| 11. |
- 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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| 12. |
- Punkkinen, M.P.J., et al.
(författare)
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Surface properties of 3d transition metals
- 2011
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Ingår i: Philosophical Magazine. - : Informa UK Limited. - 1478-6435 .- 1478-6443. ; 91:27, s. 3627-3640
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Tidskriftsartikel (refereegranskat)abstract
- Using the projector augmented wave method within density functional theory, we present a systematic study of the layer relaxation, surface energy and surface stress of 3d transition metals. Comparing the calculated trends for the surface energy and stress with those obtained for 4d and 5d metals we find that magnetism has a significant effect on the surface properties. Enhanced surface magnetic moments decrease the size of the surface relaxation, lower the surface energy and surface stress, leading to compressive stress in Cr and Mn.
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| 13. |
- Ropo, M., et al.
(författare)
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First-principles atomistic study of surfaces of Fe-rich Fe-Cr
- 2011
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 23:26, s. 265004-
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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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| 14. |
- 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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| 15. |
- 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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| 16. |
- 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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| 17. |
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| 18. |
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| 19. |
- Kwon, S. K., et al.
(författare)
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Surface energy and stress release by layer relaxation
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:23
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Tidskriftsartikel (refereegranskat)abstract
- The surface energy (gamma) and surface stress (tau) for semi-infinite close-packed surfaces of 4d transition metals have been calculated using ab initio total-energy methods. The moderate agreement between the present and former theoretical data for tau indicates the high level of numerical difficulty associated with such calculations. For the most close-packed surfaces, the present unrelaxed tau values follow the typical trend characteristic for the cohesive energy in nonmagnetic transition-metal series, whereas the relaxed tau values group around similar to 1 mJ/m(2), obtained for Y, Zr, and Ag, and similar to 3 mJ/m(2), calculated for Nb, Mo, Tc, Ru, Rh, and Pd. We have found that the average surface energy reduction upon layer relaxation is around 4%. At the same time, a large part of the surface stress is released during the surface relaxation process. To explain the observed behaviors, we have established a simple relationship, which connects the variations of gamma and tau to the layer relaxation. This relation reveals the principal factors determining the difference between the surface energy and stress release rates at 4d transition-metal surfaces.
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| 20. |
- Kwon, S. K., et al.
(författare)
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Surface Energy and Stress Release by Layer Relaxation
- 2005
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Ingår i: Physical Review B. ; 72, s. 235423-
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Tidskriftsartikel (refereegranskat)abstract
- The influence of carbon impurities on the properties of iron phases (bcc, hcp, dhcp, fcc) has been studied using the first-principles projector augmented-wave (PAW) method for a wide pressure range. It is shown that the presence of ~6 at. % of interstitial carbon has a little effect on the calculated structural sequence of the iron phases under high pressure. The bcc → hcp transition both for pure iron and iron containing carbon takes place around 9 GPa. According to the enthalpies comparison, the solubility of carbon into the iron solid is decreased by high pressure. The coexistence of iron carbide (Fe3C) + pure hcp Fe is most stable phase at high pressure compared with other phases. Based on the analysis of the pressure-density dependences for Fe3C and hcp Fe, we suggest that there might be some fraction of iron carbide present in the core.
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