Small-radius hole polarons with intrinsic structure: continuous one-dimensional motion and anomalous mobility

Abstract

The theory of a conventional small-radius polaron, i.e., a self-trapped excess electron or hole in a dielectric or semiconductor, describes a hopping motion with overcoming a barrier whose height is near the polaron binding energy E b (assumed to be great enough to provide a strong localization). This notion does not hold for a polaron with an intrinsic degree of freedom that enable it to move continuously without hops. The role of intrinsic degrees of freedom can be played by the coordinates of two or three lattice atoms among which a hole is distributed; such hole polarons with E b∼1 eV (called also two-site holes) are well-known for rare-gas and alkali-halide crystals. A weak temperature dependence and great absolute values of the mobility of two-site holes in rare-gas crystals, measured by Le Comber et al., drastically contradict to the mentioned traditional notion. It is shown that intrinsic degrees of freedom change the character of the self-trapped hole motion in such a way that no barrier of the scale of E b appears (the actual barrier is two orders of magnitude lower). The mobility of a self-trapped hole, calculated with allowance for its intrinsic structure within a realistic one-dimensional model, qualitatively agrees with experiment.