Kalenyuk O. Nonlinear electromagnetic microwave properties of perfect epitaxial high temperature superconducting YBa2Cu3O7-d films

Thesis is devoted to research of microwave properties of high temperature superconducting epitaxial thin YBa2Cu3O7- (YBCO) films both in linear and nonlinear regimes. Study of temperature, magnetic field and power dependences of the microwave surface impedance of YBCO films was carried out in a frequency range, in a linear regime (with respect to microwave field) at low microwave power and in a nonlinear regime at higher power levels. It is shown that obtained temperature dependences of the surface impedance point on the anisotropy of energy gap in the quasiparticles excitation spectrum, which is typical for d-wave Copper pairing of the mixed type (s+id). Investigation of the films microwave impedance in the presence of applied dc magnetic field demonstrates, that homogeneously distributed Abrikosov vortices give an additional contribution to the surface resistance which linearly increases with magnetic induction value. For the case of non-homogeneous distribution of Abrikosov vortices thehysteretic effect in the surface resistance dependence on applied magnetic field value is observed. This effect can be explained on the base of critical state model for a thin superconducting film in applied dc magnetic field. Study of nonlinear surface impedance dependence on microwave field and/or current amplitude reveals, that in experiments performed on coplanar resonators the non-linearity origin is related to microwave-induced Abrickosov vortices, which enter the film through its edges. This type of nonlinear microwave response can be described in the framework of modified critical state model, which can be applied for microwave magnetic field component with an additional account for nonuniform distribution of microwave currents along the resonator.