Self trapping from degenerate bands (spin S = 1) and related phenomena

Abstract

A theory of the self-trapping barrier in crystal with degenerate bands (spin S = 1) is developed for holes and Frenkel excitons. It is shown that band degeneracy leads to spontaneous symmetry breaking of the barrier, which acquires a prolate or oblate shape. The dependence of the barrier height on a number of parameters, such as the ratio of the effective masses, the deformation potentials, and others is investigated. The barrier-height scale is determined in all cases by the mass of the heavy hole (exciton). The mechanism of formation of quasimolecular self-trapped holes and excitons is discussed. Similar results were obtained for the strong-coupling polaron, for which lowering of the symmetry leads to the appearance of two rotational degrees of freedom. The results are applied to a number of other problems: fluctuation levels in semiconductors, the Urbach rule, and others. It is shown also that in systems with multicomponent order parameters the critical nuclei may be nonspherical (e.g., cigar- or disk-shaped).