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Sökning: swepub > Johansson Börje > Refereegranskat > Kollar J.

  • Resultat 1-10 av 34
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1.
  • Kádas, Krisztina, et al. (författare)
  • Structural stability of beta-beryllium
  • 2007
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 75:3
  • Tidskriftsartikel (refereegranskat)abstract
    • Using density-functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the stability of the body-centered-cubic phase of Be (beta-Be). The elastic constants and Debye temperature of beta-Be are calculated over a wide volume range and compared to those obtained for the low-temperature hexagonal phase (alpha-Be). A significant difference in the anisotropy of the bcc and hcp structures is found. In line with experiments, we predict that the hcp -> bcc phase transition occurs at 240 GPa at 0 K and 239 GPa at ambient temperature. We find that the cubic shear constant C '=(C-11-C-12)/2 rapidly decreases for volumes above similar to 1.05V(0), where V-0 is the zero temperature equilibrium volume for beta-Be. At 1.17V(0), the stability condition C-'> 0 is violated and the bcc phase becomes mechanically unstable. We demonstrate that at 0 K, the softening of beta-Be near its experimental volume of 1.063V(0) is related to an electronic topological transition due to the increased number of occupied s states near the Fermi level compared to that at V-0. This softening turns out to be important for the stability of the bcc phase before melting. The disclosed electronic topological transition is found to be present in other analogous hexagonal metals as well.
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2.
  • Ropo, M., et al. (författare)
  • First-principles atomistic study of surfaces of Fe-rich Fe-Cr
  • 2011
  • Ingår i: Journal of Physics : Condensed Matter. - 0953-8984. ; 23:26, s. 265004
  • Tidskriftsartikel (refereegranskat)abstract
    • The surface properties of Fe-rich ferromagnetic Fe-Cr alloys are investigated using a first-principles quantum-mechanical method. In dilute alloys, the surfaces are dominated by Fe, whereas the Cr-containing surfaces become favorable when the bulk Cr concentration exceeds the limit of similar to 10 atomic per cent. The abrupt change in the surface behavior is the consequence of complex competing magneto-chemical interactions between the alloying atoms. Considering the quantities of various features: equilibrium surface profiles, chemical potentials, segregation energies, surface energies, magnetic moments, mixing energies and pair interactions, within a wider range of bulk and surface concentrations enables us to build a comprehensive picture of the physics of Fe-Cr surfaces. Using the present achievements many previously controversial results can now be merged into a consistent model of Fe-rich Fe-Cr alloys.
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3.
  • Laukkanen, P., 0, et al. (författare)
  • Anomalous Bismuth-Stabilized (2×1) Reconstructions on GaAs(100) and InP(100) Surfaces
  • 2008
  • Ingår i: Physical Review Letters. ; 100
  • Tidskriftsartikel (refereegranskat)abstract
    • First-principles phase diagrams of bismuth-stabilized GaAs- and InP(100) surfaces demonstrate for the first time the presence of anomalous (2 x 1) reconstructions, which disobey the common electron counting principle. Combining these theoretical results with our scanning-tunneling-microscopy and photoemission measurements, we identify novel (2 x 1) surface structures, which are composed of symmetric Bi-Bi and asymmetric mixed Bi-As and Bi-P dimers, and find that they are stabilized by stress relief and pseudogap formation.
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4.
  • Punkkinen, M. P. J., et al. (författare)
  • Bismuth-stabilized (2x1) and (2x4) reconstructions on GaAs(100) surfaces : Combined first-principles, photoemission, and scanning tunneling microscopy study
  • 2008
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 78:19
  • Tidskriftsartikel (refereegranskat)abstract
    • Bismuth adsorbate-stabilized (2x1) and (2x4) reconstructions of the GaAs(100) surfaces have been studied by first-principles calculations, valence-band and core-level photoelectron spectroscopies, and scanning tunneling microscopy. It is demonstrated that large Bi atom size leads to the formation of the pseudogap at the Fermi energy and to the lower energy of an adsorbate-derived surface band, which contributes to the stabilization of the exceptional Bi/GaAs(100)(2x1) reconstruction. It is proposed that the Bi/GaAs(100)(2x4) reconstructions include asymmetric mixed Bi-As dimers, in addition to the Bi-Bi dimers. Based on the calculations, we solve the atomic origins of the surface core-level shifts (SCLSs) of the Bi 5d photoemission spectra from the Bi/GaAs(100)(2x4) surfaces. This allows for resolving the puzzle related to the identification of two SCLS components often found in the measurements of the Bi 5d and Sb 4d core-level emissions of the Bi/III-V and Sb/III-V(100)(2x4) surfaces. Finally, the reason for the absence of the common (2x4)-beta 2 structure and additional support for the stability of the (2x1) structure on the Bi/III-V(100) surfaces are discussed in terms of Bi atom size and subsurface stress.
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5.
  • Airiskallio, E., et al. (författare)
  • High temperature oxidation of Fe-Al and Fe-Cr-Al alloys : The role of Cr as a chemically active element
  • 2010
  • Ingår i: Corrosion Science. - 0010-938X. ; 52:10, s. 3394-3404
  • Tidskriftsartikel (refereegranskat)abstract
    • Good high-temperature corrosion resistance of Fe-Al alloys in oxidizing environments is due to the alpha-Al2O3 film which is formed on the surface provided temperature is above 900 degrees C and the Al-content of the alloy exceeds the critical value. Ab initio calculations combined with experiments on Fe-13Al, Fe-18Al, Fe-23Al and Fe-10Cr-10Al alloys show that the beneficial effect of Cr on the oxidation resistance is significantly related to bulk effects. The comparison of experimental and calculated results indicates a clear correlation between the Fe-Cr chemical potential difference and the formation of the protective oxide scales. (C) 2010 Elsevier Ltd. All rights reserved.
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6.
  • Airiskallio, E, et al. (författare)
  • Third element effect in the surface zone of Fe-Cr-Al alloys
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 81:3
  • Tidskriftsartikel (refereegranskat)abstract
    • The third element effect to improve the high temperature corrosion resistance of the low-Al Fe-Cr-Al alloys is suggested to involve a mechanism that boosts the recovering of the Al concentration to the required level in the Al-depleted zone beneath the oxide layer. We propose that the key factor in this mechanism is the coexistent Cr depletion that helps to maintain a sufficient Al content in the depleted zone. Several previous experiments related to our study support that conditions for such a mechanism to be functional prevail in real oxidation processes of Fe-Cr-Al alloys.
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7.
  • Kádas, Krisztina, et al. (författare)
  • Surface relaxation and surface stress of 4d transition metals
  • 2006
  • Ingår i: Surface Science. - 0039-6028. ; 600:2, s. 395-402
  • Tidskriftsartikel (refereegranskat)abstract
    • Using the density functional theory formulated within the framework of the exact muffin-tin orbitals method, we present a systematic study of the top layer relaxation and surface stress of 4d transition metals. Our calculations predict layer contractions for most surfaces. We also find that the relaxations of the close packed surfaces decrease with increasing atomic number through the 4d series. We propose that the relaxation is mainly due to the reduction of the number of sp electrons in the surface layer relative to bulk. The surface stress is found to be very sensitive to the relaxation and, therefore, an accurate determination of the layer relaxation is necessary for obtaining reliable values for the surface stress. Comparing the top layer relaxations for the close packed surfaces, we see essential deviations between data derived in different ab initio calculations. At the same time, the overall trend for the present surface stress of 4d metals is in reasonable agreement with recent full-potential data.
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8.
  • Kadas, K., et al. (författare)
  • Temperature-dependent elastic properties of alpha-beryllium from first principles
  • 2007
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 76:23
  • Tidskriftsartikel (refereegranskat)abstract
    • Using density functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the temperature dependence of the structural parameters and the elastic properties of the hexagonal closed-packed phase of Be (alpha-Be). We find that the elastic constants follow a normal behavior with temperature: decrease with increasing temperature with a slightly increasing slope. Up to the melting point, the monocrystalline elastic constants decrease by an average of 16% and the polycrystalline elastic constants by 10%. These trends contradict the large temperature factor observed in high-temperature direct pulse ultrasonic experiments. At the same time, the low-temperature pulse echo measurements confirm the present theoretical findings. Our results call for further accurate experimental studies on the elastic properties of alpha-Be at high temperatures.
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9.
  • Kokko, K., et al. (författare)
  • Surface core-level shift of Pd at the AgcPd1-c(111) surface : Nonlinear subsurface effects
  • 2007
  • Ingår i: Surface Science. - 0039-6028. ; 601:23, s. 5419-5423
  • Tidskriftsartikel (refereegranskat)abstract
    • The surface core-level binding-energy shift (SCLS) of Pd at the AgcPd1-c(111) surface is calculated as a function of bulk concentration of the alloy. The equilibrium volume and the surface concentration profile used in the calculations refer to the 0 K case. The SCLSs are evaluated within the Z + 1 approximation. The results are analysed using the mixing enthalpy of the alloy and the bulk and surface chemical potentials. A relation of the SCLS to the bulk concentration is considered. This relation is shown to be mediated by the surface concentration profile which induces the observed nonlinear behaviour. The results are interpreted using a simple model for the alloy electronic structure.
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10.
  • Kollar, J, et al. (författare)
  • Metal surfaces : Surface, step and kink formation energies
  • 2000
  • Ingår i: Physica status solidi. B, Basic research. - 0370-1972. ; 217:1, s. 405-418
  • Tidskriftsartikel (refereegranskat)abstract
    • We review the surface, step, and kink energies in monoatomic metallic systems. A systematic comparison is given between the theoretical results based on density functional theory and available experimental data. Our calculated values are used to predict the equilibrium shapes of small metal particles, monoatomic surface islands, and the instability of different surface geometries.
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