Microwave surface resistance of YBCO superconducting thin films under high DC magnetic field

摘要

We investigated the dc magnetic field and temperature dependences of the microwave surface resistance (R_s) of YBa_2Cu _3O_y (YBCO) superconducting thin films with various thicknesses. We used YBCO thin films with 300, 500, and 700 nm deposited on a MgO (1 0 0) substrate by the thermal co-evaporation method. The R_s was measured using the dielectric resonator method at 21.8 GHz, and a high dc magnetic field of up to 12 T was applied parallel to the c-axis of the YBCO thin films during R_s measurements. The R_s value of YBCO thin films increased when dc magnetic field was applied. These relationships could be explained by using the two-fluid model for high frequency and low magnetic field limits. We defined the viscous drag coefficient η under the low magnetic limit and the high magnetic field limit as the η_(low) (B, T, d) and η_(high) (B, T, d), respectively. The η_(low) values were approximately two orders of magnitude greater than η_(high) values. Also, all η_(low) and η_(high) value decreased or saturated as the temperature and film thickness increased. These results show that η under the low and high magnetic field limit depended on the temperature and film thickness.