Active HF and microwave noise spectroscopy for characterization of superconducting materials

Most superconducting materials under a suitable DC current bias emanate excess noise which may extend from low frequencies up to the terahertz region. The microwave noise in some granular high-T_c films at 12 GHz can be as high as 10 pW in a 100-MHz bandwidth. The authors studied this noise in Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O thin films grown on different substrates in the temperature range from 4.2 to 100 K in the frequency bands 0-2 GHz and 11-12.5 GHz, using current bias and magnetic field as experimental variables. Analysis of the measurements provides valuable information on the superconductor crystalline microstructure, the parameters of the intrinsic Josephson clusters, the synchronization of the radiation from clusters, and the properties of the intergranular inclusions. In particular, the degree of granularity, the strength of the pinning centers, and the size and the geometry of weak links and of weak-link clusters can be deduced from the noise amplitude and frequency dependence measured as a function of the bias current and externally applied magnetic field. Noise emission tests can be easily performed along with standard superconducting transition measurements, providing instant microstructural information without the need for a sophisticated electron microscope