Model for self-trapping of excitons with internal degrees of freedom

A new model of the small polaron for studying the self-trapping and the off-center instability of excitons is presented. The model consists of an exciton with two internal states, 1s and 2p, coupled with two lattice distortion modes, the symmetric and the antisymmetric modes. The stable and metastable states of the exciton-lattice system are calculated numerically within the adiabatic approximation. The obtained phase diagram shows the inter-relation between the two types of symmetry breaking, namely the breaking of the translational symmetry (self-trapping) and the breaking of inversion symmetry (off-center instability) in a unified way.