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  • 1. Ahuja, R.
    et al.
    Eriksson, O.
    Johansson, Börje
    Theoretical high-pressure studies of Cs metal2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6301, no 1Article in journal (Refereed)
  • 2. Ahuja, R.
    et al.
    Eriksson, O.
    Johansson, Börje
    Theoretical study of the high-pressure orthorhombic TlI-type phase in NaBr and NaI2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6309, no 9Article in journal (Refereed)
  • 3. Ahuja, R.
    et al.
    Rekhi, S.
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Theoretical prediction of a phase transition in gold2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6321, no 21Article in journal (Refereed)
  • 4. Akerman, J. J.
    et al.
    Venturini, E. L.
    Siegal, M. P.
    Yun, S. H.
    Karlsson, U. O.
    Rao, K. V.
    KTH, Superseded Departments, Materials Science and Engineering.
    Two-stage crossover from thermal to quantum flux creep of dilute vortex ensembles in various high-T-c superconducting thin films2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 64, no 9Article in journal (Refereed)
    Abstract [en]

    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.

  • 5. Akerman, J. J.
    et al.
    Yun, S. H.
    Karlsson, U. O.
    Rao, K. V.
    KTH, Superseded Departments, Materials Science and Engineering.
    Dislocation-mediated creep of highly separated vortices in a-axis-oriented HgBa2CaCu2O6+delta thin films2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 64, no 2Article in journal (Refereed)
    Abstract [en]

    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.

  • 6. ANDERSEN, JN
    et al.
    KARLSSON, Ulf
    SURFACE CORE-LEVEL SHIFTS OF INAS(110)1990In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 41, no 6, p. 3844-3846Article in journal (Refereed)
  • 7. ANDERSSON, CBM
    et al.
    ANDERSEN, JN
    PERSSON, PES
    KARLSSON, Ulf O
    KTH, School of Information and Communication Technology (ICT).
    SURFACE ELECTRONIC-STRUCTURE OF INAS(110)1993In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 47, no 4, p. 2427-2430Article in journal (Refereed)
    Abstract [en]

    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.

  • 8.
    Andersson, David A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Simak, S. I.
    Skorodumova, N. V.
    Abrikosov, I. A.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Modeling of Ce2, Ce2O3 ,and CeO2-x in the LDA+U formalism2007In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 75, no 3, p. 035109-Article in journal (Refereed)
    Abstract [sv]

    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.

  • 9.
    Andersson, David A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Simak, S. I.
    Skorodumova, N. V.
    Abrikosov, I. A.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Theoretical study of CeO(2) doped with tetravalent ions2007In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 76, no 7, 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).

  • 10.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Börjesson, Lars
    Physics Department, Chalmers University of Technology.
    Jarlborg, T
    Departement de Physique de la Matière Condensée, Université de Genève.
    Phuong, HV
    Physics Department, Chalmers University of Technology.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    RELATION BETWEEN THE CU(1)-O(4) DISTANCE AND T(C) IN Y1-2XCAXTHXBA2CU3O7-DELTA - RAMAN-SCATTERING AND CALCULATION OF THE ELECTRON-PHONON INTERACTION1992In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 46, no 10, p. 6501-6504Article in journal (Refereed)
    Abstract [en]

    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.

  • 11.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    MAGNETORESISTANCE MEASUREMENTS ON POLYCRYSTALLINE YBA2CU3O7-DELTA1991In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 44, no 14, p. 7722-7725Article in journal (Refereed)
    Abstract [en]

    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.

  • 12.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    Wen, TL
    Arrheniuslaboratoriet, Stockholms Universitet.
    Hegedüs, Zsolt
    Arrheniuslaboratoriet, Stockholms Universitet.
    Nygren, Mats
    Arrheniuslaboratoriet, Stockholms Universitet.
    CA AND PR SUBSTITUTION IN Y-BASED AND SM-BASED 1-2-3 COMPOUNDS1993In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 48, no 10, p. 7590-7597Article in journal (Refereed)
    Abstract [en]

    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.

  • 13. Babaev, Egor
    Characteristic length scales and formation of vortices in the Ginzburg-Landau-Higgs model in the presence of a uniform background charge2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6317, no 17Article in journal (Refereed)
    Abstract [en]

    In this Brief Report we consider a nonlocal Ginzburg-Landau-Higgs model in the presence of a neutralizing uniform background charge. We show that such a system possesses vortices that feature a strong radial electric field. We estimate the basic properties of such an object and characteristic length scales in this model.

  • 14. Babaev, Egor
    Thermodynamics of the crossover from weak- to strong-coupling superconductivity2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6318, no 18Article in journal (Refereed)
    Abstract [en]

    In this paper we study an evolution of low-temperature thermodynamical quantities for an electron gas with a delta -function attraction as the system crosses over from weak-coupling (BCS-type) to strong-coupling (Bose-type) superconductivity in three and two dimensions.

  • 15. Basylko, S. A.
    et al.
    Kundrotas, P. J.
    Onischouk, V. A.
    Tornau, E. E.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Coulomb gap in a model with finite charge-transfer energy2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6302, no 2Article in journal (Refereed)
    Abstract [en]

    The Coulomb gap in a donor-acceptor model with finite charge-transfer energy Delta describing the electronic system on the dielectric side of the metal-insulator transition is investigated by means of computer simulations on two- and three-dimensional finite samples with a random distribution of equal amounts of donor and acceptor sites. Rigorous relations reflecting the symmetry of the model presented with respect to the exchange of donors and accepters are derived. In the immediate neighborhood of the Fermi energy mu. the single-particle density of states g(epsilon) is determined solely by finite size effects, and g(epsilon) further away from mu is described by an asymmetric power law with a nonuniversal exponent, depending on the parameter Delta.

  • 16.
    Belonoshko, Anatoly B.
    et al.
    KTH, Superseded Departments, Physics.
    Gutierrez, G.
    Ahuja, Rajeev
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Molecular dynamics simulation of the structure of yttria Y2O3 phases using pairwise interactions2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6418, no 18Article in journal (Refereed)
    Abstract [en]

    We have studied the structure of yttria (Y2O3) by means of ab initio and molecular dynamics methods. The suggested simple model for the interatomic interaction is shown to produce reasonable results at moderate pressures for a wide range of temperatures. The calculated x-ray structure factor is in good agreement with experimental data obtained by the x-ray levitation technique at the temperature of 2526 K. The quality of the agreement decreases with increasing temperature. We demonstrate that it is not necessary to assume nonstoichiometry of liquid yttria, as was done in a recent publication, to obtain agreement with experiment. The structure of liquid yttria can be considered as a mixture of 4- and 6-coordinated Y atoms. We also show the possibility of a light amorphous yttria phase, which possibly can be obtained by quenching from a vapor instead of conventional amorphous yttria quenched from a liquid.

  • 17.
    Belonoshko, Anatoly B.
    et al.
    KTH, Superseded Departments, Physics.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Dong, Qian
    KTH, Superseded Departments, Materials Science and Engineering.
    Hultquist, Gunnar
    KTH, Superseded Departments, Materials Science and Engineering.
    Leygraf, Christofer
    KTH, Superseded Departments, Materials Science and Engineering.
    First-principles study of hydrogen diffusion in α-Al 2O3 and liquid alumina2004In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 69, no 2, p. 243021-243026Article in journal (Refereed)
    Abstract [en]

    We have studied the energetics and mobility of neutral hydrogen in alumina Al2O3 using ab initio density-functional calculations. The mobility of hydrogen was studied in corundum (α-Al2O 3) as well as in liquid alumina. Using both static as well as molecular-dynamics calculations, and applying classical transition state theory, we derive the temperature-dependent diffusivity of hydrogen in α-Al 2O3 as D(T)=(21.7 × 10-8 m 2/s)exp(-1.24 eV/kT). The corresponding diffusivity of hydrogen in liquid/amorphous alumina, derived directly from ab initio molecular dynamics calculations, is D(T)=(8.71 × 10-7 m2/s)exp(-0.91 eV/kT). The computed diffusivity compares very well to experimental data. We conclude that diffusion of neutral hydrogen through the bulk of alumina is a good approximation of the mechanism for hydrogen mobility in corrosion scales. The representation of grain-boundary structures by amorphous alumina is, probably, realistic at higher temperatures.

  • 18.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Davis, Sergio
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Department of Physics, Uppsala University.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Simak, Sergei
    Burakovsky, Leonid
    Preston, D. L.
    Xenon melting: Density functional theory versus diamond anvil cell experiments2006In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 74, p. 054114-Article in journal (Refereed)
    Abstract [en]

    We performed two-phase ab initio density functional theory based molecular dynamics simulations of Xe melting and demonstrated that, contrary to claims in the recent literature, the pressure dependence of the Xe melting curve is consistent with the corresponding-states theory as well as with the melting curve obtained earlier from classical molecular dynamics with a Xe pair potential. While at low pressure the calculated melting curve is in perfect agreement with reliable experiments, our calculated melting temperatures at higher pressures are inconsistent with those from the most recent diamond anvil cell experiment. We discuss a possible explanation for this inconsistency.

  • 19.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Davis, Sergio
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Skorodumova, Natalia
    Department of Physics, Uppsala University.
    Lundow, Per-Håkan
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Properties of the fcc Lennard-Jones crystal model at the limit of superheating2007In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 76, p. 064121-Article in journal (Refereed)
    Abstract [en]

    The face-centered-cubic (fcc) Lennard-Jones (LJ) model can be considered as a representative model of a simple solid. We investigate the mechanism of melting at the limit of superheating in the fcc LJ solid by means of the procedure recently developed by us [Phys. Rev. B 73, 012201 (2006)]. Insight into the mechanism of melting was gained by studying diffusion and defects in the fcc LJ solid by means of molecular dynamics simulations. We found that the limit of superheating achieved by us is likely to be the highest so far. We also found that the size of the cluster which ignites the melting is very small (down to five to six atoms, depending on the size of the supercell) and closely correlates with the linear size of a supercell when the number of atoms varies between 500 and 13 500.

  • 20.
    Bergkvist, Sara
    et al.
    KTH, Superseded Departments, Physics.
    Henelius, Patrik
    KTH, Superseded Departments, Physics.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Ground state of the random-bond spin-1 Heisenberg chain2002In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 66, p. 134407-1-134407-9Article in journal (Refereed)
    Abstract [en]

    Stochastic series expansion quantum Monte Carlo is used to study the ground state of the antiferromagnetic spin-1 Heisenberg chain with bond disorder. Typical spin- and string-correlation functions behave in accordance with real-space renormalization group predictions for the random-singlet phase. The average string-correlation function decays algebraically with an exponent of -0.378(6), in very good agreement with the prediction of -(3-root5)/2similar or equal to-0.382, while the average spin-correlation function is found to decay with an exponent of about -1, quite different from the expected value of -2. By implementing the concept of directed loops for the spin-1 chain we show that autocorrelation times can be reduced by up to two orders of magnitude.

  • 21.
    Bergqvist, Lars
    et al.
    Institut für Festkörperforschung, Forschungszentrum Jülich.
    Belhadji, Brahim
    Picozzi, Silvia
    Dederichs, Peter H.
    Volume dependence of the Curie temperatures in diluted magnetic semiconductors2008In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 77, no 1, p. 014418-Article in journal (Refereed)
    Abstract [en]

    Using electronic structure methods and statistical methods, we have studied theoretically the volume dependence of the exchange interactions and Curie temperatures in the diluted magnetic semiconductors. In both Mn-doped GaAs and Mn-doped InAs, the calculated Curie temperatures from numerical exact Monte Carlo simulations are more or less constant for a large volume interval. We have compared the exchange mechanisms in Mn-doped GaAs using both the local density approximation (LDA) and the LDA+U method. It is demonstrated that the magnetic properties are understood within Zener’s p-d exchange model for the LDA+U, while in LDA they reflect a mixture between double and p-d exchange mechanisms.

  • 22.
    Bergqvist, Lars
    et al.
    Uppsala University.
    Eriksson, Olle
    Kudrnosky, Josef
    Drchal, V.
    Bergman, Anders
    Nordström, Lars
    Turek, Ilja
    Magnetic properties and disorder effects in diluted magnetic semiconductors2005In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 72, no 19, p. 195210-Article in journal (Refereed)
    Abstract [en]

    We present calculations of the exchange interactions and critical temperatures for several diluted magnetic semiconductor systems. It is shown that the exchange interactions are dominated by short-ranged interactions that have a strong directional dependence. Using a combination of first-principles calculations of the exchange interactions together with Monte Carlo simulations of the classical Heisenberg model, in which the positional disorder and spin fluctuations are properly included, the calculated critical temperatures are in good agreement with experimantal observations. It is shown that agreement between theory and experiment, as regards ordering temperatures, is obtained only when the magnetic atoms are randomly positioned in a simulation cell which proves that disorder effects play a very important role. The effect of strong electron-electron interaction has been studied by means of the LSDA+U scheme. We investigate in detail the nature of the anisotropic exchange interactions by means of a Fermi surface analysis.

  • 23. BERNHOFF, H
    et al.
    SODERHOLM, S
    KARLSSON, Ulf O
    KTH, School of Information and Communication Technology (ICT).
    FLODSTROM, SA
    QVARFORD, M
    ANDERSEN, JN
    NYHOLM, R
    LINDAU, I
    SYNCHROTRON-RADIATION SOFT-X-RAY PHOTOEMISSION-STUDY OF LEAD ON BI2CASR2CU2O81992In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 46, no 10, p. 6488-6494Article in journal (Refereed)
    Abstract [en]

    In this paper we present a study of the interaction of Pb with a clean single-crystal Bi2CaSr2Cu2O8 superconductor surface based on photoemission and low-energy electron diffraction (LEED). Deposition of Pb on a Bi2CaSr2Cu2O8 Crystal kept at room temperature gives rise to the formation of metallic Bi and oxidized Pb at the interface. This behavior could not be observed when the crystal was kept at 100 K during Pb deposition. For all investigated Pb overlayers on a cold crystal (100 K), surface-sensitive photoelectron spectroscopy revealed the growth of a covering metallic Pb overlayer film. The growth at 100 K, contrary to the growth at room temperature, preserved the original LEED 5 X 1 pattern even for Pb depositions corresponding to a 24-angstrom thick overlayer indicating epitaxial growth. Furthermore, a rigid 0.4-eV shift of the valence band and the Bi 5d core levels is observed upon initial Pb deposition and is tentatively attributed to electron doping.

  • 24. Bjornangen, T.
    et al.
    Axnas, J.
    Eltsev, Y.
    Rydh, A.
    Rapp, Östen.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    In-plane anisotropy and possible chain contribution to magnetoconductivity in YBa2Cu3O7-delta2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6322, no 22Article in journal (Refereed)
    Abstract [en]

    The magnetoresistivity of untwinned YBa2Cu3O7-delta Single crystals was measured in magnetic fields up to 12 T-c for B parallel toc and B parallel to ab. at temperatures up to T-c + 84 K and T-c + 50 K, respectively. Two issues were addressed; determination of the in-plane coherence length anisotropy gamma=xi (b)/xi (a) and discussion of the role of the CuO chains in the magnetoconductivity Delta sigma. A fluctuation conductivity theory that allows for an in-plane coherence length anisotropy (xi (a)not equal xi (b)) as well as an extension of the usual Aronov-Hikami-Larkin theory that does not, were used in analyses. The results suggest that the anisotropy gamma is close to 1. The analyses also suggest a contribution to the magnetoconductivity from the CuO chains of a sign which depends on the direction between held and current.

  • 25. Bouzerar, G.
    et al.
    Kudrnovsky, Josef
    Bergqvist, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Bruno, P.
    Ferromagnetism in diluted magnetic semiconductors: A comparison between ab initio mean-field, RPA, and Monte Carlo treatments2003In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 68, p. 081203-Article in journal (Refereed)
    Abstract [en]

    We have determined from first principles the Curie temperature in the diluted magnetic semiconductor (Ga1-x-yMnxAsy) As alloys, where the compensation effect by As antisites is taken into account. The disorder due to random positions of Mn impurities and As antisites is taken into account in the framework of the coherent potential approximation. We demonstrate that a simple mean-field approximation (MFA) already gets Tc accurately. In particular, it is shown that the calculated Tc in the random-phase approximation and Monte Carlo simulations differ only weakly from the MFA values. The reason is that a proper treatment of the disorder beyond the virtual-crystal approximation leads, in this doped material, to a flat magnon dispersion in a large part of the Brillouin zone.

  • 26. Carniato, S.
    et al.
    Luo, Yi
    KTH, Superseded Departments, Biotechnology.
    Ågren, Hans
    KTH, Superseded Departments, Biotechnology.
    Theoretical study of the near-edge Cu L x-ray absorption spectrum of copper phthalocyanine2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6308, no 8Article in journal (Refereed)
    Abstract [en]

    Copper tetraazaporphyrin is used as a model for copper phthalocyanine in theoretical simulations of the features observed at the Cu L-2,L-3 near-edge x-ray absorption spectra. Theoretical results obtained by a density-functional theory approach provided interpretations of the different features observed experimentally, including the so-called white line and the satellite structures, leading to the conclusion that the solid phase arrangement exerts no significant influence on the shape of the Cu L-2,L-3 copper molecular spectrum. The calculations show that the unoccupied 3d levels are essentially located in the plane (3d(x2y2)) of the molecule with a small contribution that has an out-of-plane 3d(z2-y2) and 3d(xz,yz) character. The angular dependence of the copper unoccupied electronic levels could be used to confirm the orientation of the copper tetraazaporphyrin on the substrates.

  • 27. CHIAIA, G
    et al.
    QVARFORD, M
    LINDAU, I
    SODERHOLM, S
    KARLSSON, Ulf O
    KTH, School of Information and Communication Technology (ICT).
    FLODSTROM, SA
    LEONYUK, L
    NILSSON, A
    MARTENSSON, N
    CLUSTER-CONFIGURATION-INTERACTION ANALYSIS OF CU 2P AND VALENCE-BAND PHOTOEMISSION MEASUREMENTS ON BI2SR2CACU2O8 AND BI2SR2CUO6 SUPERCONDUCTORS1995In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 51, no 2, p. 1213-1222Article in journal (Refereed)
  • 28.
    Claesson, Thomas
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Månsson, Martin
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Önsten, Anneli
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Shi, Ming
    Swiss Light Source, Paul Scherrer Institut, Switzerland.
    Sassa, Yasmine
    Laboratory for Neutron Scattering, Zürich, Paul Scherrer Institut.
    Pailhés, Stephane
    Laboratory for Neutron Scattering, Zürich, Paul Scherrer Institut.
    Chang, Johan
    Laboratory for Neutron Scattering, Zürich, Paul Scherrer Institut.
    Bendounan, Azzedin
    Synchrotron SOLEIL, L'Orme des Merisiers, France.
    Patthey, Luc
    Swiss Light Source, Paul Scherrer Institut, Switzerland.
    Mesot, Joël
    Paul Scherrer Institute, ETH Zürich, Switzerland.
    Muro, Takayuki
    Japan Synchrotron Radiation Research Institute.
    Matsushita, Tomohiro
    Japan Synchrotron Radiation Research Institute.
    Kinoshita, Toyohiko
    Japan Synchrotron Radiation Research Institute.
    Nakamura, Tetsuya
    Japan Synchrotron Radiation Research Institute.
    Momono, Naoki
    Department of Physics, Hokkaido University, Japan.
    Oda, Migaku
    Department of Physics, Hokkaido University, Japan.
    Ido, Masayuki
    Department of Physics, Hokkaido University, Japan.
    Tjernberg, Oscar
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    The electronic structure of La(1.48)Nd(0.4)Sr(0.12)CuO(4) probed by high- and low-energy angle-resolved photoelectron spectroscopy: Evolution with probing depth2009In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 80, no 9, p. 094503-1-094503-6Article in journal (Refereed)
    Abstract [en]

    We present angle-resolved photoelectron spectroscopy data probing the electronic structure of the Nd-substituted high-T-c cuprate La1.48Nd0.4Sr0.12CuO4. Data have been acquired at low and high photon energies, h nu=55 and 500 eV, respectively. The two extracted Fermi surfaces show significant differences. The differences can be attributed to either the change in probing depth suggesting dissimilarity of the intrinsic electronic structure between surface and bulk regions, or a considerable c-axis dispersion signaling a strong interlayer coupling. At both photon energies, considerable spectral weight is observed at all points along the Fermi surface and the intensity distribution as well as Fermi-surface shape observed at low as well as high photon energy is markedly different from what has been previously reported for La1.28Nd0.6Sr0.12CuO4 by Zhou [Science 286, 268 (1999)]. Document Type: Article

  • 29. Dalibor, T
    et al.
    Pensl, G
    Nordell, N
    Schoner, A
    Electrical properties of the titanium acceptor in silicon carbide1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 55, no 20, p. 13618-13624Article in journal (Refereed)
  • 30.
    Delczeg-Czirjak, Erna Krisztina
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg, Lorand
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Eriksson, O.
    Division for Materials Theory, Division of Physics and Materials Science, Uppsala University.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab initio study of structural and magnetic properties of Si-doped Fe(2)P2010In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 82, no 8, p. 085103-Article in journal (Refereed)
    Abstract [en]

    Ab initio electronic-structure methods are used to study the properties of Fe(2)P(1-x)Si(x) in ferromagnetic and paramagnetic states. The site preference and lattice relaxation are calculated with the projector augmented wave method as implemented in the Vienna ab initio simulation package. The paramagnetic state is modeled by the disordered local magnetic moment scheme, and the chemical and magnetic disorder is treated using the coherent potential approximation in combination with the exact muffin-tin orbital formalism. The calculated lattice parameters, atomic positions, and magnetic properties are in good agreement with the experimental and other theoretical results. In contrast to the observation, for the ferromagnetic state the body centered ortho-rhombic structure (bco, space group I (mm2) under bar) is predicted to have lower energy than the hexagonal structure (hex, space group P (6) over bar 2m). The zero-point spin fluctuation energy difference is found to be large enough to stabilize the hex phase. For the paramagnetic state, the hex structure is calculated to be the stable phase and the computed total energy versus composition indicates a hex to bco crystallographic phase transition with increasing Si content. The phonon vibrational free energy, estimated from the theoretical equation of state, turns out to stabilize the hexagonal phase, whereas the electronic and magnetic entropies favor the low symmetry orthorhombic structure.

  • 31.
    Delczeg-Czirjak, Erna Krisztina
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg, Lorand
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ropo, Matti
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kokko, K.
    Department of Physics and Astronomy, University of Turku, Finland.
    Punkkinen, M.P.J
    Department of Physics and Astrononmy, University of Turku, Finland.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ab initio study of the elastic anomalies in Pd-Ag alloys2009In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 79Article in journal (Refereed)
    Abstract [en]

    Ab initio total-energy calculations, based on the exact muffin-tin orbital method, are used to determine the elastic properties of Pd1−xAgx random alloys in the face-centered-cubic crystallographic phase. The compositional disorder is treated within the coherent-potential approximation. The single crystal and polycrystalline elastic constants and the Debye temperature are calculated for the whole range of concentration, 0≤x≤1. It is shown that the variation in the elastic parameters of Pd-Ag alloys with chemical composition strongly deviates from a simple linear or parabolic trend. The complex electronic origin of these anomalies is demonstrated.

     

  • 32.
    Gel'mukhanov, Faris
    et al.
    KTH, Superseded Departments, Biotechnology.
    Privalov, Timofei
    KTH, Superseded Departments, Chemistry.
    Ågren, Hans
    KTH, Superseded Departments, Biotechnology.
    Temperature dependence and Debye-Waller factors for resonant x-ray Raman scattering in solids2000In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 62, no 21, p. 13996-14005Article in journal (Refereed)
    Abstract [en]

    Resonant x-ray Raman scattering is a strongly coherent process. The well defined phase relations between scattering channels through the core excited states localized at different atoms result in a conservation of the electronic crystal momentum. However, as we show here, the zero-point and thermal vibrations dephase these scattering channels and lead to incoherent contributions with different spectral shapes. The relative strength of the coherent and incoherent contributions are found to strongly depend on temperature via a Debye-Waller factor. This results in a characteristic temperature dependence of the spectral profile of the resonant x-ray Raman scattering.

  • 33. GISLASON, HP
    et al.
    YANG, BH
    LINNARSSON, M
    KTH, School of Information and Communication Technology (ICT).
    SHIFTING PHOTOLUMINESCENCE BANDS IN HIGH-RESISTIVITY LI-COMPENSATED GAAS1993In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 47, p. 9418-9424Article in journal (Refereed)
    Abstract [en]

    It is shown that Li diffusion of GaAs can give rise to semi-insulating samples with electrical resistivity as high as 10(7) OMEGAcm in undoped, n-type, and p-type starting materials. The optical properties of the compensated samples are correlated with the depletion of free carriers caused by the Li diffusion. The radiative recombination of the Li-compensated samples is dominated by emissions with excitation-dependent peak positions that shift to lower energies with increasing compensation. The photoluminescence properties are characteristic of fluctuations of the electrostatic potential in strongly doped, compensated crystals.

  • 34. GOTHELID, M
    et al.
    BJORKQVIST, M
    GREHK, TM
    LELAY, G
    KARLSSON, Ulf O
    KTH, School of Information and Communication Technology (ICT).
    METAL-SEMICONDUCTOR FLUCTUATION IN THE SN ADATOMS IN THE SI(111)-SN AND GE(111)-SN (ROOT-3X-ROOT-3)R30-DEGREES RECONSTRUCTIONS1995In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 52, no 20, p. 14352-14355Article in journal (Refereed)
    Abstract [en]

    The two components of the Sn 4d core level in the Si(111)-Sn and Ge(111)-Sn. (root 3 X root 3)R30 degrees structures are proposed to arise from semiconductor-metal fluctuations in the Sn adatom layer. Adsorption of potassium on the Si(111)-Sn (root 3 X root 3)R30 degrees surface suppresses the metallic component and shifts the tin into a purely semiconducting phase with a filled dangling bond state.

  • 35. GOTHELID, M
    et al.
    HAMMAR, M
    BJORKQVIST, M
    KARLSSON, Ulf O
    KTH, School of Information and Communication Technology (ICT).
    FLODSTROM, SA
    WIGREN, C
    LELAY, G
    GEOMETRY OF THE GE(111)-AU(ROOT-3X-ROOT-3)R 30-DEGREES RECONSTRUCTION1994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, no 7, p. 4470-4475Article in journal (Refereed)
    Abstract [en]

    A structure model for the Ge(111)-Au(root 3X root 3)R30 degrees surface reconstruction is proposed based on scanning tunneling microscopy (STM) and photoelectron spectroscopy on the Ge 3d and Au 4f core lines. The basic unit is a Au3Ge molecule binding in one-third of the T-1 substrate sites with a gold trimer pointing out of the surface. This leaves two-thirds of a monolayer of unoccupied T-1 sites which make up a hexagonal honeycomb pattern. Two types of STM images have been obtained which are explained within this model, where either the trimers or the substrate Ge atoms are probed depending on the specific tip conditions in combination with the sample bias voltage. Furthermore, small insets of a metallic (1X1) structure are found at low gold coverage together with a distorted (2X2) surface structure.

  • 36. GOTHELID, M
    et al.
    HAMMAR, M
    KARLSSON, Ulf O
    KTH, School of Information and Communication Technology (ICT).
    WIGREN, C
    LELAY, G
    STRUCTURAL AND ELECTRONIC EVOLUTION ON THE GE(111)-AG SURFACE1995In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 52, no 19, p. 14104-14110Article in journal (Refereed)
    Abstract [en]

    High-resolution core-level photoelectron spectroscopy has been used to study three different silver induced surface reconstructions on the Ge(111) surface. At the lowest coverage a (4 X 4) structure is formed, which displays a similar Ge 3d core-level line shape as the clean c(2 X 8) surface. Details in the spectra are discussed with respect to possible models. The Ge(111)-Ag (root 3 X root 3)R30 degrees structure Ge 3d spectrum is dominated by a very strong contribution assigned to the two topmost Ge layers in a missing top layer structure, similar to the Si(111)-Ag (root 3 X root 3)R30 degrees surface. A weak bulk peak is present on the high-binding-energy side of the spectrum, while a third contribution assigned to Ge in phase boundaries is included in the fit on the lower-binding-energy side. A comparison with results obtained from the Ge(111)-Au root 3 structure points to substantial differences between the two noble-metal-induced root 3 reconstructions on the Ge(111) surface. Finally, after further deposition of silver at room temperature, the root 3 geometry is locally broken creating a (6 X 6) structure and a new surface-related peak emerges on the low-binding-energy side of the Ge 3d spectrum, which was interpreted as being due to Ge atoms floating on top of the outermost surface layer. The valence band also revealed the existence of small metallic silver islands.

  • 37. GREHK, TM
    et al.
    JOHANSSON, LSO
    KARLSSON, Ulf O
    KTH, School of Information and Communication Technology (ICT).
    FLODSTROM, AS
    ADSORPTION OF POTASSIUM ON THE SI(111)ROOT-3X-ROOT-3R30-DEGREES - B-SURFACE - OBSERVATION OF AN INSULATING SURFACE AT SUBMONOLAYER COVERAGE1993In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 47, no 20, p. 13887-13890Article in journal (Refereed)
    Abstract [en]

    The Si(111) square-root 3 X square-root 3R 30-degrees:B surface with submonolayer coverages of potassium has been studied with angle-resolved photoelectron spectroscopy. Deposition of potassium leads to the formation of a state 0.7 eV below the Fermi level. The energy position of this state showed only a minor dependence on the actual potassium coverage and the state displayed a small dispersion of 0.1 eV when probed along the GAMMABAR-MBAR' direction in the square-root 3 X square-root 3R 30-degrees-surface Brillouin zone. These results support the idea that a mainly covalent bonding exists between the potassium atoms and the Si surface.

  • 38. Han, XJ.
    et al.
    Bergqvist, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Dederichs, Peter H.
    Muller-Krumbhaar, Heinz
    Christie, Jaime
    Scandolo, Sandro
    Tangney, Paul
    Polarizable interatomic force field for TiO2 parametrized using density functional theory2010In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 81, p. 134108-Article in journal (Refereed)
    Abstract [en]

    We report a classical interatomic force field for TiO2, which has been parametrized using density functional theory forces, energies, and stresses in the rutile crystal structure. The reliability of this classical potential is tested by evaluating the structural properties, equation of state, phonon properties, thermal expansion, and some thermodynamic quantities such as entropy, free energy, and specific heat under constant volume. The good agreement of our results with ab initio calculations and with experimental data, indicates that our force field describes the atomic interactions of TiO2 in the rutile structure very well. The force field can also describe the structures of the brookite and anatase crystals with good accuracy.

  • 39.
    He, Sailing
    et al.
    KTH, School of Information and Communication Technology (ICT), Centres, Zhejiang-KTH Joint Research Center of Photonics, JORCEP. KTH, Superseded Departments, Alfvén Laboratory.
    Ruan, Zhichao
    KTH, School of Information and Communication Technology (ICT), Centres, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
    Chen, Long
    KTH, School of Information and Communication Technology (ICT), Centres, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
    Shen, Jianqi
    KTH, School of Information and Communication Technology (ICT), Centres, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
    Focusing properties of a photonic crystal slab with negative refraction2004In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 70, no 11, p. 115113-Article in journal (Refereed)
    Abstract [en]

    A layered Korringa-Kohn-Rostoker method is exploited to study the subwavelength imaging through a slab of rods-in-air photonic crystal. Both the intensity and phase spectra of transmission are investigated. The high transmission of evanescent waves arises due to the excitation of some slab-guided bound modes and the high coupling between the incident evanescent field and some bulk-guided Bloch modes. Through a study of the phase spectrum of transmission, it is shown that the self-collimation effect occurs at smaller incident angles whereas the negative refraction effect occurs at relatively larger incident angles. The existence of imaging aberrations is also explained with the phase spectrum. The focusing properties of the photonic crystal slab are mainly due to the negative-refraction effect for large incident angles, rather than the self-collimation effect.

  • 40.
    He, Shuang
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Hunan University, China.
    Peng, Ping
    Gorbatov, Oleg I.
    Nosov Magnitogorsk State Technical University.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Materials Center Leoben Forschung GmbH, Austria.
    Effective interactions and atomic ordering in Ni-rich Ni-Re alloys2016In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 94, no 2, article id 024111Article in journal (Refereed)
    Abstract [en]

    Interatomic interactions and ordering in fcc Ni-rich Ni-Re alloys are studied by means of first-principles methods combined with statistical mechanics simulations based on the Ising Hamiltonian. First-principles calculations are employed to obtain effective chemical and strain-induced interactions, as well as ordering energies and enthalpies of formation of random and ordered Ni-Re alloys. Based on the nonmagnetic enthalpies of formation, we speculate that the type of ordering can be different in alloys with Re content less than 10 at.%. We demonstrate that effective chemical interactions in this system are quite sensitive to the alloy composition, atomic volume, and magnetic state. In statistical thermodynamic simulations, we have used renormalized interactions, which correctly reproduce ordering energies obtained in the direct total energy calculations. Monte Carlo simulations for Ni 0.91 Re 0.09   alloy show that there exists a strong ordering tendency of the (112 0)  type leading to precipitation of the D1 a   ordered structure at about 940 K. Our results for the atomic short-range order indicate, however, that the presently applied theory overestimates the strength of the ordering tendency compared to that observed in the experiment.

  • 41. He, Z Q
    et al.
    Khazmi, Y O
    Kanski, J
    Ilver, L
    Nilsson, P O
    Karlsson, Ulf O
    KTH, School of Information and Communication Technology (ICT).
    As overlayer on GaAs(110) studied with photoemission1995In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 52, no 23, p. 16602-16607Article in journal (Refereed)
    Abstract [en]

    As-terminated GaAs(110) surfaces were prepared on ex situ cleaved substrates by molecular-beam epitaxy. The surface stoichiometry was controlled by postgrowth As deposition. Photoemission from a surface covered with a monolayer As was investigated in detail using synchrotron radiation. Two different surface components were found in core-level spectra, which are interpreted as due to adatoms bonding to the surface anions and cations. In the valence-band spectra several surface states were identified, in analogy with previous reports on the isoelectronic Sb/GaAs(110) system. The polarization dependence is not the same, however, which leads us to the conclusion that the adlayer bonding mechanisms are different in the two cases.

  • 42. Henelius, Patrik
    et al.
    Sandvik, A. W.
    Sign problem in Monte Carlo simulations of frustrated quantum spin systems2000In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 62, no 2, p. 1102-1113Article in journal (Refereed)
    Abstract [en]

    We discuss the: sign problem arising in Monte Carlo simulations of frustrated quantum spin systems. We show that for a class of semifrustrated systems [Heisenberg models with ferromagnetic couplings J(z)(r) < 0 along the; axis and antiferromagnetic couplings J(yx)(r) = -J(z)(r) in the xy plane, for arbitrary distances, 1. the sign problem present for algorithms operating in the z basis can be solved within a recent operator-loop formulation of the stochastic series expansion method [a cluster algorithm for sampling the diagonal matrix elements of the power series expansion of exp(-beta H) to all orders]. The solution relies on the identification of operator loops which change the configuration sign when updated (merons) and is similar to the meron-cluster algorithm recently proposed by Chandrasekharan and Wiese for solving the sign problem fur a class of fermion models [Phys. Rev. Lett. 83, 3116 (1999]. Some important expectation values, e.g., the internal energy, can be evaluated in the subspace with no merons, where the weight function is positive definite. Calculations of other expectation values require sampling of configurations with only a small number of merons (typically zero or two), with an accompanying sign problem which is not serious. We also discuss problems which arise in applying the meron concept to more general quantum spin models with frustrated interactions.

  • 43. HIMPSEL, FJ
    et al.
    KARLSSON, Ulf O
    KTH, School of Engineering Sciences (SCI).
    MCLEAN, AB
    TERMINELLO, LJ
    DEGROOT, FMF
    ABBATE, M
    FUGGLE, JC
    YARMOFF, JA
    THOLE, BT
    SAWATZKY, GA
    FINE-STRUCTURE OF THE CA 2P X-RAY-ABSORPTION EDGE FOR BULK COMPOUNDS, SURFACES, AND INTERFACES1991In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 43, no 9, p. 6899-6907Article in journal (Refereed)
    Abstract [en]

    The fine structure of the Ca 2p soft-x-ray-absorption edge is studied for a variety of bulk compounds (Ca metal, CaSi2, CaO, and CaF2), for surfaces and interfaces [CaF2(111), BaF2 on CaF2(111), Ca and CaF2 on Si(111)], and for defects (F centers in CaF2). The observed multiplet structure is explained by atomic calculations in a crystal field [cubic O(h) for the bulk and threefold C3-nu for the (111) surfaces and interfaces]. While the bulk spectra are isotropic, the surface and interface spectra exhibit a pronounced polarization dependence, which is borne out by the calculations. This effect can be used to become surface and/or interface selective via polarization-modulation experiments, even for buried interfaces. A change in valence from Ca2+ to Ca1+ causes a downwards energy shift and extra multiplet lines according to the calculation. The energy shift is observed for F centers at the CaF2 surface and for the CaF2/Si(111) interface.

  • 44. Hofmeister, H.
    et al.
    Dutta, J.
    Hofmann, H.
    Atomic structure of amorphous nanosized silicon powders upon thermal treatment1996In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 54, no 4, p. 2856-2862Article in journal (Refereed)
    Abstract [en]

    Amorphous silicon powders prepared by plasma-enhanced chemical vapor deposition, of 8-24-nm-sized particles agglomerated into larger aggregates were annealed in a reducing atmosphere to study the phase transformation behavior of these particles. High-resolution electron microscopy revealed a very rough surface, with structural details of 1 to 2 nm, of the as-prepared single powder particles. Upon l h annealing at temperatures between 300 and 600 °C circular contrast features, 1.5-2.5 nm in size, are observed in the amorphous particles, hinting to the formation of a medium-range order. A distinct onset of crystallization is achieved at 700 °C, with structures ranging from very small crystalline ordered regions of 2.5-3.5 nm in size, to fast-grown multiply twinned crystallites. Rapid progress of crystallization, mainly caused by growth twinning, is observed upon annealing at 800 °C. At 900 °C, almost completely crystalline particles are formed. The particles having lattice characteristics of diamond cubic silicon frequently exhibit a faulted structure, because of multiple twinning events. They are covered by an amorphous oxide shell of a 1.5 to 2 nm thickness, which is found to develop with the onset of crystallization. Size and surface roughness of the as-prepared powders are widely preserved throughout all stages of heating, and practically no sintering occurs up to 900 °C.

  • 45.
    Holm, William
    et al.
    KTH, Superseded Departments, Physics.
    Andersson, Magnus
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    Kulikov, M.A.
    Academy of Sciences of Russia, Moscow.
    Makarenko, I.N.
    Academy of Sciences of Russia, Moscow.
    Anisotropic fluctuation magnetoconductivity in a yba2cu3O7-delta single-crystal above tc1993In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 48, no 6, p. 4227-4230Article in journal (Refereed)
    Abstract [en]

    The in- and out-of-plane magnetoresistance of a single crystal of YBa2Cu3O7-delta has been measured at temperatures above T(c) in magnetic fields up to 12 T. A Montgomery-type contact configuration was used with corrections for the finite size of the contacts taken into account. For temperatures up to 2.5 K above T(c) the data are well described by both the Aronov-Hikami-Larkin theory for a layered superconductor (applicable only for the in-plane magnetoconductivity) and the Ginzburg-Landau theory for a three-dimensional anisotropic superconductor (applicable for both the in-plane and the out-of-plane magnetoconductivity). All three analyses give consistent results for the coherence lengths.

  • 46.
    Hudl, Matthias
    et al.
    Department of Engineering Sciences, Uppsala University.
    Nordblad, Per
    Department of Engineering Science, Uppsala University.
    Björkman, Torbjörn
    Department of Physics and Astronomy, Uppsala University.
    Eriksson, Olle
    Department of Physics and Astronomy, Uppsala University.
    Häggström, Lennart
    Department of Physics and Astronomy, Uppsala University.
    Sahlberg, Martin
    Department of Materials Chemistry, Uppsala Univeristy.
    Andersson, Yvonne
    Department of Materials Chemistry, Uppsala University.
    Delczeg-Czirjak, Erna Krisztina
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Order-disorder induced magnetic structures of FeMnP0.75Si0.252011In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 83, no 13, p. 134420-Article in journal (Refereed)
    Abstract [en]

    We report on the synthesis and structural characterization of the magnetocaloric FeMnP0.75Si0.25 compound. Two types of samples (quenched and slowly cooled) were synthesized and characterized structurally and magnetically. We have found that minor changes in the degree of crystallographic order causes large changes in the magnetic properties. The slow-cooled sample, with a higher degree of order, is antiferromagnetic. The quenched sample has a net moment of 1.26 mu(B) per formula unit and ferrimagnetic behavior. Theoretical calculations give rather large values for the Fe and Mn magnetic moments, both when occupied on the tetrahedral and the pyramidal lattice sites.

  • 47. Hugosson, Håkan Wilhelm
    et al.
    Eriksson, O.
    Jansson, U.
    Johansson, Börje
    Phase stabilities and homogeneity ranges in 4d-transition-metal carbides: A theoretical study2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6313, no 13Article in journal (Refereed)
    Abstract [en]

    First-principles full-potential linear muffin-tin orbital calculations have been used to study the 4d-transition-metal cal bides ZrC, NbC, and MoC. The experimental phase diagrams at T = 0 of the refractory compounds ZrC, NbC, and MoC have been reproduced with great accuracy from first principles theory. The energy of for marion for these compounds has been calculated for several phases and stoichiometries in order to understand the differences in phase stabilities and the changes in homogeneity ranges found between these systems is explained. The results can be regarded as theoretical zero-temperature phase stability diagrams for the three compounds containing not only the experimentally verified but also hypothetical phases and many of the experimental properties and trends are reproduced and explained. A study of the changes and differences in electronic structure and bonding of the studied compounds, phases and stoichiometries is also presented. As a part of this study the hexagonal Me2C (Me being Zr, Nb, or Mo) phases were studied and the theoretical structures, with relaxed interlayer distances and lattice parameters, were obtained. The phase stabilities and electronic structure of the experimentally reported orthorhombic Nb2C and Mo2C phases were also studied.

  • 48. Janin, E.
    et al.
    von Schenck, H.
    Göthelid, Mats
    KTH, Superseded Departments, Materials Science and Engineering.
    Karlsson, U. O.
    Svensson, M.
    Bridge-bonded atomic oxygen on Pt(110)2000In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 61, no 19, p. 13144-13149Article in journal (Refereed)
    Abstract [en]

    We use hybrid density-functional theory (DFT) and scanning tunneling microscopy (STM) to study oxygen adsorption on the Pt(110) surface. In the STM images oxygen appear as bright protrusions on the ridges of the missing row reconstructed surface. DFT calculations of binding energies for atomic oxygen on a bridge site shows a clear preference over hollow sites.

  • 49. Johansson, J.
    et al.
    Haviland, David B.
    KTH, Superseded Departments, Physics.
    Random background charges and Coulomb blockade in one-dimensional tunnel junction arrays2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6301, no 1Article in journal (Refereed)
    Abstract [en]

    We have numerically studied the behavior of one-dimensional tunnel junction arrays when random background charges are included using the orthodox theory of single-electron tunneling. Random background charge distributions are varied in both amplitude and density. The use of a uniform array as a transistor is discussed both with and without random background charges. An analytic expression for the gain near zero gate voltage in a uniform array with no background charges is derived. The gate modulation with background charges present is simulated.

  • 50. JOHANSSON, LI
    et al.
    HAKANSSON, KL
    WINCOTT, PL
    KARLSSON, Ulf O
    KTH, School of Information and Communication Technology (ICT).
    CHRISTENSEN, AN
    SURFACE-SHIFTED CORE LEVELS IN MO3SI (100) AND (110)1991In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 43, no 15, p. 12355-12363Article in journal (Refereed)
    Abstract [en]

    High-resolution photoemission studies of core levels in Mo3Si have been carried out using synchrotron radiation. Surface-shifted Si 2p components were observed on annealed (100) and (110) crystal faces and were unambiguously identified in adsorption experiments of hydrogen and oxygen. The surface core-level shifts were extracted using a curve-fitting procedure. For the (110) surface one shifted component was identified having a surface shift of -1.02(1) eV. For the (100) surface two shifted components were found to be necessary in order to model the experimental spectrum, the stronger component having a shift of -0.68(2) eV and the weaker a shift of -1.03(4) eV. No surface-shifted components could be identified in the Mo 4p photoelectron spectrum. Upon oxygen adsorption a chemically shifted Si 2p component was observed, indicating silicon oxidation, while no chemically shifted component appeared in the Mo 4p spectrum. In the Si 2p spectrum pronounced photoelectron-diffraction effects were observed both as a function of photoelectron kinetic energy and emission angle. These findings are presented and discussed.

123 1 - 50 of 118
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