A study of the microwave power dependence in high temperature superconducting thin films

Abstract

In this thesis we aimed to examine the nonlinear behaviour of High Temperature Superconducting (HTS) thin films subject to increasing microwave fields. Although there are many possible causes of nonlinear microwave loss in HTS thin films, they broadly split between magnetic and thermal mechanisms. We discussed the relevant current literature and also provided two phenomenological models that highlighted some aspects of behaviour observed in experiment. In order to investigate the behaviour of films in large microwave fields we utilised an existing experimental rig for the measurement of microwave loss. This consisted of a parallel plate resonator, with a fundamental TE10 mode at 5GHz operating within a cryostat of a commercially available vibrating sample magnetometer. Measurements could therefore be made using dc fields up to 8T and the rig was modified to provide automated power measurements also. We carried out studies that investigated the effects of micro-structure upon the dc field losses as well as the power dependence. In one study, the microwave losses and pinning properties of a set of films were measured prior to ion irradiation and then compared with the results taken after irradiation by heavy and light ions. We also investigated the microwave properties of a series of films grown with varying degrees of cation disorder. This study highlighted the problems associated with HTS thin film growth and the importance of crystal structure to the high frequency performance of films. In a third study, we attempted to write a macroscopic Josephson Junction using a focused ion beam, and searched for microwave Josephson effects in the parallel plate resonator. Finally, we carefully analyzed the microwave losses of a set of good quality films searching for evidence of particular nonlinear microwave mechanisms. We used complex impedance plane analysis where the r-parameters from the experimental data were compared to those predicted by theory (r(H_r_f,H_d_c) #DELTA#X_s(H_r_f,H_d_c)/#DELTA#R_s(H_r_f,H_d_c)). Moreover, the form of the temperature dependence of the critical rf fields, B*(T), of these films was studied and a preliminary investigation into the thermal properties of the films is reported. (author)Available from British Library Document Supply Centre-DSC:DXN035315 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo