Microwave surface resistance of YBa2Cu3O7 thin films on LaAlO3 substrates

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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.