Microwave properties of a composite superconducting structure in the mixed state

A theoretical study on microwave properties of a composite layered structure of twin high-T_c superconducting thin-films separated by a nonlinear dielectric is presented. Microwave reflectance and transmittance are calculated within the framework of the self-consistent treatment of vortex dynamics for the superconducting films in the mixed state. The effects of nonlinear field- and temperature-dependent dielectric permittivity of SrTiO₃ are specifically discussed. Two significant peaks arising from nonlinearity of the permittivity are found in the temperature-dependent microwave reflectance and transmittance. As for the field-dependent microwave response, we suggest that the strongly field-dependent nonlinear permittivity of SrTiO₃ at low temperature does not affect the microwave transmittance and reflectance.