Self-consistent electronic structure of tantalum (001): Evidence for the primary role of surface states in driving reconstructions on tungsten (001)

The surface linearized augmented-plane-wave method is used to perform self-consistent scalar-relativistic local-density-functional calculations for the Ta(001) surface. Essentially, all of the surface states and resonances which were found on W(001) are also found on Ta(001), except that on Ta(001) most of these surface states are unoccupied; most differences between W and Ta can be described by rigid-band behavior. Unlike tungsten, where the Fermi energy falls in the center of a large surface-state peak in the surface-atom d-band projected density of states, this same peak in tantalum lies well above the Fermi energy. The sensitivity of surface states to the potential is demonstrated by the existence on Ta(001) of a highly localized and occupied -Γ₁ surface state at the center of the Brillouin zone. This state is virtually identical, in all of its properties, to one found on the W(001) surface. On the basis of this calculation for Ta(001) and earlier W(001) calculations, it is concluded that surface states on W do indeed play a role in destabilizing W(001) against the observed reconstructions. The experimental observation that Ta(001) does not reconstruct is explained in this calculation by the fact that the relevant surface states are unoccupied. This is also evident in the absence of d-band "dangling bonds" in the contour plots of the Ta charge density near the surface. The theoretical work function for Ta(001) is 4.3 eV, in good agreement with the experimental value of 4.15 eV. The theoretical value for the surface core-level shift is 0.96 eV to greater binding energy, which is about 0.5 eV larger than the experimental value of van der Veen et al. obtained for Ta(111), but the sign of the shift agrees with experiment.