Calculations of Electronic Properties of Metal Surfaces and Superlattices

Doktorsavhandling, 1993

Investigations of some aspects of electronic properties of metal surfaces and superlattices are presented in this Thesis. The calculations are based on Density-Functional methods. The first topic is resonances at free-electron-like metal surfaces. A model is presented for formation of electronic resonances in the surface density of states of metals, at energies far from projected bulk band gaps. The model employs a jellium potential for the surface, giving the correct image-like asymptotic behaviour far from the surface, where ion-core lattice effects are included perturbatively. The model is applied to the clean Al(111) surface with and without the presence of external uniform fields, in order to compare with inverse photoemission (IPE) and scanning tunneling spectroscopy (STS) data for that surface. Peaks in the conductance curves, recorded in STS experiments, are demonstrated to have a one-to-one correspondence to peaks in the density of states. Specifically, a "crystal-derived" resonance below the metal vacuum level for finite fields is shown to map onto a "crystal-derived" image resonance, which should influence the analysis and interpretation of IPE spectra. The second subject is properties of metal superlattices. Calculations of the electronic structure and total energies of specific superlattices, consisting of Mo and V, are presented with emphasis on hydrogen solubility in, and elastic moduli of, these materials. It is found that hydrogen solubility is affected by the electron transfer across the interfaces and by volume changes of the constituent metals upon coherent growth of the superlattice. In the case of Mo/V, where hydrogen dissolves in bulk V, but not in bulk Mo, these effects compensate each other and do not affect hydrogen solubility in the V layers. The elastic constants of Mo/V systems are calculated in order to examine the effects of superlattice modulation wavelength L, since elastic anomalies are frequently reported for metal superlattices. No dependence of the bulk modulus on L is found for L * 9 ]. It is further demonstrated that continuum elasticity models account surprisingly well for the effective elastic constants of Mo/V. An estimation of the effects of disordered interfaces on the Young modulus, based on continuum models, gives a possible softening up to 30%.