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  • 51.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of YBa2Cu4O8 dominated by phonon-phonon interactions1993In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 48, no 5, p. 3575-3578Article in journal (Refereed)
    Abstract [en]

    The thermal conductivity κ of dense sintered ceramic YBa2Cu4O8 in the range 30–310 K has been measured. At 100 K, κ is 10 W m-1 K-1, approaching the in-plane κ of single crystals of other high-Tc materials. κ decreases rapidly with increasing T to 7.4 W m-1 K-1 at 300 K. Fitting standard models for κ(T) to the data we find that κ is limited mainly by phonon-phonon interactions. Depending on the model used, the best fit is found for effective values of FTHETADebye between 155 and 185 K, characteristic for the acoustic phonon branches, indicating that such phonons carry most of the heat. Finally, we suggest a possible way to test the electron-phonon model for the electrical and thermal conductivities in high-Tc materials.

  • 52.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Niska, John
    Loberg, Bengt
    Thermal conductivity of polycrystalline YBa2Cu4O81994In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 49, no 6, p. 4189-4198Article in journal (Refereed)
    Abstract [en]

    We have measured the thermal conductivity κ and the thermal diffusivity a of a dense bulk ceramic polycrystalline sample of YBa2Cu4O8 (1:2:4) in the temperature range 30–300 K. We find κ≊10 W m-1 K-1 at 100 K, significantly higher than in ceramic YBa2Cu3O7-δ (1:2:3) and approaching the in-plane value for single-crystal 1:2:3, and decreasing to 7.6 W m-1 K-1 at 300 K. The data for this sample can be described by standard theories for phonon thermal conductivity of crystalline materials with boundary, phonon, and electron scattering. The higher κ in 1:2:4 as compared to 1:2:3 is, in this model, due to the smaller point defect scattering in the former. The fitted parameters for the three scattering mechanisms all agree with independent estimates based on simple models; inserting data for electric resistivity, grain size, carrier density, and lattice properties we can predict κ and its T dependence to within about 20%. We also discuss models for the phonon and electron thermal conductivities in some detail, including some second-order effects such as inelastic electron scattering and a T-dependent carrier density.

  • 53. Andersson, C B M
    et al.
    Karlsson, Ulf O
    KTH, School of Information and Communication Technology (ICT).
    Hakansson, M C
    Olsson, L O
    Ilver, L
    Nilsson, P O
    Kanski, J
    Persson, P E S
    Electronic structure of InAs((1)over-bar-(1)over-bar-(1)over-bar)2x2 and InSb((1)over-bar-(1)over-bar-(1)over-bar)2x2 studied by angle-resolved photoelectron spectroscopy1996In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 54, no 3, p. 1833-1840Article in journal (Refereed)
    Abstract [en]

    The electronic structure of molecular-beam-epitaxy-grown InAs((111) over bar)2x2 and InSb((111) over bar)2x2 surfaces is investigated by angle-resolved photoelectron spectroscopy. Valence band spectra, and dispersions of five surface-related structures, are presented. The qualitative similarities of data from the two surfaces indicate that they are very similar, with respect to atomic and electronic structure. Comparisons with other (111) surfaces support the identification of the surface-related structures.

  • 54. Andersson, D. A.
    et al.
    Simak, S. I.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Abrikosov, I. A.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Modeling of CeO2, Ce2O3, and CeO2-x in the LDA plus U formalism2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 3, p. 035109-Article in journal (Refereed)
    Abstract [en]

    The electronic structure and thermodynamic properties of CeO2 and Ce2O3 have been studied from first principles by the all-electron projector-augmented-wave (PAW) method, as implemented in the ab initio total-energy and molecular-dynamics program VASP (Vienna ab initio simulation package). The local density approximation (LDA)+U formalism has been used to account for the strong on-site Coulomb repulsion among the localized Ce 4f electrons. We discuss how the properties of CeO2 and Ce2O3 are affected by the choice of U as well as the choice of exchange-correlation potential, i.e., the local density approximation or the generalized gradient approximation. Further, reduction of CeO2, leading to formation of Ce2O3 and CeO2-x, and its dependence on U and exchange-correlation potential have been studied in detail. Our results show that by choosing an appropriate U it is possible to consistently describe structural, thermodynamic, and electronic properties of CeO2, Ce2O3, and CeO2-x, which enables modeling of redox processes involving ceria-based materials.

  • 55. Andersson, D. A.
    et al.
    Simak, S. I.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Abrikosov, I. A.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Theoretical study of CeO2 doped with tetravalent ions2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 17, p. 174119-Article in journal (Refereed)
    Abstract [en]

    We have used density functional theory calculations within the LDA+U formulation to investigate how small amounts of dissolved SiO2, GeO2, SnO2, or PbO2 affect the redox thermodynamics of ceria (CeO2). Compared to pure ceria, reduction is facilitated and the reducibility increases in the sequence of CeO2-SnO2, CeO2-GeO2, and CeO2-SiO2, which correlates with the decrease of the ionic radii of the solutes. For low solute concentrations, there is an inverse relation between high reducibility and the solution energy of tetravalent solutes. CeO2-PbO2 is unique in the sense that the initial reduction occurs by Pb(IV)double right arrow Pb(II) instead of the usual Ce(IV)double right arrow Ce(III) reaction. Among the investigated ceria compounds, CeO2-PbO2 has the lowest reduction energy and rather low solution energy. We have studied how the solution and reduction energies depend on the concentration of Si, Ge, Sn, Pb, Ti, Zr, Hf, and Th solute ions. While the solution energy increases monotonously with concentration, the reduction energy first decreases, as compared to pure ceria (except for Th, which exhibits a small increase), and with further increase of solute concentration, it either remains almost constant (Zr, Hf, and Th) or slightly increases (Ti, Si, Ge, and Sn).

  • 56.
    Andersson, David A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Thermodynamics of structural vacancies in titanium monoxide from first principles calculations2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 14, p. 144101-Article in journal (Refereed)
    Abstract [en]

    The structure, stability and electronic properties of the low oxygen oxides of titanium, TiOx with 1/3 <= x <= 3/2, have been studied by means of accurate first-principles calculations. In both stoichiometric and nonstoichiometric TiO there are large fractions of vacant lattice sites. These so-called structural vacancies are essential for understanding the properties and phase stability of titanium oxides. Structures with an ordered arrangement of vacancies were treated with a plane wave pseudo-potential method, while calculations for structures with disordered vacancies were performed within the framework of the Korringa-Kohn-Rostoker Green's function technique. The relaxed structural parameters in general compare well with experimental data, though some discrepancies exist for stoichiometric TiO in the ideal B1 structure, i.e., without any vacancies. The equation of state as well as the elastic properties are also derived. A monoclinic, vacancy-containing, structure of stoichiometric TiO is confirmed to be stable at low temperature and pressure. Experimentally a transition from a stoichiometric cubic structure with disordered vacancies to the ideal B1 structure without any vacancies has been observed at high pressure. It is discussed how this experimental observation relates to the present theoretical results for defect-containing and defect-free TiO.

  • 57.
    Andersson, David A.
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Simak, S. I.
    Monovacancy and divacancy formation and migration in copper: a first-principles theory2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 11, p. 115108-Article in journal (Refereed)
    Abstract [en]

    The formation and migration of monovacancies and divacancies in copper have been studied from first-principles in order to resolve the discrepancies between previously published experimental and theoretical data. The monovacancy and divacancy formation, migration and binding enthalpies as well as the formation volumes have been calculated in the framework of a plane-wave pseudopotential implementation of the density functional theory, with full structural relaxations included. The monovacancy and divacancy formation entropies have been estimated from experimental data by performing a least-squares analysis. We show that the complete set of first-principles data, taking into account the presence of both vacancies and divacancies as well as the temperature dependence of the formation enthalpies and entropies allow one to reproduce the Arrhenius plot of the total vacancy concentration and the diffusion coefficient, both in good agreement with the most accurate experiments.

  • 58. Andersson, K.
    et al.
    Haviland, David B.
    KTH, Superseded Departments, Physics.
    Escape from a zero-current state in a one-dimensional array of Josephson junctions2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, no 9Article in journal (Refereed)
    Abstract [en]

    A long one-dimensional array of small Josephson junctions exhibits Coulomb blockade of Cooper pair tunneling. This zero-current state exists up to a switching voltage, V-sw, where there is a sudden onset of current. In this paper we present histograms showing how V-sw changes with temperature for a long array and calculations of the corresponding escape rates. Our analysis of the problem is based on the existence of a voltage-dependent energy barrier and we do not make any assumptions about its shape. The data divide into two temperature regimes, the higher of which can be explained with the Kramers thermal escape model. At low temperatures the escape becomes independent of temperature.

  • 59.
    Andersson, Magnus
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Comment on “Superconducting state of Ca-VII below a critical temperature of 29 K at a pressure of 216 GPa”2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 21Article in journal (Refereed)
    Abstract [en]

    In a recent paper, Sakata et al. [ Phys. Rev. B 83 220512 (2011)] report on superconductivity at 29 K in Ca under high pressure. Here, I argue that their method to define the onset of the superconducting transition temperature from resistivity measurements is misleading and that the correct value for the onset of the transition based on resistivity data shown in their paper should be 21 K. The discrepancy between these two temperatures is explained by superconducting fluctuations occurring at temperatures above the superconducting transition temperature.

  • 60.
    Andersson, Magnus
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    de Boissieu, M.
    Bruehne, S.
    Drescher, C.
    Assmus, W.
    Ohahshi, S.
    Tsai, A. P.
    Mihalkovic, M.
    Krajci, M.
    Rapp, Östen
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Electronic and structural properties of Laves-phase MgZn(2) of varying chemical disorder2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 2, p. 024202-Article in journal (Refereed)
    Abstract [en]

    The C14 Laves-phase MgZn(2) has been investigated from 30 to 36 at. % Mg. In this way chemical disorder can be monitored over a limited concentration range and the influence on electron properties can be investigated. Our studies include thermodynamic calculations of atomic configurations of Mg and Zn at off-stoichiometric compositions, electronic-transport measurements, and electronic band-structure calculations of MgZn(2). The disorder introduced by alloying was found to be substitutional for all C14 alloys, and to have a markedly stronger effect on resistivity and magnetoresistance, Delta rho(B)/rho(0), on the Mg-rich side due to strain introduced when Mg substitutes for Zn. rho(T) and Hall constant were characteristic for weakly disordered binary alloys. Delta rho/rho of MgZn(2) was large, reached 6 at 4.2 K and 8 T, and decreased strongly at off-stoichiometric compositions. The results are discussed in view of the band-structure results and in terms of relations between atomic order and electronic properties. Several properties were found to resemble pure Zn. An empirical correlation over more than six orders of magnitude in Delta rho/rho was found for Zn and Zn-based alloys.

  • 61.
    Andersson, Magnus
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Feuerbacher, M.
    Rapp, Östen
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Magnetoresistance and Hall effect of the complex metal alloy Mg2Al32008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 2Article in journal (Refereed)
    Abstract [en]

    Unusual electronic transport properties have been found in the rhombohedral beta'-phase of the complex metallic alloy Mg2Al3. The magnetoresistance (MR) is 2 orders of magnitude larger than in the related cubic beta-Mg2Al3-phase and Kohler's rule is strongly violated in the beta'-phase at higher temperatures. Above about 100 K the Hall coefficient R-H of the beta- and beta'-phases are similar and free-electron-like, while in the beta'-phase, R-H changes sign with decreasing temperature at low fields. We have inquired into the sources of these transport anomalies, but have not been able to clearly identify the grounds. Several conventional mechanisms for a large magnetoresistance are discussed, and found not to be applicable. The different properties in the beta- and the beta'-phases are puzzling since the magnitudes of the electrical resistivities are similar and omega(c)tau (cyclotron frequencyxscattering time) is equally small in both phases. The similar temperature range in which anomalies occur in the beta'-phase in the resistivity, the Hall effect, and the magnetoresistance indicates an electronic transition or a change of the electron structure in this phase below about 100 K.

  • 62.
    Andersson, Magnus
    et al.
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Fivat, Patrice
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Fabrega, Lourdes
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Obara, Haruhiko
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Decroux, Michel
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Triscone, Jean-Marc
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Fischer, Oystein
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Vortex solid-to-liquid transition in DyBa2Cu3O7-delta/(Y0.45Pr0.55)Ba2Cu3O7-delta multilayers1996In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 54, no 1, p. 675-681Article in journal (Refereed)
    Abstract [en]

    Current-voltage (I-V) characteristics of a coupled multilayer system consisting of thin (24 Angstrom) superconducting layers of DyBa2Cu3O7-delta each separated by 96 Angstrom of (Y0.45Pr0.55)Ba2Cu3O7-delta have been studied in the mixed state. Analyses of vortex glass scaling and signs of the clean system melting transition in the dynamics of our multilayers both suggest that the vortex system is close to three-dimensional for magnetic fields up to several T. Therefore, the theoretically predicted crossover when in-plane vortex correlations become more important than out-of-plane vortex correlations (occurring at a few tenths of a T in our multilayers) does not correspond to a complete decoupling of the superconducting layers in these materials.

  • 63.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Rydh, Andreas
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Rapp, Östen
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Scaling of the vortex-liquid resistivity in optimally doped and oxygen-deficient YBa2Cu3O7-delta single crystals2001In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 63, no 18, p. 1845111-1845119Article in journal (Refereed)
    Abstract [en]

    The consequences of a recently proposed model [A. Rydh. O. Rapp. and M. Andersson. Phys. Rev. Lett. 83. 1850 (1999)] for the vortex-liquid resistivity close to a vortex liquid-to-glass transition are anal-zed in detail. We find a detailed quantitative agreement between the model and resistivity measurements on disordered, optimally doped YBa2Cu3O7-delta single crystals. For temperatures below the superconducting transition temperature. a scaling of all measured resistivity curves in magnetic field (0<B12 T) is obtained. Possible ways of slightly modifying the model in order to fully describe resistivity measurements on more anisotropic underdoped oxygen-deficient YBa2Cu3O7-delta single crystals are suggested. The physical interpretation of the model and its connection to other models of the vortex-liquid resistivity are discussed. in particular, we find close connections to vortex glass models and to a generalized Coulomb gas model.

  • 64.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Chobal, O.
    Rizak, I.
    Rizak, V.
    Sabadosh, V.
    Effects of pressure and temperature on the thermal conductivity of Sn2P2S62011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 13, article id 134121Article in journal (Refereed)
    Abstract [en]

    The thermal conductivity kappa of the ferroelectric, paraelectric, and incommensurate phases of polycrystalline Sn2P2S6 has been measured in the 0.1-0.7 GPa range. The thermal conductivity kappa of the ferroelectric phase decreases with increasing pressure p. This unusual behavior, which is found in only a few other phases, is attributed to a negative Gruneisen parameter. The temperature T dependence of kappa for the ferroelectric phase (kappa similar to T-1) is well described by the Debye model for kappa, with three-phonon Umklapp scattering serving as the dominant scattering mechanism near and above the Debye temperature (similar to 100 K) up to a few tenths of degrees below the ferro- to paraelectric phase transition, where kappa(T) gradually changes and becomes temperature independent upon further heating. The thermal conductivity of the paraelectric and incommensurate phases was temperature independent and indistinguishable. Possible causes for the unusually weak T dependence at high temperatures and implications of the p dependence of kappa are discussed.

  • 65.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Filinchuk, Yaroslav
    Dmitriev, Vladimir
    Quwar, Issam
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Phase coexistence and hysteresis effects in the pressure-temperature phase diagram of NH3BH32011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 2, p. 024115-Article in journal (Refereed)
    Abstract [en]

    The potential hydrogen storage compound NH3BH3 has three known structural phases in the temperature and pressure ranges 110–300 K and 0–1.5 GPa, respectively. We report here the boundaries between, and the ranges of stability of, these phases. The phase boundaries were located by in situ measurements of the thermal conductivity, while the actual structures in selected areas were identified by in situ Raman spectroscopy and x-ray diffraction. Below 0.6 GPa, reversible transitions involving only small hysteresis effects occur between the room-temperature tetragonal plastic crystal I4mm phase and the low-temperature orthorhombic Pmn21 phase. Transformations of the I4mm phase into the high-pressure orthorhombic Cmc21 phase, occurring above 0.8 GPa, are associated with very large hysteresis effects, such that the reverse transition may occur at up to 0.5 GPa lower pressures. Below 230 K, a fraction of the Cmc21 phase is metastable to atmospheric pressure, suggesting the possibility that dense structural phases of NH3BH3, stable at room temperature, could possibly be created and stabilized by alloying or by other methods. Mixed orthorhombic Pmn21/Cmc21 phases were observed in an intermediate pressure-temperature range, but a fourth structural phase predicted by Filinchuk et al. [ Phys. Rev. B 79 214111 (2009)] was not observed in the pressure-temperature ranges of this experiment. The thermal conductivity of the plastic crystal I4mm phase is about 0.6 W m−1 K−1 and only weakly dependent on temperature, while the ordered orthorhombic phases have higher thermal conductivities limited by phonon-phonon scattering.

  • 66.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Johari, Gyan P.
    Collapse of an ice clathrate under pressure observed via thermal conductivity measurements2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, p. 174201-Article in journal (Refereed)
    Abstract [en]

    Irreversible transformation of the tetrahydrofuran ice clathrate at 130 K was studied by measuring thermal conductivity k with increase in pressure p. Initially, k increases slowly with p up to 0.75 GPa where it levels off, is roughly constant up to 0.95 GPa, then decreases up to 1.05 GPa. Pressure collapses the clathrate structure, plausibly beginning with lattice distortion, and k increases at 1.05 GPa in a sharp sigmoid-shape manner due to large densification until the transformation is complete at 1.25 GPa. This is the opposite of that found for ice whose k decreases first slowly with increase in p and then rapidly in an inverted sigmoidshape manner [O. Andersson and H. Suga, Phys. Rev. B 65, 140201 (2002)]. At 1.08 GPa and 131 K, k increases with time t (s) according to exp(t /2945), which is also the opposite of the collapse of ice [G. P. Johari and O. Andersson, Phys. Rev. B 70, 184108 (2004)]. The difference in its behavior is attributed to strong phonon scattering from the tetrahydrofuran guest molecules. k of the collapsed clathrate is 30% less than that for the collapsed ice, which is comparable with the 25% lesser k of the tetrahydrofuran-water solution from k of water at ambient pressure. On depressurizing at 130 K, k decreases progressively more rapidly and k of the collapsed state at 0.3 GPa is slightly lower than that of the as-made clathrate, showing that its original structure is not recovered.

  • 67.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of C60 at pressures up to 1 GPa and temperatures in the range 50-300 K1996In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 54, no 5, p. 3093-3100Article in journal (Refereed)
    Abstract [en]

    The thermal conductivity λ of C60 shows anomalies near 260 K and 90 K which are associated with the well-established phase transition and glass transition, respectively. Both transition temperatures increase with pressure, at the rates 120 K GPa-1 and 62 K GPa-1, respectively. With increasing temperature, λ of the simple cubic (sc) phase increased below 170 K (glasslike behavior) but decreased above. The glasslike behavior of λ is probably due to a substantial amount of lattice defects. Possible reasons for the change of sign of dλ/dT near 170 K are discussed. In the face centered cubic (fcc) phase (T≳260 K at atmospheric pressure) λ was almost independent of temperature, a behavior which is far from that of an ordered crystal (λ∝T-1 for T≳Debye temperature). This result can be attributed to the molecular orientational disorder of the fcc phase. The relaxation behavior associated with the glassy state and its unusually strong dependence on thermal history are discussed briefly, and data which support a previously reported relaxation model are presented. At room temperature, the density dependencies of λ, (∂ lnλ/∂ lnρ)T, were 5.5 and 9.5 for the fcc and sc phases, which are values typical for an orientationally disordered phase and a normal crystal phase, respectively.

  • 68.
    Andreasson, Björn Pererik
    et al.
    Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland.
    Janousch, M.
    Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland.
    Staub, U.
    Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland.
    Todorova, T.
    Condensed Matter Theory Group, Paul Scherrer Institut, Villigen, Switzerland.
    Delley, B.
    Condensed Matter Theory Group, Paul Scherrer Institut, Villigen, Switzerland.
    Meijer, G. I.
    IBM Research, Zurich Research Laboratory, Rüschlikon, Switzerland.
    Pomjakushina, E.
    Laboratory for Developments and Methods, Paul Scherrer Institut, Villigen, Switzerland.
    Detecting oxygen vacancies in SrTiO3 by 3d transition-metal tracer ions2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 21, p. Article number: 212103-Article in journal (Refereed)
    Abstract [en]

    X-ray absorption experiments on 3d transition-metal tracer ions in SrTiO3 are presented. The absorption spectra of the tracer-ion changed upon reduction in the SrTiO3. This change is due to an oxygen vacancy created at the tracer-ion site. This finding is supported by density-functional theory calculations, which prove that the oxygen vacancies preferentially are created at the tracer-ion sites. Using the chemical sensitivity of x-ray absorption spectroscopy, tracer ions can be used to detect oxygen vacancies in SrTiO3 and possibly in other oxide systems. © 2009 The American Physical Society.

  • 69. Andreasson, Jakob
    et al.
    Holmlund, Joakim
    Knee, Christopher S.
    Käll, Mikael
    Börjesson, Lars
    Naler, Stefan
    Bäckström, Joakim
    Ruebhausen, Michael
    Azad, Abul Kalam
    Uppsala University, Disciplinary Domain of Science and Technology, För teknisk-naturvetenskapliga fakulteten gemensamma enheter, The Studsvik Neutron Research Laboratory.
    Eriksson, Sten-G.
    Uppsala University, Disciplinary Domain of Science and Technology, För teknisk-naturvetenskapliga fakulteten gemensamma enheter, The Studsvik Neutron Research Laboratory.
    Franck-Condon higher order lattice excitations in the LaFe1-xCrxO3 (x=0, 0.1, 0.5, 0.9, 1.0) perovskites due to Fe-Cr charge transfer effects2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 10, p. 103402-Article in journal (Refereed)
    Abstract [en]

    First and higher order lattice excitations in the B-site disordered perovskites LaFe1-xCrxO3 (x=0, 0.1, 0.5, 0.9, and 1) and La0.835Sr0.165Fe0.5Cr0.5O3-delta are investigated using temperature dependent and polarized inelastic light scattering [lambda=515 nm (2.41 eV) and 676 nm (1.83 eV)] on oriented crystallites. A peak at approximately 2.4 eV in the imaginary part of the dielectric function of LaFe0.5Cr0.5O3 is assigned to a charge transfer from Fe3+ (d(5)) to Cr3+ (d(3)) ions, coupled with the appearance of an intense A(g)-like mode at approximately 700 cm(-1) in the Raman data. This excitation is identified as a symmetric oxygen breathing mode activated by the Fe-Cr charge transfer through an orbital coupling mechanism. Higher order scattering (up to seventh order) of the intrinsic Raman active symmetric breathing mode is also explained by an orbital-mediated electron-phonon coupling, similar to the Franck-Condon effect observed in the Jahn-Teller active-perovskite-structured manganite LaMaO(3). These results show that the Franck-Condon mechanism is a more common mechanism for resonant higher order scattering in solids than previously believed and propose the LaFe1-xCrxO3 system as a model system for electron-phonon coupling and higher order Raman scattering in solids.

  • 70.
    Andreasson, Jakob
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Cell and Molecular Biology.
    Holmlund, Joakim
    Rauer, Ralf
    Kaell, Mikael
    Boerjesson, Lars
    Knee, Christopher S.
    Eriksson, Annika K.
    Eriksson, Sten-G.
    Ruebhausen, Michael
    Chaudhury, Rajit P.
    Electron-phonon interactions in perovskites containing Fe and Cr studied by Raman scattering using oxygen-isotope and cation substitution2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 23, p. 235103-Article in journal (Refereed)
    Abstract [en]

    We use temperature-dependent inelastic light scattering to study the origin of the strong multiphonon scattering of a local oxygen breathing mode present in the mixed B-site orthorhombic (space group Pnma) perovskite LaFe0.5Cr0.5O3 but absent in isostructural LaFeO3 and LaCrO3. It is seen that the multiphonon scattering is critically sensitive to the presence of both Fe and Cr ions on the B site. These results support our interpretation that the multiphonon scattering is activated by local electron-phonon interactions according to the Franck-Condon picture following an Fe-Cr charge transfer. Further, O-18 substitution is performed on the x=0, 0.04, and 0.5 compounds and clearly shows that all modes appearing above the first-order phonon-scattering region in these compounds originate from higher-order oxygen stretching vibrations. In particular this is the case for the strong second-order scattering dominating the scattering response in LaFeO3. Accordingly we propose that these modes are generated by infrared-active longitudinal optical (IR LO) two-phonon and combination scattering activated by Frohlich interaction. For x=0.02 and 0.04 the characteristic IR LO two-phonon and Franck-Condon multiphonon-scattering profiles mix. We also study the influence of isovalent cation substitution and Sr doping in AFe(0.5)Cr(0.5)O(3) (A=La, Nd, and Gd) and La1-ySryFe0.5Cr0.5O3-delta (y=0, 0.16, and 0.5) on the strong electron-phonon coupling present in LaFe0.5Cr0.5O3. The Franck-Condon effect in LaFe0.5Cr0.5O3, is not significantly affected by isovalent A-site substitution, despite the increasing orthorhombic distortion associated with decreasing A-site ionic radii. On the contrary, aliovalent Sr doping causes a rapid decrease in the Franck-Condon scattering. This shows that the strong electron-phonon coupling in these compounds is highly sensitive to local lattice and electronic decoherence but insensitive to global lattice distortions. Finally, a preliminary assignment of the A(g) and B-2g phonon modes in AFe(0.5)Cr(0.5)O(3) (A=La, Nd, and Gd) is made based on the present observations and published results for LaCrO3 and AMnO(3). The modes associated with oxygen octahedral tilt and bending vibrations are heavily influenced by the magnitude of the orthorhombic distortion.

  • 71.
    Andreasson, Jakob
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Cell and Molecular Biology.
    Holmlund, Joakim
    Singer, Stefan G.
    Knee, Christopher S.
    Rauer, Ralf
    Schulz, Benjamin
    Kall, Mikael
    Ruebhausen, Michael
    Eriksson, Sten-G
    Börjesson, Lars
    Lichtenstein, Alexander
    Electron-lattice interactions in the perovskite LaFe0.5Cr0.5O3 characterized by optical spectroscopy and LDA plus U calculations2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 7, p. 075103-Article in journal (Refereed)
    Abstract [en]

    We use resonance Raman scattering (incident photon energies between 1.8 and 4.13 eV), LDA+U calculations, spectroscopic ellipsometry, and oblique IR reflectivity to characterize the strong electron-phonon interactions in the disordered perovskite LaFe0.5Cr0.5O3. When the photon energy coincides with a Cr to Fe Mott-Hubbard transfer gap around 2.4 eV the electron-phonon interaction is manifested by a Franck-Condon effect with exceptional first-and higher order scattering of a local oxygen breathing mode. At higher incident energies we observe a superposition of Franck-Condon scattering and Frohlich interaction induced infrared active longitudinal optical two-phonon scattering activated mainly by O to Fe charge transfer. Our results establish LaFe0.5Cr0.5O3 as a model compound for research on electron-phonon interactions in strongly correlated complex systems and show that Franck-Condon scattering in complex solids is not limited to Jahn-Teller active compounds.

  • 72.
    Aperis, Alex
    et al.
    Department of Physics, National Technical University of Athens, Athens, Greece.
    Kotetes, P.
    Varelogiannis, G.
    Oppeneer, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Small-q phonon-mediated unconventional superconductivity in the iron pnictides2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 9, p. 092505-Article in journal (Refereed)
    Abstract [en]

    We report self-consistent calculations of the gap symmetry for iron-based high-temperature superconductors using realistic small-q phonon-mediated pairing potentials and four-band energy dispersions. When both electron and hole Fermi surface pockets are present, we obtain the nodeless s± state that was first encountered in a spin-fluctuation mechanism picture. Nodal s± as well as other gap structures such as dx2−y2, s±+dx2−y2, and even a p-wave triplet state, are accessible upon doping within our phononic mechanism. Our results resolve the conflict between phase-sensitive experiments reporting a gap changing sign, attributed previously only to a nonphononic mechanism, and isotope effect measurements proving the involvement of phonons in the pairing.

  • 73.
    Aperis, Alex
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Multiband full-bandwidth anisotropic Eliashberg theory of interfacial electron-phonon coupling and high-Tc superconductivity in FeSe/SrTiO32018In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 97, article id 060501(R)Article in journal (Refereed)
    Abstract [en]

    We examine the impact of interfacial phonons on the superconducting state of FeSe/SrTiO3 developing a material's specific multiband, full bandwidth, and anisotropic Eliashberg theory for this system. Our self-consistent calculations highlight the importance of the interfacial electron-phonon interaction, which is hidden behind the seemingly weak-coupling constant λm=0.4, in mediating the high Tc, and explain other puzzling experimental observations, such as the s-wave symmetry and replica bands. We discover that the formation of replica bands has a Tc decreasing effect that is nevertheless compensated by deep Fermi-sea Cooper pairing which has a Tc enhancing effect. We predict a strong-coupling dip-hump signature in the tunneling spectra due to the interfacial coupling.

  • 74.
    Aradi, B.
    et al.
    Budapest University of Technology and Economy.
    Gali, A.
    Research Group of the Hungarian Academy of Sciences.
    Deák, P.
    Budapest University of Technology and Economy.
    Lowther, J. E.
    University of the Witwatersrand.
    Son, N. T.
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials . Linköping University, The Institute of Technology.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials . Linköping University, The Institute of Technology.
    Ab initio density-functional supercell calculations of hydrogen defects in cubic SiC2001In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 63, no 245202Article in journal (Refereed)
    Abstract [en]

    Based on ab initio density-functional calculations in supercells of 3C-SiC, the stable configurations of hydrogen and dihydrogen defects have been established. The calculated formation energies are used to give semiquantitative estimates for the concentration of hydrogen in SiC after chemical vapor deposition, low temperature H-plasma anneal, or heat treatment in high temperature hydrogen gas. Vibration frequencies, spin distributions, and occupation levels were also calculated in order to facilitate spectroscopic identification of these defects. (V+nH) complexes are suggested as the origin of some of the signals assigned earlier to pure vacancies. Qualitative extrapolation of our results to hexagonal polytypes explains observed electrical passivation effects of hydrogen.

  • 75. Arapan, S. C.
    et al.
    Korepov, S. V.
    Liberman, M. A.
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Conductance of a disordered double quantum wire in a magnetic field: Boundary roughness scattering2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, no 11Article in journal (Refereed)
  • 76.
    Arapan, Sergiu
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    High-pressure phase transformations in carbonates2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 18, p. 184115-Article in journal (Refereed)
    Abstract [en]

    High-pressure phase transitions sequences in CaCO3, SrCO3, and BaCO3 are studied by first-principle electronic structure calculations. Each of the carbonates undergoes the aragonite to postaragonite phase transition with pressure in agreement with the experimental observation of Ono et al. However, the postaragonite to post-postaragonite phase transition, predicted by Oganov et al. and later observed in CaCO3, is unlikely to occur in SrCO3 and BaCO3. Hence, the concept that isostructural compounds will exhibit the same type of pressure-induced phase transitions has limitations. A change of the hybridization of the carbon atom from sp(2) to sp(3) within the Pmcn phase occurs in each of compounds, thus the carbonates are likely to transform at very high pressure to structures with tetrahedral CO4-4 carbonate group.

  • 77. Arapan, Sergiu
    et al.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    High-pressure phase transformations in carbonates2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 18, p. 184115-Article in journal (Refereed)
    Abstract [en]

    High-pressure phase transitions sequences in CaCO3, SrCO3, and BaCO3 are studied by first-principle electronic structure calculations. Each of the carbonates undergoes the aragonite to postaragonite phase transition with pressure in agreement with the experimental observation of Ono et al. However, the postaragonite to post-postaragonite phase transition, predicted by Oganov et al. and later observed in CaCO3, is unlikely to occur in SrCO3 and BaCO3. Hence, the concept that isostructural compounds will exhibit the same type of pressure-induced phase transitions has limitations. A change of the hybridization of the carbon atom from sp(2) to sp(3) within the Pmcn phase occurs in each of compounds, thus the carbonates are likely to transform at very high pressure to structures with tetrahedral CO4-4 carbonate group.

  • 78. Arapan, Sergiu
    et al.
    Simak, Sergei I.
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling. Uppsala University, Sweden.
    Volume-dependent electron localization in ceria2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 12, article id 125108Article in journal (Refereed)
    Abstract [en]

    We have performed a numerical study of the process of electron localization in reduced ceria. Our results show that different localized charge distributions can be attained in a bulk system by varying the lattice parameter. We demonstrate that the effect of electron localization is mainly determined by lattice relaxation and an accurate account for the effects of electronic correlation is necessary to achieve localized charge distribution.

  • 79. Arapan, Sergiu
    et al.
    Simak, Sergei I.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Volume-dependent electron localization in ceria2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 12, article id 125108Article in journal (Refereed)
    Abstract [en]

    We have performed a numerical study of the process of electron localization in reduced ceria. Our results show that different localized charge distributions can be attained in a bulk system by varying the lattice parameter. We demonstrate that the effect of electron localization is mainly determined by lattice relaxation and an accurate account for the effects of electronic correlation is necessary to achieve localized charge distribution.

  • 80.
    Arapan, Sergiu
    et al.
    National Institute Mat Science, Japan; Moldavian Academic Science, Moldova.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Skorodumova, Natalia V.
    KTH Royal Institute Technology, Sweden; Uppsala University, Sweden.
    Volume-dependent electron localization in ceria2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 12, p. 125108-Article in journal (Refereed)
    Abstract [en]

    We have performed a numerical study of the process of electron localization in reduced ceria. Our results show that different localized charge distributions can be attained in a bulk system by varying the lattice parameter. We demonstrate that the effect of electron localization is mainly determined by lattice relaxation and an accurate account for the effects of electronic correlation is necessary to achieve localized charge distribution.

  • 81. Araujo, C. M.
    et al.
    Blomqvist, A.
    Scheicher, R. H.
    Chen, P.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Superionicity in the hydrogen storage material Li2NH: Molecular dynamics simulations2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 17Article in journal (Refereed)
    Abstract [en]

    We have employed ab initio molecular dynamics simulations in an attempt to study a temperature-induced order-disorder structural phase transformation that occurs in Li2NH at about 385 K. A structural phase transition was observed by us in the temperature range 300-400 K, in good agreement with experiment. This transition is associated with a melting of the cation sublattice (Li+), giving rise to a superionic phase, which in turn is accompanied by an order-disorder transition of the N-H bond orientation. The results obtained here can contribute to a better understanding of the hydrogen storage reactions involving Li2NH and furthermore broaden its possible technological applications toward batteries and fuel cells.

  • 82.
    Araujo, Moyses
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Blomqvist, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Chen, Ping
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Superionicity in the hydrogen storage material Li2NH: Molecular dynamics simulations2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 17, p. 172101-Article in journal (Refereed)
    Abstract [en]

    We have employed ab initio molecular dynamics simulations in an attempt to study a temperature-induced order-disorder structural phase transformation that occurs in Li2NH at about 385 K. A structural phase transition was observed by us in the temperature range 300-400 K, in good agreement with experiment. This transition is associated with a melting of the cation sublattice (Li+), giving rise to a superionic phase, which in turn is accompanied by an order-disorder transition of the N-H bond orientation. The results obtained here can contribute to a better understanding of the hydrogen storage reactions involving Li2NH and furthermore broaden its possible technological applications toward batteries and fuel cells.

  • 83. Araújo, C. Moysés
    et al.
    Ahuja, Rajeev
    Talyzin, Alexandr V
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Pressure-induced structural phase transition in NaBH42005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 5, p. 054125-Article in journal (Refereed)
    Abstract [en]

    We present a combined experimental and theoretical study of the technologically important NaBH4 compound under high pressure. Using Raman spectroscopy at room temperature, we have found that NaBH4 undergoes a structural phase transformation starting at 10.0 GPa with the pure high-pressure phase being established above 15.0 GPa. In order to compare the Raman data recorded under high pressure with the low-temperature tetragonal phase of NaBH4, we have also performed a cooling experiment. The known order-disorder transition from the fcc to the tetragonal structure was then observed. However, the new high pressure phase does not correspond to this low-temperature structure. Using first-principle calculations based on the density functional theory, we show that the high-pressure phase corresponds to the alpha-LiAlH4–type structure. We have found a good agreement between the measured and calculated transition pressures. Additionally, we present the electronic structure of both the fcc and the high-pressure phases.

  • 84.
    Arhammar, C
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Araujo, Moyses
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Energetics of Al doping and intrinsic defects in monoclinic and cubic zirconia: First-principles calculations2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 11, p. 115208-Article in journal (Refereed)
    Abstract [en]

    First-principles theory within the supercell approach has been employed to investigate Al doping and intrinsic defects in monoclinic and cubic zirconia. The effect of oxygen chemical potential and Fermi level on the formation energy and on the defect concentration have been taken into account. The formation of oxygen vacancies is found to be energetically more favorable in the cubic than in the monoclinic phase under the same oxygen chemical potential and Fermi energy. In both phases, substitutional Al decays from neutral charge state into the charge state -1, with the transition energy just above to the top of the valence band. Our findings indicate that by confining the Fermi energy to the region between the middle of the band gap and the bottom of the conduction band, high Al solubility could be achieved, although formation of Al is likely followed by the formation of interstitial oxygen. Furthermore, the concentration of Al with charge state -1 along with the equilibrium Fermi energy have been calculated in a self-consistent procedure. Here, the possible compensating defects with the relevant charge states have been considered. The obtained concentrations of Al and oxygen vacancies follow the experimental trend but underestimates experimental data. When the formation of defect clusters, composed by two substitutional Al and one oxygen vacancy, are considered, good quantitative agreement with experimental values of both Al and oxygen vacancy concentration is achieved. The results suggest that defect clusters will be formed as a result of Al doping in cubic phase of ZrO2, whereas the concentration of defect clusters is negligible in the monoclinic phase, both in accordance with experiment.

  • 85.
    Armiento, Rickard
    et al.
    Massachusetts Institute of Technology, Cambridge, MA, USA.
    Kozinsky, Boris
    Robert Bosch LLC, Cambridge, MA, USA.
    Fornari, Marco
    Central Michigan University, Mount Pleasant, MI, USA .
    Ceder, Gerbrand
    Massachusetts Institute of Technology, Cambridge, MA, USA.
    Screening for high-performance piezoelectrics using high-throughput density functional theory2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 1, p. 014103-Article in journal (Refereed)
    Abstract [en]

    We present a large-scale density functional theory (DFT) investigation of the ABO(3) chemical space in the perovskite crystal structure, with the aim of identifying those that are relevant for forming piezoelectric materials. Screening criteria on the DFT results are used to select 49 compositions, which can be seen as the fundamental building blocks from which to create alloys with potentially good piezoelectric performance. This screening finds all the alloy end points used in three well-known high-performance piezoelectrics. The energy differences between different structural distortions, deformation, coupling between the displacement of the A and B sites, spontaneous polarization, Born effective charges, and stability is analyzed in each composition. We discuss the features that cause the high piezoelectric performance of the well-known piezoelectric lead zirconate titanate (PZT), and investigate to what extent these features occur in other compositions. We demonstrate how our results can be useful in the design of isovalent alloys with high piezoelectric performance.

  • 86.
    Armiento, Rickard
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Kozinsky, Boris
    Research and Technology Center, Robert Bosch LLC, Cambridge, Massachusetts, USA.
    Hautier, Geoffroy
    Université catholique de Louvain, Belgium.
    Fornari, Marco
    Central Michigan University, Mount Pleasant, Michigan, USA.
    Ceder, Gerbrand
    Massachusetts Institute of Technology, Cambridge, USA.
    High-throughput screening of perovskite alloys for piezoelectric performance and thermodynamic stability2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 13, p. 134103-Article in journal (Refereed)
    Abstract [en]

    We screen a large chemical space of perovskite alloys for systems with optimal properties to accommodate a morphotropic phase boundary (MPB) in their composition-temperature phase diagram, a crucial feature for high piezoelectric performance. We start from alloy end points previously identified in a high-throughput computational search. An interpolation scheme is used to estimate the relative energies between different perovskite distortions for alloy compositions with a minimum of computational effort. Suggested alloys are further screened for thermodynamic stability. The screening identifies alloy systems already known to host an MPB and suggests a few others that may be promising candidates for future experiments. Our method of investigation may be extended to other perovskite systems, e.g., (oxy-)nitrides, and provides a useful methodology for any application of high-throughput screening of isovalent alloy systems.

  • 87.
    Armiento, Rickard
    et al.
    University of Bayreuth, Germany .
    Kümmel, Stephan
    University of Bayreuth, Germany .
    Körzdörfer, Thomas
    University of Bayreuth, Germany .
    Electrical response of molecular chains in density functional theory: Ultranonlocal response from a semilocal functional2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 16, p. 165106-Article in journal (Refereed)
    Abstract [en]

    An exchange potential functional is constructed from semi-local quantities and is shown to reproduce hydrogen chain polarizabilities with the same accuracy as exact exchange methods. We discuss the exchange potential features that are essential for accurate polarizability calculations, i.e., derivative discontinuities and the potential step structure. The possibility of a future generalization of the methods into a complete semi-local exchange-correlation functional is discussed.

  • 88.
    Armiento, Rickard
    et al.
    KTH, Superseded Departments, Physics.
    Matsson, A. E.
    Interface Sci. Department MS 1415, Sandia National Laboratories, Albuquerque.
    Subsystem functionals: investigating the exchange energy per particle2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 16, p. 1651171-16511717Article in journal (Refereed)
    Abstract [en]

    A viable way of extending the successful use of density-functional theory into studies of even more complex systems than are addressed today has been suggested by Kohn and Mattsson [W. Kohn and A. E. Mattsson, Phys. Rev. Lett. 81, 3487 (1998); A. E. Mattsson and W. Kohn, J. Chem. Phys. 115, 3441 (2001)], and is further developed in this work. The scheme consists of dividing a system into subsystems and applying different approximations for the unknown (but general) exchange-correlation energy functional to the different subsystems. We discuss a basic requirement on approximative functionals used in this scheme; they must all adhere to a single explicit choice of the exchange-correlation energy per particle. From a numerical study of a model system with a cosine effective potential, the Mathieu gas, and one of its limiting cases, the harmonic oscillator model, we show that the conventional definition of the exchange energy per particle cannot be described by an analytical series expansion in the limit of slowly varying densities. This indicates that the conventional definition is not suitable in the context of subsystem functionals. We suggest alternative definitions and approaches to subsystem functionals for slowly varying densities and discuss the implications of our findings on the future of functional development.

  • 89.
    Armiento, Rickard
    et al.
    KTH, Superseded Departments, Physics.
    Mattsson, A. E.
    Compl. Mat. and Molecular Biology, Sandia National Laboratories, Albuquerque.
    Alternative separation of exchange and correlation in density-functional theory2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 24, p. 2451201-2451205Article in journal (Refereed)
    Abstract [en]

    It has recently been shown that local values of the conventional exchange energy per particle cannot be described by an analytic expansion in the density variation. Yet, it is known that the total exchange-correlation (XC) energy per particle does not show any corresponding nonanalyticity. Indeed, the nonanalyticity is here shown to be an effect of the separation into conventional exchange and correlation. We construct an alternative separation in which the exchange part is made well behaved by screening its long-ranged contributions, and the correlation part is adjusted accordingly. This alternative separation is as valid as the conventional one, and introduces no new approximations to the total XC energy. We demonstrate functional development based on this approach by creating and deploying a local-density-approximation-type XC functional. Hence, this work includes both the theory and the practical calculations needed to provide a starting point for an alternative approach towards improved approximations of the total XC energy.

  • 90.
    Armiento, Rickard
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Mattsson, A. E.
    Computational Materials and Molecular Biology MS 1110, Sandia National Laboratories, Albuquerque.
    Functional designed to include surface effects in self-consistent density functional theory2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 8, p. 085108-Article in journal (Refereed)
    Abstract [en]

    We design a density-functional-theory (DFT) exchange-correlation functional that enables an accurate treatment of systems with electronic surfaces. Surface-specific approximations for both exchange and correlation energies are developed. A subsystem functional approach is then used: an interpolation index combines the surface functional with a functional for interior regions. When the local density approximation is used in the interior, the result is a straightforward functional for use in self-consistent DFT. The functional is validated for two metals (Al, Pt) and one semiconductor (Si) by calculations of (i) established bulk properties (lattice constants and bulk moduli) and (ii) a property where surface effects exist (the vacancy formation energy). Good and coherent results indicate that this functional may serve well as a universal first choice for solid-state systems and that yet improved functionals can be constructed by this approach.

  • 91.
    Armiento, Rickard
    et al.
    Royal Institute of Technology, AlbaNova University Center, Stockholm, Sweden .
    Mattsson, Ann E.
    Sandia National Laboratories, Albuquerque, New Mexico, USAS.
    Alternative separation of exchange and correlation in density-functional theory2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 24, p. 245120-Article in journal (Refereed)
    Abstract [en]

    It has recently been shown that local values of the conventional exchange energy per particle cannot be described by an analytic expansion in the density variation. Yet, it is known that the total exchange-correlation (XC) energy per particle does not show any corresponding nonanalyticity. Indeed, the nonanalyticity is here shown to be an effect of the separation into conventional exchange and correlation. We construct an alternative separation in which the exchange part is made well behaved by screening its long-ranged contributions, and the correlation part is adjusted accordingly. This alternative separation is as valid as the conventional one, and introduces no new approximations to the total XC energy. We demonstrate functional development based on this approach by creating and deploying a local-density-approximation-type XC functional. Hence, this work includes both the theory and the practical calculations needed to provide a starting point for an alternative approach towards improved approximations of the total XC energy.

  • 92.
    Armiento, Rickard
    et al.
    Royal Institute of Technology, AlbaNova University Center, Stockholm, Sweden.
    Mattsson, Ann E.
    Sandia National Laboratories, Albuquerque, NM, USA.
    Functional designed to include surface effects in self-consistent density functional theory2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 8, p. 085108-Article in journal (Refereed)
    Abstract [en]

    We design a density-functional-theory (DFT) exchange-correlation functional that enables an accurate treatment of systems with electronic surfaces. Surface-specific approximations for both exchange and correlation energies are developed. A subsystem functional approach is then used: an interpolation index combines the surface functional with a functional for interior regions. When the local density approximation is used in the interior, the result is a straightforward functional for use in self-consistent DFT. The functional is validated for two metals (Al, Pt) and one semiconductor (Si) by calculations of (i) established bulk properties (lattice constants and bulk moduli) and (ii) a property where surface effects exist (the vacancy formation energy). Good and coherent results indicate that this functional may serve well as a universal first choice for solid-state systems and that yet improved functionals can be constructed by this approach.

  • 93.
    Armiento, Rickard
    et al.
    Royal Institute of Technology, Stockholm Center for Physics, Astronomy and Biotechnology, Stockholm, Sweden.
    Mattsson, Ann E.
    Sandia National Laboratories, Albuquerque, New Mexico , USA.
    Subsystem functionals in density-functional theory: Investigating the exchange energy per particle2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 16, p. 085108-Article in journal (Refereed)
    Abstract [en]

    A viable way of extending the successful use of density-functional theory into studies of even more complex systems than are addressed today has been suggested by Kohn and Mattsson [W. Kohn and A. E. Mattsson, Phys. Rev. Lett. 81, 3487 (1998); A. E. Mattsson and W. Kohn, J. Chem. Phys. 115, 3441 (2001)], and is further developed in this work. The scheme consists of dividing a system into subsystems and applying different approximations for the unknown (but general) exchange-correlation energy functional to the different subsystems. We discuss a basic requirement on approximative functionals used in this scheme; they must all adhere to a single explicit choice of the exchange-correlation energy per particle. From a numerical study of a model system with a cosine effective potential, the Mathieu gas, and one of its limiting cases, the harmonic oscillator model, we show that the conventional definition of the exchange energy per particle cannot be described by an analytical series expansion in the limit of slowly varying densities. This indicates that the conventional definition is not suitable in the context of subsystem functionals. We suggest alternative definitions and approaches to subsystem functionals for slowly varying densities and discuss the implications of our findings on the future of functional development.

  • 94.
    Arnalds, Unnar B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Papaioannou, Evangelos Th.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hase, Thomas P. A.
    Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom.
    Raanaei, Hossein
    Department of Physics, Persian Gulf University, Bushehr 75168, Iran.
    Andersson, Gabriella
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Charlton, Timothy R.
    ISIS, Harwell Science and Innovation Campus, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Oxon OX11 0QX, United Kingdom.
    Langridge, Sean
    ISIS, Harwell Science and Innovation Campus, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Oxon OX11 0QX, United Kingdom.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Magnetic structure and diffracted magneto-optics of patterned amorphous multilayers2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 14, p. 144434-1-144434-8, article id 144434Article in journal (Refereed)
    Abstract [en]

    We present magneto-optical Kerr effect measurements of patterned arrays of Co68Fe24Zr8 / Al2O3 amorphous multilayers. The multilayers were patterned in two dimensions into two different arrangements of circular and ellipsoidal islands. Magnetization loops were recorded in a longitudinal geometry using both the specularly reflected beam as well as diffracted beams scattered off the patterned films. The magnetization of the patterned structures is significantly different from the magnetization of a continuous multilayer owing to the lateral confinement of the pattern and the introduction of additional dipolar coupling between the layers at the edges of the islands. By investigating the magnetic response at the different diffraction orders from the two different configurations of islands we are able to observe the magnetization at different length scales and determine the magnetic response of the circular and ellipsoidal islands individually.

  • 95.
    Arnalds, Unnar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hase, Thomas
    University of Warwick.
    Papaioannou, Evangelos
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Raanaei, Hossein
    Persian Gulf University.
    Abrudan, Radu
    Ruhr-Universitat Bochum.
    Charlton, Timothy
    ISIS, Rutherford Appleton Laboratory.
    Langridge, Sean
    ISIS, Rutherford Appleton Laboratory.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    X-ray resonant magnetic scattering from patterned multilayers2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 6, p. 064426-Article in journal (Refereed)
    Abstract [en]

    We report on x-ray resonant magnetic scattering from laterally patterned arrays of amorphous Co68Fe24Zr8/Al2O3 multilayers. The arrays are composed of circular and ellipsoidal elements which display distinct individual magnetic responses enabling the investigation of the dependence of the observed magnetization on the scattering condition. We focus our attention to special points in reciprocal space, relating to the lateral and perpendicular structure of the samples, thereby revealing the magnetic structure of the multilayered arrays. This allows a comparison of the observed magnetization under different scattering conditions to magneto-optical measurements. The scattering data are supported by micromagnetic simulations which further enhance our understanding of the intricate charge and magnetic scattering from three dimensional patterns.

  • 96.
    Arnardottir, K. B.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kyriienko, O.
    Shelykh, I. A.
    Hall effect for indirect excitons in an inhomogeneous magnetic field2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 24, p. 245311-Article in journal (Refereed)
    Abstract [en]

    We study the effect of an inhomogeneous out-of-plane magnetic field on the behavior of two-dimensional (2D) spatially indirect excitons. Due to the difference of the magnetic field acting on electrons and holes, the total Lorentz force affecting the center of mass motion of an indirect exciton appears. Consequently, an indirect exciton acquires an effective charge proportional to the gradient of the magnetic field. The appearance of the Lorentz force causes the Hall effect for neutral bosons, which can be detected by measurement of the spatially inhomogeneous blueshift of the photoluminescence using a counterflow experiment.

  • 97. Arnaudov, B
    et al.
    Paskova, Tanja
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Evtimova, S
    Valcheva, E
    Heuken, M
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Multilayer model for Hall effect data analysis of semiconductor structures with step-changed conductivity2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, no 4Article in journal (Refereed)
    Abstract [en]

    We present a multilayer model for analysis of Hall effect data of semiconductor structures composed of sublayers with different thicknesses and contacts placed on the top surface. Based on the circuit theory we analyze the contributions of the conductivity of every sublayer and derive general expressions for the conductivity and carrier mobility of a multilayer planar sample. The circuit analysis is performed taking into account the fact that the sample sublayers are partially connected in parallel to each other by series resistances formed in areas lying below the contacts from each upper layer. In order to solve the inverse problem of determining the electrical parameters of one of the sublayers, a procedure for analysis of the Hall effect data is proposed. The model is simplified for a structure composed of two layers with the same type of conductivity, and is used to determine the electrical parameters of GaN films grown on relatively thick GaN buffers.

  • 98.
    Arnaudov, B
    et al.
    Faculty of Physics, Sofia University, Sofia, Bulgaria.
    Paskova, Tanja
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Goldys, EM
    Semiconductor Science and Technology Laboratories, Macquarie University, Sydney, Australia.
    Evtimova, S
    Faculty of Physics, Sofia University, Sofia, Bulgaria.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Modeling of the free-electron recombination band in emission spectra of highly conducting n-GaN2001In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 64, no 4Article in journal (Refereed)
    Abstract [en]

    We simulate the spectral distribution of the free-electron recombination band in optical emission spectra of GaN with a free-carrier concentration in the range of 5 x 10(17)-1 x 10(20) cm(-3). The influence of several factors, such as nonparabolicity, electron-electron interaction. and electron-impurity interaction on both the spectral and energy position and the effective gap narrowing are taken into account. The calculated properties of the free-electron-related emission bands are used to interpret the experimental photoluminescence and cathodoluminescence spectra of GaN epitaxial layers.

  • 99. Arnaudov, B
    et al.
    Paskova, Tanja
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Paskov, Plamen
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Magnusson, Björn
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Valcheva, E
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Lu, H
    Schaff, WJ
    Amano, H
    Akasaki, I
    Energy position of near-band-edge emission spectra of InN epitaxial layers with different doping levels2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, no 11Article in journal (Refereed)
    Abstract [en]

    We studied the shape and energy position of near-band-edge photoluminescence spectra of InN epitaxial layers with different doping levels. We found that the experimental spectra of InN layers with moderate doping level can be nicely interpreted in the frames of the "free-to-bound" recombination model in degenerate semiconductors. For carrier concentrations above n>5x10(18) cm(-3) the emission spectra can also be modeled satisfactorily, but a contribution due to a pushing up of nonequilibrium holes over the thermal delocalization level in the valence band tails should be considered in the model. The emission spectra of samples with low doping level were instead explained as a recombination from the bottom of the conduction band to a shallow acceptor assuming the same value of the acceptor binding energy estimated from the spectra of highly doped samples. Analyzing the shape and energy position of the free-electron recombination spectra we determined the carrier concentrations responsible for the emissions and found that the fundamental band gap energy of InN is E-g=692+/-2 meV for an effective mass at the conduction-band minimum m(n0)=0.042m(0).

  • 100.
    Aryasetiawan, F.
    et al.
    Research Institute for Computational Sciences, AIST, Tsukuba Central 2, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan / CREST, Japan Science and Technology Agency, Japan.
    Karlsson, Krister
    University of Skövde, School of Life Sciences.
    Jepsen, O.
    Max Planck Institut für Festkörperforschung, D-705 06 Stuttgart, Germany.
    Schönberger, U.
    Max Planck Institut für Festkörperforschung, D-705 06 Stuttgart, Germany.
    Calculations of Hubbard U from first-principles2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 12, p. Article number 125106-Article in journal (Refereed)
    Abstract [en]

    The Hubbard U of the 3d transition metal series as well as SrVO3, YTiO3, Ce, and Gd has been estimated using a recently proposed scheme based on the random-phase approximation. The values obtained are generally in good accord with the values often used in model calculations but for some cases the estimated values are somewhat smaller than those used in the literature. We have also calculated the frequency-dependent U for some of the materials. The strong frequency dependence of U in some of the cases considered in this paper suggests that the static value of U may not be the most appropriate one to use in model calculations. We have also made comparison with the constrained local density approximation (LDA) method and found some discrepancies in a number of cases. We emphasize that our scheme and the constrained local density approximation LDA method theoretically ought to give similar results and the discrepancies may be attributed to technical difficulties in performing calculations based on currently implemented constrained LDA schemes.

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