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Sökning: L773:0163 1829

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1.
  • da Silva, AF, et al. (författare)
  • Impurity resistivity of the double-donor system Si : P,Bi
  • 1999
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 60:23, s. 15824-15828
  • Tidskriftsartikel (refereegranskat)abstract
    • The electrical resistivity of the shallow double-donor system Si:P,Bi, prepared by ion implantation, was investigated in the temperature range from 1.7 to 300 K. Good agreement was obtained between the measured resistivities and resistivities calculated by a generalized Drude approach for the same temperatures and dopant concentrations. The critical impurity concentration for the metal-nonmetal transition for the double-doped Si:P,Bi system was found to lie between the critical concentrations of the two single-doped systems, Si:P and Si:Bi. [S0163-1829(99)11747-8].
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2.
  • Aboelfotoh, M. O., et al. (författare)
  • Schottky-barrier behavior of metals on n- and p-type 6H-SiC
  • 2003
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:7, s. 075312-
  • Tidskriftsartikel (refereegranskat)abstract
    • The Schottky-barrier height of a number of metals (Ti, Ni, Cu, and Au) on n- and p-type Si-terminated 6H-SiC has been measured in the temperature range 150-500 K. It is found that the barrier height to n-type 6H-SiC does not exhibit a temperature dependence, while for p-type 6H-SiC the change in the barrier height with temperature follows very closely the change in the indirect energy gap in 6H-SiC. These results are inconsistent with models of Schottky-barrier formation based on the concept of a charge neutrality level. Furthermore, the present results cannot be reconciled with a defect pinning mechanism, contrary to the conclusions of earlier studies on III-V compound semiconductors. We suggest that chemical bonding at the metal-semiconductor interface plays an important role in determining the Schottky-barrier height.
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3.
  • Abrashev, M. V., et al. (författare)
  • Raman spectroscopy of CaMnO(3) : Mode assignment and relationship between Raman line intensities and structural distortions
  • 2002
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 65:18, s. 1-9
  • Tidskriftsartikel (refereegranskat)abstract
    • Polarized Raman spectra of oriented finely twinned CaMnO(3) thin films and nonpolarized Raman spectra of CaMnO(3) ceramics were studied at room temperature using several excitation laser wavelengths. The selection rules for the polarized Raman spectra, obtained from samples consisting of finely twinned orthorhombic domains, were calculated and the symmetry of all observed Raman lines was determined. The relationship between the intensity of the Raman lines and the distortions in the ABO(3) perovskites with GdFeO(3)-type structure is discussed. These distortions can be described as superposition of four simple basic distortions: two MnO(6) octahedral tilts, Jahn-Teller deformation of Mn(3+) O(6) octahedra, and shift of the A ions from their sites in the ideal perovskite. Twenty of the 24 Raman-allowed modes in the real GdFeO(3)-type structure have counterparts in only one of the four simpler structures, obtained by a single basic distortion. The assignment of the Raman lines of CaMnO(3) to definite atomic vibrations, most of them activated by a single basic distortion, was made in close comparison with the results of lattice dynamical calculations and the Raman spectra of isostructural LaMnO(3) and CaGeO(3).
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4.
  • Abrikosov, IA, et al. (författare)
  • Applicability of the coherent-potential approximation in the theory of random alloys
  • 1998
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMERICAN PHYSICAL SOC. - 0163-1829. ; 57:22, s. 14164-14173
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • The applicability of the coherent-potential approximation (CPA) for the description of electronic properties of completely random alloys is investigated. This is done by calculating the density of states and the total energy for different systems and by c
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5.
  • Abrikosov, IA, et al. (författare)
  • Locally self-consistent Green's function approach to the electronic structure problem
  • 1997
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMER INST PHYSICS. - 0163-1829. ; 56:15, s. 9319-9334
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • The locally self-consistent Green's function (LSGF) method is an order-N method for calculation of the electronic structure of systems with an arbitrary distribution of atoms of different kinds on an underlying crystal lattice. For each atom Dyson's equat
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6.
  • Abrikosov, IA, et al. (författare)
  • Magnetically induced crystal structure and phase stability in Fe1-cCoc
  • 1996
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMER INST PHYSICS. - 0163-1829. ; 54:5, s. 3380-3384
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • We present an ab initio determination of the crystallographic phase stability of Fe-Co alloys as a function of concentration, using the coherent potential approximation. A bcc --> hcp phase transition is found at a concentration of 85 at.% of Co, in good
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7.
  • ABRIKOSOV, IA, et al. (författare)
  • THEORETICAL ASPECTS OF THE FECNI1-C INVAR ALLOY
  • 1995
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMER INST PHYSICS. - 0163-1829. ; 51:2, s. 1058-1063
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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8.
  • Agui, A, et al. (författare)
  • Bulk and interface Al 2p core excitons in GaAs/AlAs/GaAs heterostructures
  • 1999
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMERICAN PHYSICAL SOC. - 0163-1829. ; 59:16, s. 10792-10795
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • Aluminum 2p soft-x-ray emission and absorption spectra (XES and XAS) of a GaAs/AIAs/GaAs heterostructure semiconductor have been measured with high-energy resolution. The Al 2p XES shows a similar feature to that of AlxGa1-xAs alloys. Strong Al 2p core ex
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9.
  • Ahlberg, Martina, et al. (författare)
  • Effect of ferromagnetic proximity on critical behavior
  • 2012
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 85:22, s. 224425-1-224425-6
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the magnetic phase transition in amorphous Fe93Zr7(x angstrom)/Co95Zr5(1 angstrom) multilayers, where x = 25, 50, and 75. The extremely thin CoZr layer induces magnetic order at temperatures well above the inherent ordering temperature of Fe93Zr7. The changes in the critical exponent beta, associated with the temperature dependence of the magnetization, imply a crossover from two-to three-dimensional behavior as the FeZr thickness is reduced, consistent with a substantial magnetic induction in the FeZr layers. In addition we determined the exponents delta and gamma, of the critical isotherm and the susceptibility, respectively, and their values confirm the nonuniversal character of the phase transition. Scaling of the results yields an excellent data collapse and is found to hold in the crossover regime where the universality hypothesis is not applicable. 
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10.
  • Ahuja, R, et al. (författare)
  • Electronic and optical properties of red HgI2
  • 1996
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMER INST PHYSICS. - 0163-1829. ; 54:15, s. 10419-10424
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • Within the local-density approximation we have used the linear muffin-tin orbital method, without geometrical approximations, to calculate the electronic structure of red HgI2. Using the self-consistent potential we have calculated the energy bands and fr
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11.
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12.
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13.
  • Ahuja, R, et al. (författare)
  • Optical properties of graphite from first-principles calculations
  • 1997
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMER INST PHYSICS. - 0163-1829. ; 55:8, s. 4999-5005
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • We present theoretical results for the frequency-dependent dielectric response, both for the electric field parallel and perpendicular to the c axis of graphite. The calculations are performed using a full-potential linear muffin-tin orbital method. Our c
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14.
  • Ahuja, R, et al. (författare)
  • Theoretical and experimental study of the graphite 1s x-ray absorption edges
  • 1996
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMER INST PHYSICS. - 0163-1829. ; 54:20, s. 14396-14404
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • Theoretical calculations in combination with experiments for the pi* and sigma* x-ray absorption edges are reported for graphite. Theory and experiment agree well for the leading pi* and sigma* resonances. By comparing theoretical calculations for a singl
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15.
  • Ahuja, R, et al. (författare)
  • Theoretical confirmation of the high-pressure orthorhombic phase in strontium
  • 1998
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMERICAN PHYSICAL SOC. - 0163-1829. ; 58:13, s. 8152-8154
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • The recent experimental observation of a new phase (Sr III) at high pressure in strontium metal, with a base-centered orthorhombic crystal structure, is investigated theoretically by means of a full-potential linear muffin-tin-orbital method. Our calculat
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16.
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17.
  • Ahuja, R, et al. (författare)
  • Theoretical high-pressure studies of Cs metal - art. no. 014102
  • 2001
  • Ingår i: PHYSICAL REVIEW B. - : AMERICAN PHYSICAL SOC. - 0163-1829. ; 6301:1, s. 4102-
  • Tidskriftsartikel (refereegranskat)abstract
    • We have studied the crystal structures of cesium metal under high compressions by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital (FPLMTO) method
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18.
  • Ahuja, R, et al. (författare)
  • Theoretical high-pressure studies of silicon VI
  • 1999
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMERICAN PHYSICAL SOC. - 0163-1829. ; 60:21, s. 14475-14477
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • We have investigated Si theoretically under high compressions by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital method. Our results confirm the
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19.
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20.
  • Ahuja, R, et al. (författare)
  • Theoretical prediction of a phase transition in gold - art. no. 212101
  • 2001
  • Ingår i: PHYSICAL REVIEW B. - : AMERICAN PHYSICAL SOC. - 0163-1829. ; 6321:21, s. 2101-
  • Tidskriftsartikel (refereegranskat)abstract
    • We performed theoretical investigations on gold under high pressure through first principles self-consistent total-energy calculations within the local-density approximation as well as generalized gradient approximation using full-potential muffin-tin-orb
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21.
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22.
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23.
  • Akahama, Yuichi, et al. (författare)
  • Structural stability and equation of state of simple-hexagonal phosphorus to 280 GPa : Phase transition at 262 GPa
  • 2000
  • Ingår i: Physical Review B (Condensed Matter). - 0163-1829. ; 61:5, s. 3139-3142
  • Tidskriftsartikel (refereegranskat)abstract
    • A structural phase transition and the equation of state of the simple-hexagonal (sh) phase, P-V, of phosphorus were investigated at pressures up to 280 GPa using monochromatic synchrotron x-ray diffraction. The P-V phase, which was stable above 137 GPa, has shown a structural transition to a higher-pressure phase, P-VI, above 262 GPa, while the c/a ratio of the sh phase slightly increased with increasing pressure from 0.948 to 0.956. The structure of the P-VI phase has been proposed to be bcc. The bulk modulus (B0) and the atomic volume at atmospheric pressure (V0) of the sh phase were estimated to be 84.1 GPa and 14.2 Å3, with a fixed value of the pressure derivative of the bulk modulus (B′0).
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24.
  • Akerman, J. J., et al. (författare)
  • Dislocation-mediated creep of highly separated vortices in a-axis-oriented HgBa2CaCu2O6+delta thin films
  • 2001
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 64:2
  • Tidskriftsartikel (refereegranskat)abstract
    • Using ac susceptibility, we determine the critical current density J(c) and the flux creep activation energy U of an a-axis-oriented HgBa2CaCu2O6+delta thin film. The critical current density at helium temperatures is found to be 4.6 x 10(4) A/cm(2), i.e., about two orders of magnitude smaller than for corresponding films with c-axis orientation. The temperature and ac field dependent activation energy is consistent with dislocation-mediated flux creep and well described by U(T,H-ac)=U-o(1-t(4))H-ac(-1/2) with t=T/T-c, T-c=120K, and U-o = 0.77 eV Oe(1/2) for temperatures T>45 K and in the field range studied. The activation energy is of the same order as that found in c-axis-oriented films. Below T = 45 K the activation energy is observed to decrease as thermally assisted quantum creep becomes increasingly important.
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25.
  • Akerman, J. J., et al. (författare)
  • Two-stage crossover from thermal to quantum flux creep of dilute vortex ensembles in various high-T-c superconducting thin films
  • 2001
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 64:9
  • Tidskriftsartikel (refereegranskat)abstract
    • The thermal-to-quantum flux creep crossover at low vortex densities has been studied in YBa2Cu3O7, TlBa2CaCu2O7-delta, and HgBa2CaCu2O6+delta thin films using ac susceptibility. The crossover temperatures T-cr are 10-11, 17, and 30 K, respectively. Both thermal and quantum flux creep is suppressed as the vortex density is decreased. We observe a two-stage nature in the crossover behavior which appears to be a general property of all the three materials studied.
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26.
  • Albers, RC, et al. (författare)
  • Electronic structure and x-ray photoelectron spectroscopy of the alpha, beta, and gamma phases of Np
  • 1996
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMER INST PHYSICS. - 0163-1829. ; 54:20, s. 14405-14412
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • The electronic structure, density of states, and x-ray photoelectron spectroscopy (XPS) intensities of the alpha, beta, and gamma phases of Np have been theoretically determined from self-consistent, fully relativistic, linear muffin-tin-orbital band-stru
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27.
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28.
  • Alekseev, AY, et al. (författare)
  • Comparing conductance quantization in quantum wires and quantum Hall systems
  • 1996
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMER INST PHYSICS. - 0163-1829. ; 54:24, s. 17320-17322
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • We suggest a means to calculate the de conductance of a one-dimensional electron system described by the Luttinger model. Our approach is based on the ideas of Landauer and Buttiker on transport in ballistic channels and on the methods of current algebra.
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29.
  • Alekseev, AY, et al. (författare)
  • Nonuniversal conductance quantization in high-quality quantum wires
  • 1998
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMERICAN PHYSICAL SOC. - 0163-1829. ; 57:12, s. R6834-R6837
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • We present a theoretical analysis of recent experimental results of Yacoby et al. on transport properties of high-quality quantum wires. We suggest an explanation of observed deviations of the conductance from the universal value 2e(2)/h per channel in th
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30.
  • Alling, Björn, 1980-, et al. (författare)
  • Energetics and magnetic impact of 3d-metal doping of the half-metallic ferromagnet NiMnSb
  • 2008
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 77:14, s. 144414-
  • Tidskriftsartikel (refereegranskat)abstract
    • We have performed a theoretical study of the effect of doping the half-Heusler alloy NiMnSb with the magnetic 3d metals Cr, Mn, Fe, Co, and Ni, with respect to both energetics and magnetic properties. Starting from the formation energies, we discuss the possibility of placing the dopant on different crystallographic positions in the alloy. We calculate total and local magnetic moments, effective exchange interactions, and density of states and also outline strategies to tune the magnetic properties of the alloy. Doping of NiMnSb with Cr as well as substituting some Ni with extra Mn have the largest impact on magnetic interactions in the system while preserving its half-metallic property. Therefore, we suggest the possibility that these dopants increase the thermal stability of half-metallicity in NiMnSb, with implications for its possible usage in spintronics applications.
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31.
  • Amft, Martin, et al. (författare)
  • Catalytic activity of small MgO-supported Au clusters towards CO oxidation : A density functional study
  • 2010
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 81:19, s. 195443-
  • Tidskriftsartikel (refereegranskat)abstract
    • In order to explain the experimentally found catalytic characteristics of Au1-4 /MgO (100) we have performed a comprehensive density functional study of these systems and their ability to (co)adsorb CO and O2 molecules. Starting from the carefully determined ground-state structures we have analyzed binding mechanisms, the influence of spin-orbit coupling, and charge redistributions in Au1-4 /MgO+CO (O2). Experimentally Au1,2 /MgO were found to be inactive under a mixed atmosphere. We show that O2 strongly binds to Au1 /MgO that prevents coadsorption. Although a catalytic reaction cycle towards CO oxidation, analogous to the gas phase reaction involving Au 2-, is energetically possible for Au2 /MgO, the cluster will get blocked by a strongly bound CO. On the other hand, the catalytic activity of Au3,4 /MgO could be explained by their ability to coadsorb CO and O2, hence indicating the occurrence of a Langmuir- Hinshelwood-type reaction mechanism for these clusters.
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32.
  • Amft, Martin, et al. (författare)
  • Small gold clusters on graphene, their mobility and clustering : a DFT study
  • 2011
  • Ingår i: Physical Review B Condensed Matter. - : IOP Publishing. - 0163-1829 .- 1095-3795. ; 23:20, s. 205301-
  • Tidskriftsartikel (refereegranskat)abstract
    • Motivated by the experimentally observed high mobility of gold atoms on graphene and their tendency to form nanometer-sized clusters, we present a density functional theory study of the ground state structures of small gold clusters on graphene, their mobility and clustering. Our detailed analysis of the electronic structures identifies the opportunity to form strong gold-gold bonds and the graphene-mediated interaction of the pre-adsorbed fragments as the driving forces behind gold's tendency to aggregate on graphene. While clusters containing up to three gold atoms have one unambiguous ground state structure, both gas phase isomers of a cluster with four gold atoms can be found on graphene. In the gas phase the diamond-shaped Au-4(D) cluster is the ground state structure, whereas the Y-shaped Au-4(Y) becomes the actual ground state when adsorbed on graphene. As we show, both clusters can be produced on graphene by two distinct clustering processes. We also studied in detail the stepwise formation of a gold dimer out of two pre-adsorbed adatoms, as well as the formation of Au-3. All reactions are exothermic and no further activation barriers, apart from the diffusion barriers, were found. The diffusion barriers of all studied clusters range from 4 to 36 meV only, and are substantially exceeded by the adsorption energies of -0.1 to -0.59 eV. This explains the high mobility of Au1-4 on graphene along the C-C bonds.
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33.
  • Andersen, J. N., et al. (författare)
  • Changes in the local surface geometry with conserved adsorbate coverage and long-range order caused by annealing
  • 1992
  • Ingår i: Physical Review B (Condensed Matter). - 0163-1829. ; 46:19, s. 12784-12787
  • Tidskriftsartikel (refereegranskat)abstract
    • The ordered c(2×2) Na on Al(100) and (3 × 3) R30°K on Al(111) structures formed at either 100 K or at room temperature are studied by high-resolution core-level spectroscopy. For both systems equal alkali coverages are found at these two temperatures. The core-level spectra, however, show strong changes with temperature. This behavior leads to the surprising conclusion that annealing at room temperature causes an irreversible change in the local geometry, i.e., of the adsorption site, of the overlayer even though neither the long-range order nor the adsorbate coverage changes.
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34.
  • Andersen, J. N., et al. (författare)
  • Surface core-level shifts of some 4d-metal single-crystal surfaces : Experiments and ab initio calculations
  • 1994
  • Ingår i: Physical Review B (Condensed Matter). - 0163-1829. ; 50:23, s. 17525-17533
  • Tidskriftsartikel (refereegranskat)abstract
    • High resolution measurements are reported of the surface core-level shift of the 3d level for the Rh(111), Rh(110), Pd(111), Pd(110), and Ag(111) single-crystal surfaces. These measurements and earlier ones for the Mo(110), Rh(100), and Pd(100) surfaces are analyzed by ab initio calculations of the surface core-level shift. The calculations are found to reproduce well the trends of the experimental shifts with the 4d metal and with the crystal plane. The comparison between these experimental and theoretical results demonstrates the importance of proper inclusion of final-state effects for accurate calculations of surface core-level shifts. A core hole in a surface atom is found to be better screened than one in a bulk atom for the 4d metals to the left of Pd in the Periodic Table. The use of the Z+1 approximation to describe the core hole is investigated both by explicit use of this approximation and by performing calculations for 1s and 3d core holes, respectively. The Z+1 approximation is found to be well obeyed in the case of Ag whereas for the rest of the 4d transition metals it is less precise, introducing errors of typically 0.1 eV.
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35.
  • ANDERSEN, JN, et al. (författare)
  • SURFACE CORE-LEVEL SHIFTS OF INAS(110)
  • 1990
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 41:6, s. 3844-3846
  • Tidskriftsartikel (refereegranskat)
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36.
  • Andersson, Britt M., et al. (författare)
  • Thermal conductivity of polycrystalline YBa2Cu4O8
  • 1994
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 49:6, s. 4189-4198
  • Tidskriftsartikel (refereegranskat)abstract
    • 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.
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37.
  • ANDERSSON, CBM, et al. (författare)
  • SURFACE ELECTRONIC-STRUCTURE OF INAS(110)
  • 1993
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 47:4, s. 2427-2430
  • Tidskriftsartikel (refereegranskat)abstract
    • The electronic structure of the InAs(I 10) cleavage surface has been studied by angle-resolved photoelectron spectroscopy. The bulk band structure has been calculated utilizing the augmented plane-wave method and then the bulk bands have been projected along the lines GAMMA-XBARBAR and GAMMA-YBARBAR of the surface Brillouin zone (SBZ). Three surface-related structures have been found and their initial state versus k parallel-to dispersion along the line GAMMA-XBARBAR and the line GAMMA-YBARBAR of the SBZ has been determined. The structures are identified as A5, A4, and A3 along GAMMA-XBARBAR and as A5, A4, and C2 along GAMMA-YBARBAR.
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38.
  • Andersson, David A., et al. (författare)
  • Modeling of Ce2, Ce2O3 ,and CeO2-x in the LDA+U formalism
  • 2007
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 75, s. 035109-1-035109-6
  • Tidskriftsartikel (refereegranskat)abstract
    • 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.
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39.
  • Andersson, David A., et al. (författare)
  • Theoretical study of CeO(2) doped with tetravalent ions
  • 2007
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 76, s. 1741191-17411910
  • Tidskriftsartikel (refereegranskat)abstract
    • 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).
  •  
40.
  • Andersson, Gabriella, et al. (författare)
  • Influence of compressive biaxial strain on the hydrogen uptake of ultra-thin, single crystal vanadium layers
  • 1997
  • Ingår i: Physical Review B. - 0163-1829. ; 55:3, s. 1774-1781
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the influence of biaxial compressive strain on the thermodynamic propertied of hydrogen in thin, single-crystal vanadium layers (0.9, 1.6, and 2.0 nm). At H/V<0.1 (atomic ratio), the host-mediated H-H interaction was found to be attractive. For H/V>0.1, the interaction was found to be extremely weak in the 0.9-nm-thick layer and repulsive in the 1.6- and 2.0-nm-thick V layers. The repulsive interaction was found to increase with increasing layer thicknesses. These results are unexpected as the interaction is attractive in both bulk V and in thin vanadium layers under biaxial tensile strain for HV<0.5. Both the strained and unstrained vanadium lattices are found to have roughly the same hydrogen affinity at H/V=0.1 (0.3 eV/H). No ordered bulklike beta phase was observed at any concentration (0
  •  
41.
  • Andersson, G, et al. (författare)
  • Influence of compressive biaxial strain on the hydrogen uptake of ultrathin single-crystal vanadium layers
  • 1997
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMER INST PHYSICS. - 0163-1829. ; 55:3, s. 1774-1781
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • We have investigated the influence of biaxial compressive strain on the thermodynamic properties of hydrogen in thin, single-crystal vanadium layers (0.9, 1.6, and 2.0 nm). At H/V<0.1 (atomic ratio), the host-mediated H-H interaction was found to be attra
  •  
42.
  • Andersson, JO, et al. (författare)
  • Monte Carlo studies of the dynamics of an interacting monodispersive magnetic-particle system
  • 1997
  • Ingår i: PHYSICAL REVIEW B-CONDENSED MATTER. - : AMER INST PHYSICS. - 0163-1829. ; 56:21, s. 13983-13988
  • Tidskriftsartikel (refereegranskat)abstract
    • The influence of the dipole-dipole interaction on the dynamics of monodispersive ensembles of magnetic nanoparticles have been studied by Monte Carlo simulations. An increased interaction strength drives the system from a state with only individual partic
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43.
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44.
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45.
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46.
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47.
  • ANDERSSON, JO, et al. (författare)
  • OVERLAP LENGTH IN A CU-MN SPIN-GLASS PROBED BY AC SUSCEPTIBILITY
  • 1993
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 48, s. 13977-13980
  • Tidskriftsartikel (refereegranskat)abstract
    • An important theoretical concept in the ‘’droplet” model of spin glasses is the existence of an overlap length. Experimentally this concept was verified by dc-magnetization measurements. However, recent ac-susceptibility measurements by Lefloch et al. [Europhys. Lett. 18, 647 (1992)] have been interpreted to contradict this result. In this paper, ac-susceptibility and dc-magnetization measurements on a Cu (2% Mn) spin glass are reported. The results of the ac-susceptibility measurements are consistent with dc measurements and provide evidence for the existence of an overlap length in 3d spin glasses at low temperatures.
  •  
48.
  • Andersson, Magnus, et al. (författare)
  • CA AND PR SUBSTITUTION IN Y-BASED AND SM-BASED 1-2-3 COMPOUNDS
  • 1993
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 48:10, s. 7590-7597
  • Tidskriftsartikel (refereegranskat)abstract
    • Substitution with equal amounts of Ca and Pr has been studied mainly in Y- and Sm-based 1:2:3 samples and with some results also for Nd-based samples. Structural and chemical analysis and measurements of the superconducting T(c) and the upper critical magnetic field were performed. It was found that Ca and Pr can be dissolved in the orthorhombic structure up to about 25 at. % each in both Y- and Sm-based samples while for larger concentrations, only Pr continued to enter into the 1:2:3 structure. The depression of T(c) with Ca-Pr doping was found to be linear in concentration in contrast to the accelerated decrease in samples doped with only Pr. The rate of depression of T(c) increased in the sequence Y-, Sm-, and Nd-based hosts. The results suggest that Ca-Pr doping isolates a characteristic Pr-impurity effect in 1:2:3 samples. The critical magnetic-field slopes of Y- and Sm-based samples were almost independent of Ca-Pr content in the cosolubility region while a strong decrease was observed for larger Pr content, similar to 1:2:3 samples doped with Pr only. These results suggest an almost-temperature-independent magnetic pair-breaking effect by Pr.
  •  
49.
  • Andersson, Magnus, et al. (författare)
  • MAGNETORESISTANCE MEASUREMENTS ON POLYCRYSTALLINE YBA2CU3O7-DELTA
  • 1991
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 44:14, s. 7722-7725
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetoresistance of a polycrystalline sample of YBa2Cu3O7-delta has been measured by an ordinary resistive method at temperatures from about 1 to 15 K above T(c) and in magnetic fields up to 12 T. From our experimental results and the theory of Hikami and Larkin, we obtain values of the coherence lengths xi-ab = 15.6 angstrom and xi-c = 3.6 angstrom and of the phase-breaking time tau-phi = 1.8 x 10(-13) s.
  •  
50.
  • Andersson, Magnus, et al. (författare)
  • RELATION BETWEEN THE CU(1)-O(4) DISTANCE AND T(C) IN Y1-2XCAXTHXBA2CU3O7-DELTA - RAMAN-SCATTERING AND CALCULATION OF THE ELECTRON-PHONON INTERACTION
  • 1992
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 46:10, s. 6501-6504
  • Tidskriftsartikel (refereegranskat)abstract
    • The strong depression of T(c) with Ca-Th substitution in Y1-2xCaxThxBa2Cu3O7-delta has been investigated in a model calculation of the electronic and phononic contributions from the apex oxygen, O(4), on the electron-phonon interaction, lambda. The shift with Ca-Th substitution of the characteristic O(4) vibrational mode was measured by Raman scattering. The electronic part eta of lambda has been calculated at several distances between chain Cu(1) and O(4) in the region of the observed variation with x. Eta was found to depend strongly on this distance. The results show a small increase with x of the O(4) frequency and a strong decrease of eta. Both these factors thus contribute to a decrease of lambda. We therefore find evidence that a decreasing electron-phonon interaction is associated with the depression of T(c) in Ca-Th substituted YBa2Cu3O7-delta.
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