A study on stress affecting the microwave surface resistance of YBCO high temperature superconducting thin films

We calculate stress in YBa2Cu3O7−x/MgO (YBCO/MgO) and YBa2Cu3O7−x/LaAlO3 (YBCO/LAO) by XRD of the sample, σ1 = −1.2 GPa and σ2 = −1.4 GPa, respectively, which shows that the stress in YBCO/LAO is stronger than that in YBCO/MgO. In addition, microwave response of the two pieces of thin films is also investigated by microstrip resonator technique. Surface resistance and penetration depth of the films are obtained by analyzing S21 resonant curves of the microstrip resonator, the penetration depth λ0 = 280 nm for YBCO/MgO and λ0 = 265 nm for YBCO/LAO, and surface resistance Rs = 0.376 mΩ for YBCO/MgO and Rs = 2.660 mΩ for YBCO/LAO at 78 K, 10 GHz. The results show stronger stress in YBCO/LAO which lead to a larger microwave surface resistance than YBCO/MgO's. Some rational explanations are also analyzed and discussed in the paper.