| 1. |
- Laukkanen, P., et al.
(författare)
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Anomalous bismuth-stabilized (2x1) reconstructions on GaAs(100) and InP(100) surfaces
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 100, s. 086101-
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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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| 2. |
- Punkkinen, M. P. J., et al.
(författare)
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Bismuth-stabilized (2x1) and (2x4) reconstructions on GaAs(100) surfaces : Combined first-principles, photoemission, and scanning tunneling microscopy study
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78, s. 195304-
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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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| 3. |
- Kádas, Krisztina, et al.
(författare)
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Structural stability of beta-beryllium
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:3
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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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| 4. |
- Kádas, Krisztina, et al.
(författare)
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Temperature-dependent elastic properties of alpha-beryllium from first principles
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:23
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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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| 5. |
- Kokko, K., et al.
(författare)
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Surface core-level shift of Pd at the AgcPd1-c(111) surface : Nonlinear subsurface effects
- 2007
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 601:23, s. 5419-5423
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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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| 6. |
- Punkkinen, M. P. J., et al.
(författare)
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Surface core-level shifts of GaAs(100)(2x4) from first principles
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:11
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations show that measured surface core-level shifts (SCLSs) of the GaAs(100)(2x4) surfaces can be described within the initial state effects. The calculated As 3d and Ga 3d SCLSs for the beta 2 and alpha 2 reconstructions of the GaAs(100)(2x4) surfaces are in reasonable agreement with recent measurements. In particular, the results confirm that both the lower and the higher binding energy SCLSs, relative to the bulk emission in the As 3d photoelectron spectra, are intrinsic properties of the GaAs(100)(2x4) surfaces. The most positive and most negative As shifts are attributed to the third layer As atoms, which differs from the previous intuitive suggestions. In general, calculations show that significant SCLSs arise from deep layers, and that there are more than two SCLSs. Our previously measured As 3d spectra are fitted afresh using the calculated SCLSs. The intensity ratios of the SCLSs, obtained from the fits, show that as the heating temperature of the GaAs(100)(2x4) surface is increased gradually, the area of the alpha 2 reconstruction increases on the surface, but the beta 2 phase remains within the whole temperature range, in agreement with previous experimental findings. Our results show that the combination of the experimental and theoretical results is a prerequisite for the accurate analysis of the SCLSs of the complex reconstructed surfaces.
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| 7. |
- Ropo, M., et al.
(författare)
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Theoretical evidence of the compositional threshold behavior of FeCr surfaces
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:22, s. 220401-1-220401-4
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Tidskriftsartikel (refereegranskat)abstract
- Usingfirst-principles quantum-mechanical theory, we demonstrate that the surface chemistry ofFe-Cr alloys follows the peculiar threshold behavior characteristic of ferriticstainless steels. We find that in dilute alloys the surfacesare covered exclusively by Fe, whereas for bulk Cr concentrationabove ~10% the Cr-containing surfaces become favorable. The two distinctlydissimilar surface regimes appear as a consequence of two competingmagnetic effects: the magnetically induced immiscibility in bulk Fe-Cr alloysand the stability of magnetic surfaces.
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| 8. |
- Vitos, Levente, et al.
(författare)
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Exceptional surface stability in late transition metal alloys driven by lattice strain
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:12
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Tidskriftsartikel (refereegranskat)abstract
- Due to the lower compressibility of the close-packed crystallographic planes compared to the less close-packed ones, the open free surfaces of the late transition and noble metals are generally expected to become thermodynamically stable with increasing pressure. Surface segregation in concentrated alloys and heteroepitaxial growth are possible mechanisms to create additional lattice strain around the surface layer and, thus, alter the surface stability at ambient conditions. Here we demonstrate this phenomenon in the case of PdAg random alloys by performing ab initio density functional calculations for the surface energy and stress. Our findings reveal anomalous surface stability, so far experienced only in some magnetic transition metals, and exceptionally large excess surface stress as an indicator for surface reconstruction.
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| 9. |
- Kadas, K., et al.
(författare)
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Surface relaxation and surface stress of 4d transition metals
- 2006
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 600:2, s. 395-402
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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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| 10. |
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