meV Gap Research Articles

Electronic structure and semiconductor-semimetal transition in InAs-GaSb superlattices

Self-consistent electronic structure calculations in the envelope-function approximation are performed for InAs-GaSb superlattices, with a three-band $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$ formalism and suitable boundary conditions. The subband dispersion for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ not parallel to the growth axis, realistically computed for the first time, obeys a no-crossing rule which opens small (\ensuremath{\lesssim}10 meV) gaps between conduction-band-like and valence-band-like subbands. It is therefore argued that the semimetallic behavior observed for periods $d\ensuremath{\ge}180$ \AA{} is dominated by extrinsic effects.

Theoretical and experimental progress in intermediate valence systems (invited)

Recent experimental measurements such as susceptibility, specific heat, thermopower, and UPS on a wide range of intermetallic intermediate valence systems, e.g. YbCuAl, CeSn3, support a high degree of universality in the properties of such materials which apparently form a unique class of metallic compounds. The ratio χ(0)/γ≂g2μ2B (J+1) (J+1/2)/π2k2B is a good constant for compounds of a given rare earth (though g and J are rare earth dependent). A class of compounds of a given IV rare earth ion shows scaling of properties in terms of an effective temperature, or energy, scale TF, which is strong function of valence. We review the success of theoretical approaches such as perturbation theory, local Fermi liquid theory, the Bethe Ansatz technique, and large-N perturbation theory in understanding these experimental facts. A challenge continues to be the understanding of the ground state, in which lattice periodicity plays a fundamental role, giving rise to ραT2 in some metallic, and a several meV gap in insulating, IV compounds.

Magnetic excitations in HoFe2

Spin wave and crystal field excitations in single crystal HoFe2 have been studied using inelastic neutron scattering. At 10 K [100] is the easy magnetic direction and at this temperature the acoustic spin wave mode exhibits a 0.60 meV gap due to anisotropy. The acoustic mode is degenerate at the zone boundary with a flat optic mode of energy 8.3 meV. The lack of dispersion in this mode is a consequence of the small (<.01 meV) Ho-Ho exchange energy. A third mode is a highly dispersive optic mode which represents excitations of the iron sublattice spins and has a stiffness parameter almost identical to iron metal. A nearest neighbor linear spin wave model has been applied which represents the data of the three lowest modes well. This model predicts the remaining three allowed modes to be at energies above 200 meV. There is no measurable anisotropy in the spin wave modes with varying propagation directions. At room temperature, higher states of the Ho crystal field multiplet become populated and weaker dispersionless excitations between these levels are observed. The observed excitations are broadened beyond instrumental resolution because of contributions from several levels close together in energy. Crystal field calculations including exchange have been performed and are compared to the energies of the observed optic mode transitions.