Microwave surface resistance of high TC superconducting thin films

One of the serious material problems with high T superconductors is their granularity which greatly affects their microwave transmission (as well as absorption) properties. High quality near-epitaxial thin films of these oxide superconductors have been found to be advantageous for microwave devices due to their low conduction loss. Pulsed laser deposition (PLD) technique has been extensively used for the growth of high quality films of oxide superconductors. Work carried out at TIFR during the last several years has convincingly shown that significant improvement in microstructure, and consequently, in microwave transmission properties of YBa2Cu3O7-δ (YBCO) thin films can he realized by incorporating a small quantity (about 5%) of Ag in the YBCO target used for PLD. Microwave surface resistance of YBCO films has been measured using microstrip resonator configuration at various temperatures in the range 15-88K and in the frequency range 1.5 to 12 GHz. Due to the much improved microstructure, Ag- YBCO films have not only shown a slow surface resistance but also a good power handling capability. Au-YBCO films too have shown improved properties with the added advantage of much improved environmental stability. Here we review significant advances in the growth and microwave measurements of Ag-YBCO and Au-YBCO thin films and show why they would be of great advantage for microwave passive devices operating at 77K. (author)