[en] In the growth process of ultrathin films of vanadium oxides on Pd(111), a sequence of novel oxide phases with layer-dependent structures and oscillating oxidation states has been detected experimentally and understood theoretically. These phases are interface mediated and metastable with respect to further oxide growth. Transformation into the stable oxide configuration occurs beyond a critical thickness, where energetics combined with kinetic limitations determine the oxide multilayer structure

[en] Full text: The lead chalcogenides PbX (X=S, Se, and Te), in particular their heterostructures in nanocrystals and quantum dots, represent a highly attractive class of materials in semiconductor research. Compared to their III-V and II-VI counterparts, these narrow-gap IV-IV semiconductors exhibit unique structural, electronic and optical properties, which make them potential candidates in long-wavelength sensor devices, diode lasers and thermophotovoltaic energy converters. Concerning their technological application, central issues are (i) the opening of the band gap with decreasing size of the nanocrystal as well as its temperature dependence, (ii) the band off-sets and deformation potential of single bands close to the Fermi-level in their heterostructures, (iii) the bulk effective masses, and (iv) their dielectric constants. For instance, the size dependence of the band gap allows to tune the photoluminescence over a wide range of wavelengths in the infrared region. To address these aspects, we have performed density functional and GW calculations based on the projector augmented wave method as implemented in the VASP code. The exchange correlation potential has been treated with the generalized gradient approximation as well as by using a screened hybrid functional. In this work we present a detailed comparison of the different theoretical approaches and discuss the importance of considering relativistic effects for the Pb valence electrons (spin-orbit coupling) for the elastic (bulk moduli), electronic (band structures, band gaps, effective masses), and optical (dielectric constants) properties of bulk PbX. Further we will show results for the band off-sets obtained by supercell calculations. (author)

[en] CO adsorption on a PtCo(111) surface was studied by scanning tunneling microscopy. Comparison of images with chemical contrast of Pt and Co and images showing the CO molecules indicates that CO resides exclusively on top of Pt sites and never on Co. CO bonding is highly sensitive to the chemical environment. The probability to find CO on a Pt atom increases drastically with the number of its Co nearest neighbors. Ab initio calculations show that this ligand effect is due to different positions of the center of the Pt d band

[en] For semiconductor modeling, a major shortcoming of density functional theory is that the predicted band gaps are usually significantly too small. It is generally argued that this shortcoming is related to the fact that density functional theory is a ground state theory, and as a result, one is not allowed to associate the one-electron energies with the energies of quasi-particles. Although this fundamental objection is certainly correct, the modeling of the positioning of donor and acceptor levels in semiconductors faces serious limitations with present density functionals. Several solutions to this problem have been suggested. A particular attractive and fairly simple one is the inclusion of a small fraction of the non-local exchange in the Hamiltonian (hybrid functionals). This approach leads to sensible band gaps for most semiconductors, but fails for ionic solids. A more reliable approach is via many-electron Green's function techniques, which have made tremendous advances in recent years. Here GW calculations in various flavors are presented for small gap and large gap systems, comprising typical semiconductors (Si, SiC, GaAs, GaN, ZnO, ZnS, CdS and AlP), small gap semiconductors (PbS, PbSe, PbTe), insulators (C, BN, MgO, LiF) and noble gas solids (Ar, Ne). The general finding is that single-shot G₀W₀ calculations based on wavefunctions obtained from conventional density functional theory yield too small band gaps, whereas G₀W₀ calculations following hybrid functional calculations tend to overestimate the band gaps by roughly the same amount. This is at first sight astonishing, since the hybrid functionals yield very good band gaps themselves. The contradiction is resolved by showing that the inclusion of the attractive electron-hole interactions (excitonic effects) is required to obtain good static and dynamic dielectric functions using hybrid functionals. The corrections are usually incorporated in GW calculations using 'vertex corrections', and in fact inclusion of these vertex corrections rectifies the predicted band gaps. Finally, in order to remove the dependence on the initial wavefunctions, self-consistent GW calculations are presented, again including an approximate treatment of vertex corrections. The results are in excellent agreement with experiment, with a few per cent deviation for all materials considered. We conclude that predictive band gap engineering is now possible with the theoretical description approaching experimental accuracy

[en] Full text: The structure of the ∼ 5 AA thick aluminum oxide on Ni₃Al(111) exhibits holes at the corner of the (√67 x √67)R12.2^o unit cell, reaching down to the metal substrate. These holes are large enough to trap atoms of any kind of metal. Therefore, the ultrathin oxide film, forming a nanomesh, should be a perfect template for growing highly regular arranged metal clusters. Several metals have been deposited on the aluminum oxide by thermal deposition and the clusters grown have been studied by scanning tunneling microscopy (STM). In agreement with literature, it has been observed that the unmodified oxide is not a good template for most metals, with the exception of Pd. Pd atoms nucleate at the corner holes and, hence, show a perfect hexagonal arrangement, whereas Fe and Co clusters grow on other local defects without showing the regularity of Pd clusters. By predeposition of a Pd seed layer, however, we can create a metallic nucleation site on each corner hole and Fe as well as Co clusters form a well-ordered hexagonal arrangement on the oxide 'nanomesh'. We have also studied the morphology of the clusters and applied different methods to determine the orientation of the clusters. For low coverages ( 0.5 ML) we have studied the height distribution of Fe as well as Co clusters. We have found height differences corresponding to the interlayer distance of close-packed Co planes for Co clusters and an interlayer distance corresponding to Fe bcc (100) planes for Fe clusters. For high Co coverages (∼ 14 ML) the clusters show coarsening by coalescence of ∼ 20- 100 clusters and there are different types of clusters, where only few of them show flat close-packed facets on top. Smoke measurements have been performed for Fe as well as Co clusters below the coverage limit where coarsening occurs. These clusters show superparamagnetic behavior at temperatures above the Curie temperature (∼ 100 K) of the Ni₃Al substrate. (author)

[en] We show that quantum-mechanical molecular-dynamics simulations in a finite-temperature local-density approximation based on the calculation of the electronic ground state and of the Hellmann-Feynman forces after each time step are feasible for liquid noble and transition metals. This is possible with the use of Vanderbilt-type ''ultrasoft'' pseudopotentials and efficient conjugate-gradient techniques for the determination of the electronic ground state. Results for liquid copper and vanadium are presented

[en] Full text: Due to their relevance for catalytic reactions the investigation of Rh surfaces has been a major research field in surface science in the last years. Although most particles show no well-defined orientation under catalytic conditions, most studies have been restricted to low-indexed surface orientations. Stepped surfaces offer the means to investigate the catalytic properties approaching realistic conditions. Therefore the oxidation of a stepped Rh(553) surface has been studied using ab-initio local-density-functional calculations. The calculations were performed using the Vienna ab-initio simulation package (VASP) which is based on a plane wave basis set and PAW potentials. In the initial step the stable phases for the adsorption of O/Rh(553) at various oxygen coverages have been determined, thus allowing to establish a phase diagram. In the low-coverage limit the oxygen adsorption is dominated by the influence of the step, while for higher coverages a (10x1) reconstruction and finally the formation of a surface oxide are predicted. Yet, experimentally a reconstruction of stepped Rh surfaces has been observed for several surface orientations. Therefore the stability of the oxygen-covered Rh(553) surface versus faceting and step-bunching has been investigated in the second part of the study. In agreement with experimental data a driving force for the reconstruction of the surface towards Rh(331) and Rh(111) facets has been found for certain values of the chemical potential. A detailed explanation of the mechanisms of this driving force will be presented. (author)

[en] Density-functional studies of structural and electronic properties of transition-metal sulfides formed by 3d transition metals, based on the local spin-density approximation and including non-local corrections to the exchange-correlation functional (generalized gradient approximation), have demonstrated the importance of magneto-volume effects and magneto-structural effects, but could not achieve full agreement with experiment. A further improvement is to consider electronic correlation effects due to tightly bound and localized d-states on the transition metal atoms. With the DFT + U method used in this work, these correlation effects are taken in account and yield improved predictions for volume, magnetic moment, exchange splitting and bandgap. For MnS the semiconducting gap is correctly predicted, and for MnS₂ the high-spin AFM type-III state can be stabilized over the low-spin state. For FeS even weak correlation effects lead to better predictions for the semiconducting gap, volume and magnetic moment

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