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[en] In order to better understand the mechanisms responsible for the lowering of the work function that is observed when surface layers containing barium and oxygen are adsorbed on refractory metals such as tungsten, we have initiated a series of full-potential linearized augmented plane wave electronic structure calculations on well-defined model systems representing the Ba--O/W interface. We report here our initial results for a model in which c(2 x 2)= overlayers of upright BaO molecules have been positioned on both sides of a five-layer W(001) film. Three independent self-consistent calculations were performed involving two different Ba--O separations and two different distances between planes of Ba and surface W atoms. We find that the work function of the clean five-layer tungsten slab (4.65 eV) is lowered by ∼1.8--2 eV by the adsorption of the Ba--O surface layer in this geometry, and that this result is relatively insensitive to the Ba--O separation in the overlayer. The most important factor determining the value of the work function seems to be the position of the barium plane above the W substrate. We find evidence of significant bonding between the d-like surface states of the tungsten substrate and both the Ba d and the oxygen 2p adsorbate levels. As a result, multiple dipoles are formed at the interface and the competition between these polarized charge distributions leads to a net lowering of the work function

[en] In order to understand better the mechanisms responsible for the lowering of the work function that is observed when surface layers containing barium and oxygen are adsorbed on refractory metals such as tungsten, we have initiated a series of full-potential linearized augmented-plane-wave (FLAPW) electronic-structure calculations on well-defined model systems representing the Ba OE W interface. We report here our initial results for models in which c(2x2) monolayers of barium and oxygen have been positioned on both sides of a five-layer W(001) film, with the adsorbate atoms being placed above the fourfold-hollow sites of the tungsten surface. Two different adsorbate configurations have been investigated: ''tilted,'' where the adsorbed monolayers have been arranged so that the barium and oxygen atoms each cover a different fourfold-hollow site in the c(2x2) unit cell, and ''upright,'' where the overlayers are aligned vertically so that both adsorbates lie above the same site. We have minimized the total energy of the system to determine the optimal adsorbate positions within this set of configurations. We find that the calculated work function of the clean five-layer tungsten slab (4.65 eV) is lowered by approximately 2 eV by the adsorption of the barium and oxygen surface layers in either configuration, but the tilted structure has a significantly lower energy than does the upright. In addition, the position of the oxygen 2s state, which is very sensitive to the adsorbate geometry, strongly favors the tilted model. In both cases we find evidence of significant bonding between the d-like surface states of the tungsten substrate and both the Ba d and the oxygen 2p adsorbate levels. As a result, multiple dipoles are formed at the interface, and the competition between these polarized charge distributions leads to a net lowering of the work function

[en] We have extended the considerable computational advantages of separable, nonlocal pseudopotentials to the calculation of spin-orbit splittings in solids. We write the total ionic pseudopotential as a sum of scalar relativistic and spin-orbit contributions, where each term can be represented by a fully nonlocal potential of the separable Kleinman-Bylander (KB) form. The scalar term, which reduces to the standard KB expression for the pseudopotential in the limit where one can neglect spin-orbit interactions, is used in the local-density approximation to calculate zeroth-order electronic properties in the usual way, and spin-orbit splittings are calculated to first order using perturbation theory. We have tested our procedure by calculating the spin-orbit splittings at high symmetry points of the zinc-blende III-V semiconductors GaAs, InAs, AlSb, GaSb, and InSb. The calculated splittings in all cases are in excellent agreement with those obtained from other first-principles calculations and with experiment. Since our spin-orbit operator is fully nonlocal in both radial and angular coordinates, a considerable reduction in the labor required to calculate matrix elements has been achieved. This makes our approach ideally suited for use with ab initio molecular-dynamics techniques, which currently have become the methods of choice for exploring the electronic and structural properties of solids