Isotope effect in high temperature superconductors

Abstract

The purpose of this thesis is to investigate the isotope effect in high temperature superconductors, where experimental results show small isotope effect coefficients of 0-0.2 in contrast to the conventional superconducting theory (Bardeen, Cooper, and Schrieffer, BCS ) 1 which predicts the isotope effect coefficient to be 0.5. This leads to suggestions that there are many predominantly pairing interactions which a possible small electron-phonon mediated interaction. An effort to answer the question of the unusual isotope effect coefficients observed in high temperature superconductors might be related to the presence of the pseudogap having anisotropic symmetries at temperatures higher than the critical temperature. In this research I give an explanation for an unusual isotope effect in high temperature superconductors by considering the electron-phonon mediated interaction with an additional Coulomb repulsion interaction for isotropic and anisotropic pairing states in the presence of the pseudogap in the single particle excitation energy spectrum. The results give the exact isotope effect coefficient expression which depends on material parameters and the size of the pseudogap.