Nonlinear microwave properties of HTS thin films

This paper presents an overview of the most recent developments in the field of nonlinear microwave properties of high-temperature superconducting (HTS) thin films including both applications and fundamental science. The nonlinear microwave physics and application of HTS thin films have been extensively studied since the discovery of HTS. A number of defects (high-angle grain boundaries, oxygen vacancies) and impurities (Ni and Zn doping) have been identified as the sources of the nonlinearity. On the other hand, the growth defects responsible for poor nonlinear performance are not so well studies, mainly due to their extremely small size (sub microns) and hence the difficulty of being detected with the most of conventional microstructural techniques (AFM, SEM, TEM). Besides, although some of those techniques (TEM, STM) do offer sufficiently high resolution to detect such defects, the challenge is to correlate them directly with the microwave properties of the films.