Anharmonicity and Impurity Effects on Infrared Absorption in High Temperature Superconductors

Abstract

Taking into account the effects of anharmonicities and point impurities the quantum dynamics of phonons for high temperature superconductors is developed using Green’s function formalism via an almost complete Hamiltonian (without BCS Hamiltonian) which comprises the effects of (i) unperturbed electrons, (ii) unperturbed phonons, (iii) electron-phonon interactions, (iv) anharmonicities and (v) isotopic impurities. This is utilized to obtain the expressions for infrared absorption coefficient which can be resolved into diagonal and non-diagonal parts. Non-diagonal contribution arises only in impure crystals and vanishes in the case of pure crystal. The investigations are also made to study the dependence of infrared absorption coefficient on various parameters in the superconducting and normal regimes followed by numerical estimates for cuprate superconductor . The temperature dependence of infrared absorption coefficients and automatic emergence of pairons appears as a special feature of the theory.