Microwave surface resistance of YBa2Cu3O7 thin films on LaAlO3 substrates

Abstract

The surface resistance Rs of YBa2Cu3O7 (YBCO) thin films (0.6±0.1 μm) deposited onto 2.5-cm diam (100) LaAlO3 substrates has been measured at 22 GHz using both Cu and Nb cavities. The surface resistance falls precipitously at the superconducting transition (Tc =90 K) from a normal state value of approximately 2 Ω to a 77 K value of 13.7±1 mΩ, which is 1.6 times lower than Cu. At 4 K the surface resistance is 1±0.1 mΩ, as measured in a Nb superconducting cavity, which is an order of magnitude lower than Cu. The critical current density at 77 K is 4.5×104 A/cm2. Pole figure analyses show the ratio of c-axis to a-axis-oriented material in the film is 2.4:1. YBCO films deposited onto either LaGaO3 or LaAlO3 substrates with varying c/a ratios yield surface resistance values at 77 K that are crudely correlated with Rs. Therefore, the principal effect of orienting the material is to improve the sharpness of the high-frequency superconducting transition, consistent with the notion that the sharpness is associated with intergranular rather than intragranular properties.