Microwave properties of Tl-Ba-Ca-Cu-O thin films

Abstract

A study of the microwave properties of Tl-Ba-Ca-Cu-O (TBCCO) thin films deposited on LaAlO3 has been performed. These films have transition temperatures (Tc,R=o) within 101-105 K, and were predominantly c-axis oriented with critical current densities (Jc) of approximately 1.6 * 105 and 2.8 * 106 A cm-2 at zero field and at 77 and 4.5 K respectively. By using a power transmission method we have determined the complex conductivity ( sigma * = sigma 1 - j sigma 2) and the magnetic penetration depth ( lambda ) at 33.5 GHz. To the best of our knowledge this represents the first report published on sigma * for TBCCO films. It was observed that in the superconducting state sigma 1 deviates from the predictions of the Bardeen-Cooper-Schrieffer (BCS) theory and the phenomenological two-fluid model. Values of lambda of 613 and 555 nm were obtained at 77 and 20 K respectively. The surface resistance (Rs) of these films was determined after patterning them into a conductor-backed coplanar waveguide (CBCPW) resonators, ring resonators, and microstrip linear resonators, by measuring the quality factor (Q) at cryogenic temperatures. At 77 K and 10.8 GHz, the best of our CBCPW resonators has an unloaded quality factor (Qo) seven times that of a similar all-gold resonator. For neighbouring ring and linear resonators fabricated on the same high transition temperature superconductor film, Qo was found to be approximately three times that of a similar gold resonator at 77 K and 8 GHz. Rs values at 77 K of 6.6 and 4.5 m Omega were obtained at 10.8 and 8 GHz respectively.