
    Technical University of Munich, Germany.
    Zhang, Liding
    Technical University of Munich, Germany.
    Björk, Jonas
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Chen, Zhi
    Karlsruhe Institute Technology, Germany.
    Ruben, Mario
    Karlsruhe Institute Technology, Germany; University of Strasbourg, France.
    Barth, Johannes V.
    Technical University of Munich, Germany.
    Klappenberger, Florian
    Technical University of Munich, Germany.
    Terminal Alkyne Coupling on a Corrugated Noble Metal Surface: From Controlled Precursor Alignment to Selective Reactions2017In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 23, no 62, p. 15588-15593Article in journal (Refereed)
    Abstract [en]

    Surface-templated covalent coupling of organic precursors currently emerges as a promising route to the atom-precise fabrication of low-dimensional carbon materials. Here, we investigate the adsorption and the coupling reactions of 4,4-diethynyl-1,1:4,1-terphenyl on Au(110) under ultra-high vacuum conditions by using scanning tunneling microscopy combined with density functional theory and kinetic Monte Carlo calculations. Temperature treatment induces both 1,2,4-asymmetric cyclotrimerization and homocoupling, resulting in various reaction products, including a previously unreported, surface-templated H-shaped pentamer. Our analysis of the temperature-dependent relative product abundances unravels that 1,2,4-trimerization and homocoupling proceed via identical intermediate species with the final products depending on the competition of coupling to a third monomer versus dehydrogenation. Our study sheds light on the control of coupling reactions by corrugated surfaces and annealing protocols.

  • 163.
    Lindmaa, Alexander
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Theoretical prediction of properties of atomistic systems: Density functional theory and machine learning2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The prediction of ground state properties of atomistic systems is of vital importance in technological advances as well as in the physical sciences. Fundamentally, these predictions are based on a quantum-mechanical description of many-electron systems. One of the hitherto most prominent theories for the treatment of such systems is density functional theory (DFT). The main reason for its success is due to its balance of acceptable accuracy with computational efficiency. By now, DFT is applied routinely to compute the properties of atomic, molecular, and solid state systems.

    The general approach to solve the DFT equations is to use a density-functional approximation (DFA). In Kohn-Sham (KS) DFT, DFAs are applied to the unknown exchangecorrelation (xc) energy. In orbital-free DFT on the other hand, where the total energy is minimized directly with respect to the electron density, a DFA applied to the noninteracting kinetic energy is also required. Unfortunately, central DFAs in DFT fail to qualitatively capture many important aspects of electronic systems. Two prime examples are the description of localized electrons, and the description of systems where electronic edges are present.

    In this thesis, I use a model system approach to construct a DFA for the electron localization function (ELF). The very same approach is also taken to study the non-interacting kinetic energy density (KED) in the slowly varying limit of inhomogeneous electron densities, where the effect of electronic edges are effectively included. Apart from the work on model systems, extensions of an exchange energy functional with an improved KS orbital description are presented: a scheme for improving its description of energetics of solids, and a comparison of its description of an essential exact exchange feature known as the derivative discontinuity with numerical data for exact exchange.

    An emerging alternative route towards the prediction of the properties of atomistic systems is machine learning (ML). I present a number of ML methods for the prediction of solid formation energies, with an accuracy that is on par with KS DFT calculations, and with orders-of-magnitude lower computational cost.

    List of papers
    1. Quantum oscillations in the kinetic energy density: Gradient corrections from the Airy gas
    Open this publication in new window or tab >>Quantum oscillations in the kinetic energy density: Gradient corrections from the Airy gas
    2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 7, p. 075139-Article in journal (Refereed) Published
    Abstract [en]

    We derive a closed-form expression for the quantum corrections to the kinetic energy density in the Thomas-Fermi limit of a linear potential model system in three dimensions (the Airy gas). The universality of the expression is tested numerically in a number of three-dimensional model systems: (i) jellium surfaces, (ii) confinement in a hydrogenlike potential (the Bohr atom), (iii) particles confined by a harmonic potential in one and (iv) all three dimensions, and (v) a system with a cosine potential (the Mathieu gas). Our results confirm that the usual gradient expansion of extended Thomas-Fermi theory does not describe the quantum oscillations for systems that incorporate surface regions where the electron density drops off to zero. We find that the correction derived from the Airy gas is universally applicable to relevant spatial regions of systems of types (i), (ii), and (iv), but somewhat surprisingly not (iii). We discuss possible implications of our findings to the development of functionals for the kinetic energy density.

    Place, publisher, year, edition, pages
    American Physical Society, 2014
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-110971 (URN)10.1103/PhysRevB.90.075139 (DOI)000341238200002 ()
    Note

    Funding Agencies|Swedish Research Council (VR) [621-2011-4249]; Linnaeus Environment at Linkoping on Nanoscale Functional Materials (LiLi-NFM) - VR; U.S. Department of Energys National Nuclear Security Administration [DE-AC04-94AL85000]

    Available from: 2014-10-02 Created: 2014-10-01 Last updated: 2017-12-05
    2. Energetics of the AK13 semilocal Kohn-Sham exchange energy functional
    Open this publication in new window or tab >>Energetics of the AK13 semilocal Kohn-Sham exchange energy functional
    2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 15, article id 155143Article in journal (Refereed) Published
    Abstract [en]

    The recent nonempirical semilocal exchange functional of Armiento and Kummel [Phys. Rev. Lett. 111, 036402 (2013)], AK13, incorporates a number of features reproduced by higher-order theory. The AK13 potential behaves analogously with the discontinuous jump associated with the derivative discontinuity at integer particle numbers. Recent works have established that AK13 gives a qualitatively improved orbital description compared to other semilocal methods, and reproduces a band structure closer to higher-order theory. However, its energies and energetics are inaccurate. The present work further investigates the deficiency in energetics. In addition to AK13 results, we find that applying the local-density approximation (LDA) non-self-consistently on the converged AK13 density gives very reasonable energetics with equilibrium lattice constants and bulk moduli well described across 13 systems. We also confirm that the attractive orbital features of AK13 are retained even after full structural relaxation. Hence, the deficient energetics cannot be a result of the AK13 orbitals having adversely affected the quality of the electron density compared to that of usual semilocal functionals; an improved orbital description and good energetics are not in opposition. This is also confirmed by direct calculation of the principal component of the electric field gradient. In addition, we prove that the non-self-consistent scheme is equivalent to using a single external-potential-dependent functional in an otherwise consistent, nonvariational Kohn-Sham density-functional theory (KS DFT) scheme. Furthermore, our results also demonstrate that, while an internally consistent KS functional is presently missing, non-self-consistent LDA on AK13 orbitals works as a practical nonempirical computational scheme to predict geometries, bulk moduli, while retaining the band structure features of AK13 at the computational cost of semi-local DFT.

    Place, publisher, year, edition, pages
    AMER PHYSICAL SOC, 2016
    National Category
    Theoretical Chemistry
    Identifiers
    urn:nbn:se:liu:diva-133756 (URN)10.1103/PhysRevB.94.155143 (DOI)000390031400002 ()
    Note

    Funding Agencies|Swedish Research Council (VR) [621-2011-4249]; Linnaeus Environment at Linkoping on Nanoscale Functional Materials (LiLi-NFM) - VR; Swedish e-Science Research Centre (SeRC)

    Available from: 2017-01-09 Created: 2017-01-09 Last updated: 2017-11-29
    3. Crystal structure representations for machine learning models of formation energies
    Open this publication in new window or tab >>Crystal structure representations for machine learning models of formation energies
    2015 (English)In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 115, no 16, p. 1094-1101Article in journal (Refereed) Published
    Abstract [en]

    We introduce and evaluate a set of feature vector representations of crystal structures for machine learning (ML) models of formation energies of solids. ML models of atomization energies of organic molecules have been successful using a Coulomb matrix representation of the molecule. We consider three ways to generalize such representations to periodic systems: (i) a matrix where each element is related to the Ewald sum of the electrostatic interaction between two different atoms in the unit cell repeated over the lattice; (ii) an extended Coulomb-like matrix that takes into account a number of neighboring unit cells; and (iii) an ansatz that mimics the periodicity and the basic features of the elements in the Ewald sum matrix using a sine function of the crystal coordinates of the atoms. The representations are compared for a Laplacian kernel with Manhattan norm, trained to reproduce formation energies using a dataset of 3938 crystal structures obtained from the Materials Project. For training sets consisting of 3000 crystals, the generalization error in predicting formation energies of new structures corresponds to (i) 0.49, (ii) 0.64, and (iii) 0.37eV/atom for the respective representations.

    Place, publisher, year, edition, pages
    Wiley, 2015
    Keywords
    machine learning; formation energies; representations; crystal structure; periodic systems
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:liu:diva-120326 (URN)10.1002/qua.24917 (DOI)000357606000010 ()
    Note

    Funding Agencies|Swedish Research Council (VR) [621-2011-4249]; Linnaeus Environment at Linkoping on Nanoscale Functional Materials - VR; Swiss National Science foundation [PP00P2_138932]; Office of Science of the U.S. DOE [DE-AC02-06CH11357]; Air Force Office of Scientific Research, Air Force Material Command, USAF [FA9550-15-1-0026]

    Available from: 2015-07-31 Created: 2015-07-31 Last updated: 2017-11-01
    4. Machine Learning Energies of 2 Million Elpasolite (AB2D6) Crystals
    Open this publication in new window or tab >>Machine Learning Energies of 2 Million Elpasolite (AB2D6) Crystals
    2016 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, ISSN 031-9007, Vol. 117, no 13, article id 135502Article in journal (Refereed) Published
    Abstract [en]

    Elpasolite is the predominant quaternary crystal structure (AlNaK2F6 prototype) reported in the Inorganic Crystal Structure Database. We develop a machine learning model to calculate density functional theory quality formation energies of all ∼2×106 pristine ABC2D6 elpasolite crystals that can be made up from main-group elements (up to bismuth). Our model’s accuracy can be improved systematically, reaching a mean absolute error of 0.1  eV/atom for a training set consisting of 10×103 crystals. Important bonding trends are revealed: fluoride is best suited to fit the coordination of the D site, which lowers the formation energy whereas the opposite is found for carbon. The bonding contribution of the elements A and B is very small on average. Low formation energies result from A and B being late elements from group II, C being a late (group I) element, and D being fluoride. Out of 2×106 crystals, 90 unique structures are predicted to be on the convex hull—among which is NFAl2Ca6, with a peculiar stoichiometry and a negative atomic oxidation state for Al.

    Place, publisher, year, edition, pages
    American Physical Society, 2016
    Keywords
    Machine learning, AI, Elpasolite, Materials
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-131473 (URN)10.1103/PhysRevLett.117.135502 (DOI)000383849400010 ()
    Funder
    Swedish Research Council, 621-2011-4249,
    Note

    Funding agencies: Swiss National Science Foundation [PP00P2_138932]; Air Force Office of Scientific Research, Air Force Material Command, USAF [FA9550-15-1-0026]; NCCR MARVEL - Swiss National Science Foundation; Swiss National Supercomputing Centre (CSCS) [mr14]; Swedish R

    Available from: 2016-09-22 Created: 2016-09-22 Last updated: 2017-11-21Bibliographically approved
  • 164.
    Mikula, M.
    et al.
    Comenius University, Slovakia; Institute Mat and Machine Mech SAS, Slovakia.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr University of Bochum, Germany.
    Plasienka, D.
    Comenius University, Slovakia.
    Roch, T.
    Comenius University, Slovakia.
    Caplovicova, M.
    Slovak University of Technology Bratislava, Slovakia.
    Truchly, M.
    Comenius University, Slovakia.
    Satrapinskyy, L.
    Comenius University, Slovakia.
    Bystricky, R.
    Slovak Academic Science, Slovakia.
    Tonhauzerova, D.
    Comenius University, Slovakia.
    Vlckova, D.
    Comenius University, Slovakia.
    Kus, P.
    Comenius University, Slovakia.
    Thermally induced age hardening in tough Ta-Al-N coatings via spinodal decomposition2017In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 121, no 15, article id 155304Article in journal (Refereed)
    Abstract [en]

    We combine experiments and ab initio density functional theory calculations to investigate the evolution in structural and mechanical properties of TaAlN coatings as a function of the annealing temperature T. Formation of coherent cubic TaN- and AlN-rich nanometer-size domains, occurring during the initial stage of thermally induced phase separation within cubic NaCl-type (B1) TaAlN solid solutions, yields a monotonic increase in hardness from 29 GPa (as deposited coatings) up to a maximum of 35 GPa (+17%) reached after annealing at 1000 degrees C. Further thermal treatment at T amp;gt; 1000 degrees C leads to the transformation of metastable cubic domains into stable hexagonal TaNx and wurtzite AlN phases, thus resulting in hardness reductions. A comparison of our results with those reported in the literature reveals that TaAlN coatings are at least as hard while considerably less stiff (lower elastic moduli) than TiAlN coatings, thus indicating a substantial increase in toughness achieved upon replacing Ti with Ta in the host lattice. Present findings suggest that cubic TaAlN solid solutions are promising candidates for applications in protective coatings possessing both high-temperature hardness and toughness. Published by AIP Publishing.

  • 165.
    Ektarawong, Annop
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Max Planck Institute Eisenforsch GmbH, Germany.
    Thermodynamic stability and properties of boron subnitrides from first principles2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 6, article id 064206Article, review/survey (Refereed)
    Abstract [en]

    We use the first-principles approach to clarify the thermodynamic stability as a function of pressure and temperature of three different alpha-rhombohedral-boron-like boron subnitrides, with the compositions of B6N, B13N2, and B38N6, proposed in the literature. We find that, out of these subnitrides with the structural units of B-12(N-N), B-12(NBN), and [B-12(N-N)](0.33)[B-12(NBN)](0.67), respectively, only B38N6, represented by [B-12(N-N)](0.33)[B-12(NBN)](0.67), is thermodynamically stable. Beyond a pressure of about 7.5 GPa depending on the temperature, also B38N6 becomes unstable, and decomposes into cubic boron nitride and a-tetragonalboron- like boron subnitride B50N2. The thermodynamic stability of boron subnitrides and relevant competing phases is determined by the Gibbs free energy, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. We calculate lattice parameters, elastic constants, phonon and electronic density of states, and demonstrate that [B-12(N-N)](0.33)[B-12(NBN)](0.67) is bothmechanically and dynamically stable, and is an electrical semiconductor. The simulated x-ray powder-diffraction pattern as well as the calculated lattice parameters of [B-12(N-N)](0.33)[B-12(NBN)](0.67) are found to be in good agreement with those of the experimentally synthesized boron subnitrides reported in the literature, verifying that B38N6 is the stable composition of a-rhombohedral-boron-like boron subnitride.

  • 166.
    Feng, Qingguo
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Jönsson, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. NUST MISIS, Russia.
    Topological transitions of the Fermi surface of osmium under pressure: an LDA plus DMFT study2017In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 19, article id 033020Article in journal (Refereed)
    Abstract [en]

    The influence of pressure on the electronic structure of Os has attracted substantial attention recently due to reports on isostructural electronic transitions in this metal. Here, we theoretically investigate the Fermi surface of Os from ambient to high pressure, using density functional theory combined with dynamical mean field theory. Weprovide a detailed discussion of the calculated Fermi surface and its dependence on the level of theory used for the treatment of the electron-electron interactions. Although we confirm that Os can be classified as weakly correlated metal, the inclusion of local quantum fluctuations between 5d electrons beyond the local density approximation explains the most recent experimental reports regarding the occurrence of electronic topological transitions in Os.

  • 167.
    Karimov, A. R.
    et al.
    Russian Academic Science, Russia; National Research Nucl University of MEPhI, Russia.
    Yu, M. Y.
    Zhejiang University, Peoples R China; Ruhr University of Bochum, Germany.
    Stenflo, Lennart
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    A new class of exact solutions for Vlasov-Poisson plasmas2016In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 91, no 11, p. 114002-Article in journal (Refereed)
    Abstract [en]

    A new class of exact solutions of the Vlasov-Poisson equations based on the Jeans theorem is considered. The self-consistent solutions for the evolution of the system from given initial particle distributions make use of particular invariants of particle motion with predetermined spatial structure. The relation between the time-dependent coefficients of the latter and the macroscopic plasma quantities, or moments of the distribution function, are obtained. As an illustration, the evolution of an expanding plasma with oscillations of the electron and ion expansion fronts is presented.

  • 168.
    Brodin, G.
    et al.
    Umeå University, Sweden.
    Stenflo, Lennart
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    A new decay channel for upper-hybrid waves2016In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 91, no 10, article id 104005Article in journal (Refereed)
    Abstract [en]

    We look here at a three-wave interaction process involving only electrostatic waves in an electron plasma with stationary ions. Special attention is given to the case with an upper-hybrid wave as a pump wave, where a new decay channel is pointed out. The corresponding growth rate is calculated.

  • 169.
    Tholander, Christopher
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Tasnádi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Palisaitis, Justinas
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Jensen, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Sandström, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany.
    Zukauskaitè, Agne
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Fraunhofer Institute for Applied Solid State Physics IAF, Freiburg, Germany.
    Ab initio calculations and experimental study of piezoelectric YxIn1-xN thin films deposited using reactive magnetron sputter epitaxy2016In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 105, p. 199-206Article in journal (Refereed)
    Abstract [en]

    By combining theoretical prediction and experimental verification we investigate the piezoelectric properties of yttrium indium nitride (YxIn1-xN). Ab initio calculations show that the YxIn1-xN wurtzite phase is lowest in energy among relevant alloy structures for 0≤x≤0.5. Reactive magnetron sputter epitaxy was used to prepare thin films with Y content up to x=0.51. The composition dependence of the lattice parameters observed in the grown films is in agreement with that predicted by the theoretical calculations confirming the possibility to synthesize a wurtzite solid solution. An AlN buffer layer greatly improves the crystalline quality and surface morphology of subsequently grown YxIn1-xN films. The piezoelectric response in films with x=0.09 and x=0.14 is observed using piezoresponse force microscopy. Theoretical calculations of the piezoelectric properties predict YxIn1−xN to have comparable piezoelectric properties to ScxAl1-xN.

  • 170.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Mei, A. B.
    University of Illinois, IL 61801 USA.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Ab Initio Molecular Dynamics Simulations of Nitrogen/VN(001) Surface Reactions: Vacancy-Catalyzed N-2 Dissociative Chemisorption, N Adatom Migration, and N-2 Desorption2016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 23, p. 12503-12516Article in journal (Refereed)
    Abstract [en]

    We use density-functional ab initio molecular dynamics to investigate the kinetics of N/VN(001) surface reactions at temperatures ranging from 1600 to 2300 K. N adatoms (N-ad) on VN(001) favor epitaxial atop-V positions and diffuse among them by transiting through 4-fold hollow (FFH) sites, at which they are surrounded by two V and two N surface atoms. After several atop-V -amp;gt; FFH -amp;gt; atop-V jumps, isolated N adatoms bond strongly with an underlying N surface (N-surf) atom. Frequent N-ad/N-surf pair exchange reactions lead to N-2 desorption, which results in the formation of an anion surface vacancy. N vacancies rapidly migrate via in-plane (110) jumps and act as efficient catalysts for the dissociative chemisorption of incident N-2 molecules. During exposure of VN(001) to incident atomic N gas atoms, N-ad/N-ad recombination and desorption is never observed, despite a continuously high N monomer surface coverage. Instead, N-2 desorption is always initiated by a N adatom removing a N surface atom or by energetic N gas atoms colliding with N-ad or N-surf atoms. Similarities and differences between: N/VN(001) vs. previous N/TiN(001) results, discussed on the basis of temperature-dependent ab initio electronic structures and chemical bonding, provide insights for controlling the reactivity of NaCl-structure transition-metal nitride (001) surfaces via electron-concentration tuning.

  • 171.
    Burakovsky, L.
    et al.
    Los Alamos National Lab, NM 87545 USA.
    Burakovsky, N.
    Los Alamos National Lab, NM 87545 USA.
    Cawkwell, M. J.
    Los Alamos National Lab, NM 87545 USA.
    Preston, D. L.
    Los Alamos National Lab, NM 87545 USA.
    Errandonea, D.
    University of Valencia, Spain.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Ab initio phase diagram of iridium2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 9, p. 094112-Article in journal (Refereed)
    Abstract [en]

    The phase diagram of iridium is investigated using the Z methodology. The Z methodology is a technique for phase diagram studies that combines the direct Z method for the computation of melting curves and the inverse Z method for the calculation of solid-solid phase boundaries. In the direct Z method, the solid phases along the melting curve are determined by comparing the solid-liquid equilibrium boundaries of candidate crystal structures. The inverse Z method involves quenching the liquid into the most stable solid phase at various temperatures and pressures to locate a solid-solid boundary. Although excellent agreement with the available experimental data (to less than or similar to 65 GPa) is found for the equation of state (EOS) of Ir, it is the third-order Birch-Murnaghan EOS with B-0 = 5 rather than the more widely accepted B-0 = 4 that describes our ab initio data to higher pressure (P). Our results suggest the existence of a random-stacking hexagonal close-packed structure of iridium at high P. We offer an explanation for the 14-layer hexagonal structure observed in experiments by Cerenius and Dubrovinsky.

  • 172.
    Finzel, Kati
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    About the atomic shell structure in real space and the Pauli exclusion principle2016In: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 135, no 6, p. 148-Article in journal (Refereed)
    Abstract [en]

    It is shown that any set of eigenfunctions (1s, 2s) of a bare Coulomb Hamiltonian exhibit the same atomic shell structure pattern for the real-space indicator a(1), which is defined as the ratio between the positive kinetic energy density and the electron density. Since this model Hamiltonian excludes all effects due to the electron-electron repulsion, the appearance of the atomic shell structure is attributed to the Pauli exclusion principle that arises from the requirements for a fermionic wavefunction. Since the derivation is independent of the nuclear charge and the energy of the system, reversely imposing proper atomic shell structure behavior in the design of kinetic energy functionals mimics the Pauli exclusion principle during a variational process.

  • 173.
    Finzel, Kati
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    About the Difference Between Density Functionals Defined by Energy Criterion and Density Functionals Defined by Density Criterion: Exchange Functionals2016In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 116, no 15, p. 1187-1189Article in journal (Refereed)
    Abstract [en]

    The difference between density functionals defined by energy criterion and density functionals defined by density criterion is studied for the exchange functional. It is shown that Slater potentials are exact exchange potentials in the sense that they yield the Hartree-Fock electron density if all operators are given by local expressions. (C) 2016 Wiley Periodicals, Inc.

  • 174.
    Finzel, Kati
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Approximating the Pauli Potential in Bound Coulomb Systems2016In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 116, no 16, p. 1261-1266Article in journal (Refereed)
    Abstract [en]

    It is shown that the Pauli potential in bound Coulomb systems can in good approximation be composed from the corresponding atomic fragments. This provides a simple and fast procedure how to generate the Pauli potential in bound systems, which is needed to perform an orbital-free density functional calculation. The method is applicable to molecules and solids. (c) 2016 Wiley Periodicals, Inc.

  • 175.
    Wang, Weimin
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, Faculty of Science & Engineering.
    Olovsson, Weine
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, Faculty of Science & Engineering.
    Band structure of hydrogenated silicene on Ag(111): Evidence for half-silicane2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 8, p. 081406-Article in journal (Refereed)
    Abstract [en]

    In the case of graphene, hydrogenation removes the conductivity due to the bands forming the Dirac cone by opening up a band gap. This type of chemical functionalization is of the utmost importance for electronic applications. As predicted by theoretical studies, a similar change in the band structure is expected for silicene, the closest analog to graphene. We here report a study of the atomic and electronic structures of hydrogenated silicene with hydrogen on one side, the so-called half-silicane. The ("2 root 3 x 2 root 3") phase of silicene on Ag(111) was used in this Rapid Communication since it can be formed homogeneously across the entire surface of the Ag substrate. Low-energy electron diffraction and scanning tunneling microscopy data clearly show that hydrogenation changes the structure of silicene on Ag(111) resulting in a (1 x 1) periodicity with respect to the silicene lattice. The hydrogenated silicene also exhibits a quasiregular ("2 root 3 x 2 root 3")-like arrangement of vacancies. Angle-resolved photoelectron spectroscopy revealed two dispersive bands which can be unambiguously assigned to half-silicane. The common top of these bands is located at similar to 0.9 eV below the Fermi level. We find that the experimental bands are closely reproduced by the theoretical band structure of free-standing silicene with H adsorbed on the upper hexagonal sublattice.

  • 176.
    Ektarawong, Annop
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Max Planck Institute Eisenforsch GmbH, Germany.
    Carbon-rich icosahedral boron carbides beyond B4C and their thermodynamic stabilities at high temperature and pressure from first principles2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 94, no 5, article id 054104Article in journal (Refereed)
    Abstract [en]

    We investigate the thermodynamic stability of carbon-rich icosahedral boron carbide at different compositions, ranging from B4C to B2C, using first-principles calculations. Apart fromB4C, generally addressed in the literature, B2.5C, represented by B10C2p (C-C), where C-p and (C-C) denote a carbon atom occupying the polar site of the icosahedral cluster and a diatomic carbon chain, respectively, is predicted to be thermodynamically stable under high pressures with respect to B4C as well as pure boron and carbon phases. The thermodynamic stability of B2.5C is determined by the Gibbs free energy G as a function of pressure p and temperature T, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. The stability range of B2.5C is then illustrated through the p-T phase diagrams. Depending on the temperatures, the stability range of B2.5C is predicted to be within the range between 40 and 67 GPa. At T greater than or similar to 500 K, the icosahedral C-p atoms in B2.5C configurationally disorder at the polar sites. By investigating the properties of B2.5C, e.g., elastic constants and phonon and electronic density of states, we demonstrate that B2.5C is both mechanically and dynamically stable at zero pressure, and is an electrical semiconductor. Furthermore, based on the sketched phase diagrams, a possible route for experimental synthesis of B2.5C as well as a fingerprint for its characterization from the simulations of x-ray powder diffraction pattern are suggested.

  • 177.
    Caffrey, Nuala M.
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Armiento, Rickard
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. NUST MISIS, Russia; Tomsk State University, Russia.
    Changes in work function due to NO2 adsorption on monolayer and bilayer epitaxial graphene on SiC(0001)2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 20, article id 205411Article in journal (Refereed)
    Abstract [en]

    The electronic properties of monolayer graphene grown epitaxially on SiC(0001) are known to be highly sensitive to the presence of NO2 molecules. The presence of small areas of bilayer graphene, on the other hand, considerably reduces the overall sensitivity of the surface. We investigate how NO2 molecules interact with monolayer and bilayer graphene, both free-standing and on a SiC(0001) substrate. We show that it is necessary to explicitly include the effect of the substrate in order to reproduce the experimental results. When monolayer graphene is present on SiC, there is a large charge transfer from the interface between the buffer layer and the SiC substrate to the molecule. As a result, the surface work function increases by 0.9 eV after molecular adsorption. A graphene bilayer is more effective at screening this interfacial charge, and so the charge transfer and change in work function after NO2 adsorption is much smaller.

  • 178.
    Wang, Fei
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. University of Saarland, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. NUST MISIS, Russia; Tomsk State University, Russia.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Muecklich, Frank
    University of Saarland, Germany.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Coherency effects on the mixing thermodynamics of cubic Ti1-xAlxN/TiN(001) multilayers2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 17, p. 174201-Article in journal (Refereed)
    Abstract [en]

    In this work, we discuss the mixing thermodynamics of cubic (B1) Ti1-xAlxN/TiN(001) multilayers. We show that interfacial effects suppress the mixing enthalpy compared to bulk Ti1-xAlxN. The strongest stabilization occurs for compositions in which the mixing enthalpy of bulk Ti1-xAlxN has its maximum. The effect is split into a strain and an interfacial (or chemical) contribution, and we show that both contributions are significant. An analysis of the local atomic structure reveals that the Ti atoms located in the interfacial layers relax significantly different from those in the other atomic layers of the multilayer. Considering the electronic structure of the studied system, we demonstrate that the lower Ti-site projected density of states at epsilon(F) in the Ti1-xAlxN/TiN multilayers compared to the corresponding monolithic bulk explains a decreased tendency toward decomposition.

  • 179.
    Nowakowska, Sylwia
    et al.
    Department of Physics, University of Basel, Basel, Switzerland.
    Wäckerlin, Aneliia
    Department of Physics, University of Basel, Basel, Switzerland.
    Piquero-Zulaica, Ignacio
    Centro de Física de Materiales (CSIC/UPV-EHU)—Materials Physics Center, San Sebastián, Spain.
    Nowakowski, Jan
    Laboratory for Micro- and Nanotechnology, Paul Scherrer Institute, Villigen, PSI, Switzerland.
    Kawai, Shigeki
    Department of Physics, University of Basel, Basel, Switzerland; PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama, Japan.
    Wäckerlin, Christian
    Laboratory for Micro- and Nanotechnology, Paul Scherrer Institute, Villigen, PSI, Switzerland.
    Matena, Manfred
    Department of Physics, University of Basel, Basel, Switzerland; Donostia International Physics Center (DIPC), San Sebastián, Spain.
    Nijs, Thomas
    Department of Physics, University of Basel, Basel, Switzerland.
    Fatayer, Shadi
    Department of Physics, University of Basel, Basel, Switzerland; Departamento de Física Aplicada, Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas, Campinas, Brazil.
    Popova, Olha
    Department of Physics, University of Basel, Basel, Switzerland.
    Ahsan, Aisha
    Department of Physics, University of Basel, Basel, Switzerland.
    Mousavi, S. Fatemeh
    Department of Physics, University of Basel, Basel, Switzerland.
    Ivas, Toni
    Department of Physics, University of Basel, Basel, Switzerland.
    Meyer, Ernst
    Department of Physics, University of Basel, Basel, Switzerland.
    Stöhr, Meike
    Zernike Institute for Advanced Materials, University of Groningen, AG, Groningen, The Netherlands.
    Ortega, J. Enrique
    Centro de Física de Materiales (CSIC/UPV-EHU)—Materials Physics Center, San Sebastián, Spain; Donostia International Physics Center (DIPC), San Sebastián, Spain; Departamento Física Aplicada I, Universidad del País Vasco, San Sebastián, Spain.
    Björk, Jonas
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Chemistry. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Gade, Lutz H.
    Anorganisch-Chemisches Institut, Universität Heidelberg, Heidelberg, Germany.
    Lobo-Checa, Jorge
    Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC-Universidad de Zaragoza, Zaragoza, Spain; Departamento de Física de la Materia Condensada, Universidad de Zaragoza, Zaragoza, Spain.
    Jung, Thomas A.
    Laboratory for Micro- and Nanotechnology, Paul Scherrer Institute, Villigen, PSI, Switzerland.
    Configuring Electronic States in an Atomically Precise Array of Quantum Boxes2016In: Small, ISSN 1613-6810, E-ISSN 1613-6829, Vol. 12, no 28, p. 3757-3763Article in journal (Refereed)
    Abstract [en]

    A 2D array of electronically coupled quantum boxes is fabricated by means of on-surface self-assembly assuring ultimate precision of each box. The quantum states embedded in the boxes are configured by adsorbates, whose occupancy is controlled with atomic precision. The electronic interbox coupling can be maintained or significantly reduced by proper arrangement of empty and filled boxes.

  • 180.
    Tomita, Kota
    et al.
    University of Tokyo, Japan.
    Miyata, Tomohiro
    University of Tokyo, Japan.
    Olovsson, Weine
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Mizoguchi, Teruyasu
    University of Tokyo, Japan.
    Core-Exciton Interaction in Sodium L-2,L-3 edge Structure Investigated Using the Bethe-Salpeter Equation2016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 17, p. 9036-9042Article in journal (Refereed)
    Abstract [en]

    Despite the importance of sodium compounds for their application in sodium-ion rechargeable batteries, core-exciton interactions in sodium L-2,L-3 edge core-electron loss spectra are not well-understood. In this study, Bethe-Salpeter equation calculations of sodium L-2,L-3 edges of sodium compounds were performed to understand the relationships between the core-exciton interactions and the electronic structure of sodium compounds. It was revealed that the core-exciton interaction of sodium compounds is strongly dependent on the compounds. We found that neither band gap nor ionic charge can explain the trend; however, the transition energy shows a clear correlation to the magnitude of the core-exciton interaction. These results indicate that the magnitude of the core-exciton interaction is decided by the excited electronic structure of each compound.

  • 181.
    Sernelius, Bo E.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Core-level spectra from bilayer graphene2016In: FlatChem, ISSN 2452-2627, Vol. 1, p. 6-10Article in journal (Refereed)
    Abstract [en]

    We derive core-level spectra for doped free-standing bilayer graphene. Numerical results are presented for all nine combinations of the doping concentrations 1012 cm-2; 1013 cm-2, and 1014 cm-2 in the two graphene sheets and we compare the results to the reference spectra for monolayer graphene. We furthermore discuss the spectrum of single-particle inter-band and intra-band excitations in the ωq-plane, and show how the dispersion curves of the collective modes are modified in the bilayer system.

    The full text will be freely available from 2020-09-29 15:26
  • 182.
    Ivády, Viktor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Development of theoretical approaches for post-silicon information processing2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Despite knowing the fundamental equations in most of the physics research areas, still there is an unceasing need for theoretical method development, thanks to the more and more challenging problems addressed by the research community. The investigation of post-silicon, non-classical information processing is one of the new and rapidly developing areas that requires tremendous amount of theoretical support, new understanding, and accurate theoretical predictions. My thesis focuses on theoretical method development for solid-state quantum information processing, mainly in the field of point defect quantum bits (qubits) in silicon carbide (SiC) and diamond. Due to recent experimental breakthroughs in this field, there are diverse theoretical problems, ranging from functional development for accurate first principles description of point defects, through complete theoretical characterization of qubits, to the modeling and simulation of actual quantum information protocols, that are needed to be addressed. The included articles of this thesis cover the development of (i) hybrid-DFT+Vw approach for the first principles description of mixed correlated and uncorrelated systems, (ii) zero-field-splitting tensor calculation for solid-state quantum bit characterization, (iii) a comprehensive model for dynamic nuclear spin polarization of solid-state qubits in semiconductors, and (iv) group theoretical description of qubits and novel twodimensional materials for topologically protected states.

    List of papers
    1. Theoretical unification of hybrid-DFT and DFT plus U methods for the treatment of localized orbitals
    Open this publication in new window or tab >>Theoretical unification of hybrid-DFT and DFT plus U methods for the treatment of localized orbitals
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    2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 3, p. 035146-Article in journal (Refereed) Published
    Abstract [en]

    Hybrid functionals serve as a powerful practical tool in different fields of computational physics and quantum chemistry. On the other hand, their applicability for the case of correlated d and f orbitals is still questionable and needs more considerations. In this article we formulate the on-site occupation dependent exchange correlation energy and effective potential of hybrid functionals for localized states and connect them to the on-site correction term of the DFT+ U method. The resultant formula indicates that the screening of the onsite electron repulsion is governed by the ratio of the exact exchange in hybrid functionals. Our derivation provides a theoretical justification for adding a DFT+ U-like on-site potential in hybrid-DFT calculations to resolve issues caused by overscreening of localized states. The resulting scheme, hybrid DFT+ V-w, is tested for chromium impurity in wurtzite AlN and vanadium impurity in 4H-SiC, which are paradigm examples of systems with different degrees of localization between host and impurity orbitals.

    Place, publisher, year, edition, pages
    American Physical Society, 2014
    National Category
    Physical Sciences Chemical Sciences
    Identifiers
    urn:nbn:se:liu:diva-109879 (URN)10.1103/PhysRevB.90.035146 (DOI)000339974700005 ()
    Note

    Funding Agencies|Knut and Alice Wallenberg Foundation "Isotopic Control for Ultimate Materials Properties"; Swedish Research Council (VR) Grants [621-2011-4426, 621-2011-4249]; Swedish Foundation for Strategic Research program SRL Grant [10-0026]; Swedish National Infrastructure for Computing Grants [SNIC 001/12-275, SNIC 2013/1-331]; "Lendulet program" of Hungarian Academy of Sciences; U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]; Linnaeus Environment at Linkoping on Nanoscale Functional Materials (LiLi-NFM) - VR

    Available from: 2014-08-28 Created: 2014-08-28 Last updated: 2017-12-05
    2. Role of screening in the density functional applied to transition-metal defects in semiconductors
    Open this publication in new window or tab >>Role of screening in the density functional applied to transition-metal defects in semiconductors
    2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 20Article in journal (Refereed) Published
    Abstract [en]

    We study selected transition-metal-related point defects in silicon and silicon carbide semiconductors by a range-separated hybrid density functional (HSE06). We find that HSE06 does not fulfill the generalized Koopmans' theorem for every defect, which is due to the self-interaction error in the functional in such cases. Restoring the so-called generalized Koopmans' condition with a simple correction in the functional can eliminate this error and brings the calculated charge transition levels remarkably close to the experimental data as well as to the calculated quasiparticle levels from many-body perturbation theory.

    Place, publisher, year, edition, pages
    American Physical Society, 2013
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-93859 (URN)10.1103/PhysRevB.87.205201 (DOI)000318521500002 ()
    Note

    Funding Agencies|Swedish Foundation for Strategic Research||Swedish Research Council||Swedish Energy Agency||Swedish National Infrastructure for Computing|SNIC 011/04-8SNIC001-10-223|Knut & Alice Wallenberg Foundation||EU||European Science Foundation for Advanced Concepts in ab-initio Simulations of Materials||

    Available from: 2013-06-11 Created: 2013-06-11 Last updated: 2017-12-06
    3. Pressure and temperature dependence of the zero-field splitting in the ground state of NV centers in diamond: A first-principles study
    Open this publication in new window or tab >>Pressure and temperature dependence of the zero-field splitting in the ground state of NV centers in diamond: A first-principles study
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    2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 23, p. 235205-Article in journal (Refereed) Published
    Abstract [en]

    Nitrogen-vacancy centers in diamond (NV) attract great attention because they serve as a tool in many important applications. The NV center has a polarizable spin S = 1 ground state and its spin state can be addressed by optically detected magnetic resonance (ODMR) techniques. The m(S) = 0 and m(S) = +/- 1 spin levels of the ground state are separated by about 2.88 GHz in the absence of an external magnetic field or any other perturbations. This zero-field splitting (ZFS) can be probed by ODMR. As this splitting changes as a function of pressure and temperature, the NV center might be employed as a sensor operating at the nanoscale. Therefore, it is of high importance to understand the intricate details of the pressure and temperature dependence of this splitting. Here we present an ab initio theory of the ZFS of the NV center as a function of external pressure and temperature including detailed analysis on the contributions of macroscopic and microscopic effects. We found that the pressure dependence is governed by the change in the distance between spins as a consequence of the global compression and the additional local structural relaxation. The local structural relaxation contributes to the change of ZFS with the same magnitude as the global compression. In the case of temperature dependence of ZFS, we investigated the effect of macroscopic thermal expansion as well as the consequent change of the microscopic equilibrium positions. We could conclude that theses effects are responsible for about 15% of the observed decrease of ZFS.

    Place, publisher, year, edition, pages
    American Physical Society, 2014
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:liu:diva-113573 (URN)10.1103/PhysRevB.90.235205 (DOI)000346860700002 ()
    Note

    Funding Agencies|NIIF Supercomputer center [1090]; EU [270197, 611143]; Hungarian Academy of Sciences; Knut and Alice Wallenberg Foundation; Swedish National Infrastructure for Computing [SNIC 001/12-275, SNIC 2013/1-331]; Conicyt (Chile) Fondecyt [1141185, PIA ACT1108, PIA ACT1112]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Tomsk State University

    Available from: 2015-01-23 Created: 2015-01-23 Last updated: 2017-12-05
    4. Theoretical model of dynamic spin polarization of nuclei coupled to paramagnetic point defects in diamond and silicon carbide
    Open this publication in new window or tab >>Theoretical model of dynamic spin polarization of nuclei coupled to paramagnetic point defects in diamond and silicon carbide
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    2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 11, p. 115206-Article in journal (Refereed) Published
    Abstract [en]

    Dynamic nuclear spin polarization (DNP) mediated by paramagnetic point defects in semiconductors is a key resource for both initializing nuclear quantum memories and producing nuclear hyperpolarization. DNP is therefore an important process in the field of quantum-information processing, sensitivity-enhanced nuclear magnetic resonance, and nuclear-spin-based spintronics. DNP based on optical pumping of point defects has been demonstrated by using the electron spin of nitrogen-vacancy (NV) center in diamond, and more recently, by using divacancy and related defect spins in hexagonal silicon carbide (SiC). Here, we describe a general model for these optical DNP processes that allows the effects of many microscopic processes to be integrated. Applying this theory, we gain a deeper insight into dynamic nuclear spin polarization and the physics of diamond and SiC defects. Our results are in good agreement with experimental observations and provide a detailed and unified understanding. In particular, our findings show that the defect electron spin coherence times and excited state lifetimes are crucial factors in the entire DNP process.

    Place, publisher, year, edition, pages
    AMER PHYSICAL SOC, 2015
    National Category
    Physical Sciences Chemical Sciences
    Identifiers
    urn:nbn:se:liu:diva-121891 (URN)10.1103/PhysRevB.92.115206 (DOI)000361370600003 ()
    Note

    Funding Agencies|Knut and Alice Wallenberg Foundation "Isotopic Control for Ultimate Materials Properties"; Swedish Research Council (VR) [621-2011-4426, 621-2011-4249]; Swedish Foundation for Strategic Research program SRL [10-0026]; Grant of Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Tomsk State University Academic D. I. Mendeleev Fund Program [8.1.18.2015]; Swedish National Infrastructure for Computing Grants [SNIC 2013/1-331]; "Lendulet program" of Hungarian Academy of Sciences

    Available from: 2015-10-13 Created: 2015-10-12 Last updated: 2017-12-01
    5. Optical Polarization of Nuclear Spins in Silicon Carbide
    Open this publication in new window or tab >>Optical Polarization of Nuclear Spins in Silicon Carbide
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    2015 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, no 24, p. 247603-Article in journal (Refereed) Published
    Abstract [en]

    We demonstrate optically pumped dynamic nuclear polarization of Si-29 nuclear spins that are strongly coupled to paramagnetic color centers in 4H- and 6H-SiC. The 99% +/- 1% degree of polarization that we observe at room temperature corresponds to an effective nuclear temperature of 5 mu K. By combining ab initio theory with the experimental identification of the color centers optically excited states, we quantitatively model how the polarization derives from hyperfine-mediated level anticrossings. These results lay a foundation for SiC-based quantum memories, nuclear gyroscopes, and hyperpolarized probes for magnetic resonance imaging.

    Place, publisher, year, edition, pages
    American Physical Society, 2015
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:liu:diva-120166 (URN)10.1103/PhysRevLett.114.247603 (DOI)000356407600010 ()
    Note

    Funding Agencies|Air Force Office of Scientific Research (AFOSR); AFOSR Multidisciplinary Research Program of the University Research Initiative; National Science Foundation; Material Research Science and Engineering Center; Knut and Alice Wallenberg Foundation "Isotopic Control for Ultimate Materials Properties"; Lendulet program of the Hungarian Academy of Sciences; National Supercomputer Center in Sweden

    Available from: 2015-07-13 Created: 2015-07-13 Last updated: 2017-12-04
    6. A silicon carbide room-temperature single-photon source
    Open this publication in new window or tab >>A silicon carbide room-temperature single-photon source
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    2014 (English)In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 13, no 2, p. 151-156Article in journal (Refereed) Published
    Abstract [en]

    Over the past few years, single-photon generation has been realized in numerous systems: single molecules(1), quantum dots(2-4), diamond colour centres5 and others(6). The generation and detection of single photons play a central role in the experimental foundation of quantum mechanics(7) and measurement theory(8). An efficient and high-quality single-photon source is needed to implement quantum key distribution, quantum repeaters and photonic quantum information processing(9). Here we report the identification and formation of ultrabright, room-temperature, photostable single-photon sources in a device-friendly material, silicon carbide (SiC). The source is composed of an intrinsic defect, known as the carbon antisite-vacancy pair, created by carefully optimized electron irradiation and annealing of ultrapure SiC. An extreme brightness (2 x 10(6) counts s(-1)) resulting from polarization rules and a high quantum efficiency is obtained in the bulk without resorting to the use of a cavity or plasmonic structure. This may benefit future integrated quantum photonic devices(9).

    Place, publisher, year, edition, pages
    Nature Publishing Group, 2014
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-104643 (URN)10.1038/nmat3806 (DOI)000330182700020 ()
    Available from: 2014-02-20 Created: 2014-02-20 Last updated: 2017-12-06
    7. Dirac points with giant spin-orbit splitting in the electronic structure of two-dimensional transition-metal carbides
    Open this publication in new window or tab >>Dirac points with giant spin-orbit splitting in the electronic structure of two-dimensional transition-metal carbides
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    2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 15Article in journal (Refereed) Published
    Abstract [en]

    We investigated the structural and electrical properties of 2D MXene sheets by means of firstprinciples density functional theory (DFT) calculations. To describe the Kohn-Sham states, plane wave basis set and projector augmented wave method (PAW) were used as implemented in the Vienna ab initio Simulation Package (VASP). We applied PBE parameterization of the generalized gradient approximation of the exchange and correlation energy functional to account for many-body effects of the interacting electron system. Convergent sampling of the Brillouin-zone was achieved by a Γ-centered 15×15×1 grid. In order to model a single sheet of MXene we ensured at least 30 Å vacuum between the periodically repeated sheets. For the structural optimization 1×10−3 eV/Å force criteria was used. The relativistic spin-orbit coupling effects were also included in our simulations regarding band structure and density of states.

    Keywords
    Cone-point, MAX phase, MXene, Dirac fermion, Spin-orbit coupling
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:liu:diva-113761 (URN)10.1103/PhysRevB.92.155142 (DOI)000363512700002 ()
    Available from: 2015-05-01 Created: 2015-01-30 Last updated: 2017-12-05Bibliographically approved
  • 183.
    Jason, Peter
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Johansson, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Discrete breathers for a discrete nonlinear Schrodinger ring coupled to a central site2016In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 93, no 1, p. 012219-Article in journal (Refereed)
    Abstract [en]

    We examine the existence and properties of certain discrete breathers for a discrete nonlinear Schrodinger model where all but one site are placed in a ring and coupled to the additional central site. The discrete breathers we focus on are stationary solutions mainly localized on one or a few of the ring sites and possibly also the central site. By numerical methods, we trace out and study the continuous families the discrete breathers belong to. Our main result is the discovery of a split bifurcation at a critical value of the coupling between neighboring ring sites. Below this critical value, families form closed loops in a certain parameter space, implying that discrete breathers with and without central-site occupation belong to the same family. Above the split bifurcation the families split up into several separate ones, which bifurcate with solutions with constant ring amplitudes. For symmetry reasons, the families have different properties below the split bifurcation for even and odd numbers of sites. It is also determined under which conditions the discrete breathers are linearly stable. The dynamics of some simpler initial conditions that approximate the discrete breathers are also studied and the parameter regimes where the dynamics remain localized close to the initially excited ring site are related to the linear stability of the exact discrete breathers.

  • 184.
    Schoeneberg, Johannes
    et al.
    University of Kiel, Germany.
    Caffrey, Nuala Mai
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. University of Kiel, Germany.
    Ferriani, Paolo
    University of Kiel, Germany.
    Heinze, Stefan
    University of Kiel, Germany.
    Berndt, Richard
    University of Kiel, Germany.
    Distance- and spin-resolved spectroscopy of iridium atoms on an iron bilayer2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 11, p. 115418-Article in journal (Refereed)
    Abstract [en]

    The induced spin polarization of Ir atoms on a ferromagnetic Fe double layer on W(110) has been investigated with spin-polarized scanning tunneling microscopy. An unoccupied state is observed with a spin polarization exceeding 60% that is inverted with respect to the Fe layer. This inversion is due to the tunneling gap acting as an orbital and spin filter. Distance dependent measurements show that the spin polarization remains approximately constant over the entire experimentally accessible range, from far in the tunneling regime to 1 angstrom from the point of contact formation. This is corroborated by density functional theory calculations which show that the inversion of spin polarization occurs within 0.5 angstrom of the adatom.

  • 185.
    Sidnov, K. P.
    et al.
    National University of Science and Technology MISIS, Russia; RAS, Russia.
    Belov, D. S.
    National University of Science and Technology MISIS, Russia.
    Ponomareva, A. V.
    National University of Science and Technology MISIS, Russia.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Zharmukhambetova, A. M.
    National Research Tomsk State University, Russia.
    Skripnyak, Natalia
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. National Research Tomsk State University, Russia.
    Barannikova, S. A.
    National Research Tomsk State University, Russia; RAS, Russia.
    Rogachev, A. S.
    National University of Science and Technology MISIS, Russia; RAS, Russia.
    Rouvimov, S.
    University of Notre Dame, IN 46556 USA.
    Mukasyan, A. S.
    University of Notre Dame, IN 46556 USA.
    Effect of alloying on elastic properties of ternary Ni-Al-Ti system: Experimental validation2016In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 688, p. 534-541Article in journal (Refereed)
    Abstract [en]

    Using combustion synthesis approach we fabricated B2 NiAl intermetallic compound as well as quasibinary Ni(Al50Ti50) alloy, where half Al atoms are randomly substituted by transitional metal Ti. Youngs modulus for synthesized materials was measured and appeared to be 222 +/- 10 GPa for NiAl and 175 +/- 15 GPa for Ni(Al50Ti50) phases. Using first-principles simulations in the framework of the Density Functional Theory, we investigate the elastic properties of Ni(Al1-xTix) system, including single-crystal, as well as polycrystalline elastic moduli. Direct comparison of the experimental and theoretical values of the Youngs modulus demonstrates that the employed theoretical approach allows carefully predict elastic properties of NiAl-based intermetallics. In particular, we predict that alloying NiAl with Ti should increase the ductility of the intermetallic phase. (C) 2016 Elsevier B.V. All rights reserved.

  • 186.
    Ektarawong, Annop
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Tasnádi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Wang, Fei
    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, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Max Planck Institute Eisenforsch GmbH, Germany.
    Effects of configurational disorder on the elastic properties of icosahedral boron-rich alloys based on B6O, B13C2, and B4C, and their mixing thermodynamics2016In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 144, no 13, article id 134503Article in journal (Refereed)
    Abstract [en]

    The elastic properties of alloys between boron suboxide (B6O) and boron carbide (B13C2), denoted by (B6O)1−x(B13C2)x, as well as boron carbide with variable carbon content, ranging from B13C2 to B4C are calculated from first-principles. Furthermore, the mixing thermodynamics of (B6O)1−x(B13C2)x is studied. A superatom-special quasirandom structure approach is used for modeling different atomic configurations, in which effects of configurational disorder between the carbide and suboxide structural units, as well as between boron and carbon atoms within the units, are taken into account. Elastic properties calculations demonstrate that configurational  disorder in B13C2, where a part of the C atoms in the CBC chains substitute for B atoms in the B12 icosahedra, drastically increase the Young’s and shear modulus, as compared to an atomically ordered state, B12(CBC). These calculated elastic moduli of the disordered state are in excellent agreement with experiments. Configurational disorder between boron and carbon can also explain the experimentally observed almost constant elastic moduli of boron carbide as the carbon content is changed from B4C to B13C2. The elastic moduli of the (B6O)1−x(B13C2)x system are also practically unchanged with composition if boron-carbon disorder is taken into account. By investigating the mixing thermodynamics of the alloys, in which the Gibbs free energy is determined within the mean-field approximation for the configurational entropy, we outline the pseudo-binary phase diagram of (B6O)1−x(B13C2)x. The phase diagram reveals the existence of a miscibility gap at all temperatures up to the melting point. Also, the coexistence of B6O-rich as well as ordered or disordered B13C2-rich domains in the material prepared through equilibrium routes is predicted.

  • 187.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, USA.
    Effects of phase stability, lattice ordering, and electron density on plastic deformation in cubic TiWN pseudobinary transition-metal nitride alloys2016In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 103, p. 823-835Article in journal (Refereed)
    Abstract [en]

    We carry out density functional theory calculations to compare the energetics of layer glide, as well as stress vs. strain curves, for cubic Ti0.5W0.5N pseudobinary alloys and reference B1-structure TiN. Irrespective of the degree of ordering on the metal sublattice, the hardness and stiffness of Ti0.5W0.5, as estimated by stress strain results and resistance to layer glide, are comparable to that of the parent binary TiN, while ductility is considerably enhanced. After an initial elastic response to an applied load, the pseudobinary alloy deforms plastically, thus releasing accumulated mechanical stress. In contrast, stress continues to increase linearly with strain in TiN. Layer glide in Ti0.5W0.5N is promoted by a high valence-electron concentration which enables the formation of strong metallic bonds within the slip direction upon deformation. [1111-oriented Ti0.5W0.5N layers characterized by high local metal-sublattice ordering exhibit low resistance to slip along < 110 > directions due to energetically favored formation of (111) hexagonal stacking faults. This is consistent with the positive formation energy of < 111 >-ordered Tio.5W0.5N with respect to mixing of cubic-BI TiN and hexagonal WC-structure WN. In the cubic pseudobinary alloy, slip occurs parallel, as well as orthogonal, to the resolved applied stress at the interface between layers with the lowest friction. We suggest that analogous structural metastability (mixing cubic and hexagonal TM nitride binary phases) and electronic (high valence electron concentration) effects are responsible for the enhanced toughness recently demonstrated experimentally for cubic single-crystal pseudobinary V0.5W0.5N and V0.5MocoN epitaxial layers. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • 188.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Hellman, Olle
    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, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Max Planck Institute Eisenforsch GmbH, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. 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 dynamics2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 9, p. 094305-Article in journal (Refereed)
    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.

  • 189.
    Lindmaa, Alexander
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Armiento, Rickard
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Energetics of the AK13 semilocal Kohn-Sham exchange energy functional2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 15, article id 155143Article in journal (Refereed)
    Abstract [en]

    The recent nonempirical semilocal exchange functional of Armiento and Kummel [Phys. Rev. Lett. 111, 036402 (2013)], AK13, incorporates a number of features reproduced by higher-order theory. The AK13 potential behaves analogously with the discontinuous jump associated with the derivative discontinuity at integer particle numbers. Recent works have established that AK13 gives a qualitatively improved orbital description compared to other semilocal methods, and reproduces a band structure closer to higher-order theory. However, its energies and energetics are inaccurate. The present work further investigates the deficiency in energetics. In addition to AK13 results, we find that applying the local-density approximation (LDA) non-self-consistently on the converged AK13 density gives very reasonable energetics with equilibrium lattice constants and bulk moduli well described across 13 systems. We also confirm that the attractive orbital features of AK13 are retained even after full structural relaxation. Hence, the deficient energetics cannot be a result of the AK13 orbitals having adversely affected the quality of the electron density compared to that of usual semilocal functionals; an improved orbital description and good energetics are not in opposition. This is also confirmed by direct calculation of the principal component of the electric field gradient. In addition, we prove that the non-self-consistent scheme is equivalent to using a single external-potential-dependent functional in an otherwise consistent, nonvariational Kohn-Sham density-functional theory (KS DFT) scheme. Furthermore, our results also demonstrate that, while an internally consistent KS functional is presently missing, non-self-consistent LDA on AK13 orbitals works as a practical nonempirical computational scheme to predict geometries, bulk moduli, while retaining the band structure features of AK13 at the computational cost of semi-local DFT.

  • 190.
    Finzel, Kati
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Davidsson, Joel
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Energy-Surfaces from the Upper Bound of the Pauli Kinetic Energy2016In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 116, no 18, p. 1337-1341Article in journal (Refereed)
    Abstract [en]

    Based on the Kohn-Sham Pauli potential and the Kohn-Sham electron density, the upper bound of the Pauli kinetic energy is tested as a suitable replacement for the exact Pauli kinetic energy for application in orbital-free density functional calculations. It is found that bond lengths for strong and moderately bound systems can be qualitatively predicted, but with a systematic shift toward larger bond distances with a relative error of 6% up to 30%. Angular dependence of the energy-surface cannot be modeled with the proposed functional. Therefore, the upper bound model is the first parameter-free functional expression for the kinetic energy that is able to qualitatively reproduce binding curves with respect to bond distortions. (C) 2016 Wiley Periodicals, Inc.

  • 191.
    Wang, You-Mei
    et al.
    Hangzhou Dianzi University, Peoples R China.
    Yu, M. Y.
    Zhejiang University, Peoples R China; Ruhr University, Germany.
    Stenflo, Lennart
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Karimov, A. R.
    Russian Academic Science, Russia; National Research Nucl University of MEPhI, Russia.
    Evolution of a Cold Non-neutral Electron-Positron Plasma Slab2016In: Chinese Physics Letters, ISSN 0256-307X, E-ISSN 1741-3540, Vol. 33, no 8, p. 085205-Article in journal (Refereed)
    Abstract [en]

    Evolution of a non-neutral cold electron-positron plasma slab is investigated. Initially the slab consists of a quasineutral plasma core bounded on both sides by layers containing only positrons (or electrons). Results from a nonperturbative, or mathematically exact, analysis of the governing fluid conservation equations and the Poisson equation show that despite their equal mass and charge magnitude, the electron and positron fronts can expand separately as well as a single fluid, and that nonlinear surface oscillations can be excited on the expansion fronts.

  • 192.
    Mozafari, Elham
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Chemistry. Linköping University, Faculty of Science & Engineering.
    Shulumba, Nina
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Steneteg, Peter
    Linköping University, Department of Science and Technology, Media and Information Technology. Linköping University, Faculty of Science & Engineering.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany.
    Abrikosov, Igor A.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Materials Modeling and Development Laboratory, NUST “MISIS”, Moscow, Russia.
    Finite-temperature elastic constants of paramagnetic materials within the disordered local moment picture from ab initio molecular dynamics calculations2016In: Physical Review B, ISSN 2469-9950, Vol. 94, no 5, article id 054111Article in journal (Refereed)
    Abstract [en]

    We present a theoretical scheme to calculate the elastic constants of magnetic materials in the high-temperature paramagnetic state. Our approach is based on a combination of disordered local moments picture and ab initio molecular dynamics (DLM-MD). Moreover, we investigate a possibility to enhance the efficiency of the simulations of elastic properties using the recently introduced method: symmetry imposed force constant temperature-dependent effective potential (SIFC-TDEP). We have chosen cubic paramagnetic CrN as a model system. This is done due to its technological importance and its demonstrated strong coupling between magnetic and lattice degrees of freedom. We have studied the temperature-dependent single-crystal and polycrystalline elastic constants of paramagentic CrN up to 1200 K. The obtained results at T = 300 K agree well with the experimental values of polycrystalline elastic constants as well as the Poisson ratio at room temperature. We observe that the Young’s modulus is strongly dependent on temperature, decreasing by 14% from T = 300 K to 1200 K. In addition we have studied the elastic anisotropy of CrN as a function of temperature and we observe that CrN becomes substantially more isotropic as the temperature increases. We demonstrate that the use of Birch law may lead to substantial errors for calculations of temperature induced changes of elastic moduli. The proposed methodology can be used for accurate predictions of mechanical properties of magnetic materials at temperatures above their magnetic order-disorder phase transition.

  • 193.
    Finzel, Kati
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Ayers, Paul W.
    McMaster University, Canada.
    Functional constructions with specified functional derivatives2016In: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 135, no 12, article id 255Article in journal (Refereed)
    Abstract [en]

    A bifunctional construction depending on a specified density-potential pair and an approximate guiding electron density functional is presented. The proposed bifunctional construction properly transforms under homogeneous coordinate scaling and yields the specified functional derivative, which determines the electron density. Whereas the method is general and applicable to all functional types, it will prove especially helpful in the context of orbital-free density functional theory, where most existing approximate density functionals predict inaccurate potentials.

  • 194.
    Yalamanchili, Phani Kumar
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Wang, Fei
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. University of Saarland, Germany.
    Aboulfadl, Hisham
    University of