Modelling and experimental investigation of nonlinear properties of YBa₂Cu₃O₇ and NdBa₂Cu₃O₇ superconducting films and resonators for applications in microwave circuits

Abstract

Nonlinear effects arising in High Temperature Superconductors (HTS) under elevated electromagnetic signal power levels can significantly limit HTS superior microwave characteristics. Understanding the extent of those limitations and possibly predicting them by modelling microwave properties of high temperature superconductors can yield a substantial advantage at early stages of microwave device design and manufacturing. Therefore, one of the main objectives of this thesis is to investigate if it is feasible to expand modelling of microwave properties of superconductors including nonlinear effects in more details. In addition, an experimental investigation was conducted in this thesis focusing on the question if there is any HTS material that exhibits smaller nonlinear effects than found in the widely utilised YBa₂Cu₃O₇ (YBCO) superconductor. A novel approach towards the development of an advanced lumped element model of HTS materials that can accurately represent nonlinear effects in a microwave resonator is presented in this thesis. The theoretical model engineered by the author, more comprehensively represents superconducting phenomena than the standard two-fluid model. Simulation results demonstrated close correspondence with the measured characteristics of HTS films under test. The developed model could be suitable for advanced CAD tools applicable for design of HTS electronic circuits, that need to take into account the complex nature of nonlinear effects. In reference to the experimental investigation, microwave properties of YBa₂Cu₃O₇ thin films and improved NdBa₂Cu₃O₇ (NdBCO) thin films on MgO substrates were examined by the author. In addition, a comprehensive microwave investigation of YBCO and NdBCO microstrip resonators manufactured at Tsinghua University (China) utilising thin films, manufactured in the same laboratory (Ceraco) using the same thermal coevaporation technique, was also conducted. The measurements for the thin films and resonators were performed at frequencies of 25GHz and 2.1GHz respectively, as a function of temperature and varying RF input power (from -5dBm to +30dBm). A thorough analysis of the onset of nonlinear effects in YBCO and NdBCO thin films and resonators is presented and comparatively discussed. The performed investigations show that NdBCO thin films and NdBCO resonators demonstrated higher values of the onset of nonlinearities than YBCO thin films and YBCO resonators. Smaller nonlinear effects observed in NdBCO films and NdBCO microstrip resonators could give them an advantage over YBCO films and resonators for applications in microwave circuits.