BAND-GAP CLOSURE OF H-2-HE MIXTURES UNDER PRESSURE

We present a theoretical study on the behavior of the energy gap of H- 2-He mixtures under pressure. The calculations are performed within a large-unit-cell tight-binding formalism parametrized for solid molecul ar H-2. We investigate how the energy gap is affected at a given densi ty by the He content as well as by the orientational and the positiona l disorder of the hydrogen molecules. We find that the increase of the He content in the mixture systematically increases the energy gap, sh ifting the insulator-to-metal transition to higher pressures. For smal l values of the He content x, we also find that the lattice constant a t the metal-insulator transition scales as (1-x)(1/3). This scaling re mains approximately valid to estimate the energy gap in the insulating regime. This provides a simple rule to obtain the energy gap of H-2-H e mixtures under pressure from the energy gap of pure H-2 under pressu re. (C) 1998 American Institute of Physics. [S0021-9606(98)70434-9].