Estimate of the polaron mass in high- Tc superconductors

Abstract

I study the motion of polarons as a function of temperature in the context of a molecular crystal model in which the discreteness of the lattice is accounted for. The model is based on a non linear Schrödinger equation which can be solved perturbatively if the conditions for the existence of small polarons are assumed. The polaron bandwidth and the site jump hopping probability have been calculated versus temperature and dimensionality. The crossover temperature T d ∗ between band-like and hopping motion is reduced in low-dimensional systems due to the enhanced importance of the off-diagonal scattering processes. An Einstein phonon spectrum leads to wrong estimates of the polaron bandwidths. The first and second neighbors intermolecular force constants which parametrize the pair interactions strongly affect the values of the ground state polaron bandwidth and of the hopping probability. We discuss the relevance of the model to high- T c superconductors in which polaronic features in the transport properties have been pointed out [1]. The estimated effective polaron masses are consistent with T c values of order ≃100 K.