9623720 Sridhar This project seeks to study the response to electromagnetic waves at microwave frequencies, of new superconducting materials, such as high temperature superconductors and borocarbide superconductors. Experiments will be carried out using certain unique facilities to study microwave response under a variety of conditions and materials. This response, when suitably analyzed, will yield important information about the paired state of electrons and the motion of superconducting vortices in these materials. In addition to making fundamental contributions to the understanding of high temperature and borocarbide superconductors, this work will also have important impact on microwave applications. An understanding of the intrinsic properties of the pure systems, the the influence of purities, will provide clues to improving the loss capabilities of microwave devices. The studies of non-linear effects will be crucial to improving the high power performance of microwave devices, currently a serious limitation to the successful application of the high temperature superconducting materials in this arena. %%% The electromagnetic response of high and low temperature superconductors will be investigated using a unique experimental capability, covering a wide range of frequencies (dc to fifty gigahertz), temperature (2 to 350 Kelvin) and magnetic field (0 to 7 Tesla). The program will address key current issues, such as order parameter symmetry, quasiparticle density of states, depairing effects of a magnetic field in the Meissner and mixed states, c-axis transport including coherent/incoherent quasiparticle tunneling and possibility of intrinsic Josephson junctions. A notable feature of the high temperature superconductors is their pronounced non-linear response at moderate microwave powers and field amplitudes. An understanding of this non-linearity in terms of th e spatio-temporal dynamics of flux penetration at high frequencies will be another theme of this project. Experiments will be carried out on the non-linear impedance, mixed acdc and harmonic/intermodulation response, in both the frequency-domain and time-domain, and will be analyzed in the framework of critical state models which can have recently emerged. ***
