Microwave Surface Resistance in 123 superconducting Thin Films

摘要

The microwave surface resistance, Rs, in various 123 superconducting thin films, in situ grown by pulsed laser deposition technique on different substrates was examined. Thermal annealing of MgO substrates prior to film deposition yields YBa_2Cu_3Q_z (YBCO) films with low Rs value, ~ 0.3 mO at 30 K, 22 GHz, which is close to the Rs value reported for best YBCO films which is attributed to the improved crystallinity of YBCO films. For Y_(0.98)Ca_(0.02)Ba_2Cu_3O_z films grown on MgO, we observed much suppressed Rs value (0.1 mO at 20 K, 22 GHz), which is three times smaller as compared to the Rs value of YBCO. It is presumed to be due to the enhanced Jc in this film caused by Ca doping. R_S(T) data of under-doped (both Co substituted YBCO as well as O2 depleted YBCO films grown on LaAlO3) films reveal that in general, under-doping leads to an enhancement in residual Rs values, which may be attributed to the increased penetration depth, X, caused by the suppressed carrier content in the under-doped films. In the case of rare earth mixed (Dy_(0.4)Ho_(0.6))Ba_2Cu_3O_z films grown on LaAlO_3, at 77 K and above, they exhibit a lower Rs value when compared to YBCO film. However, below 70 K, YBCO films showed a superior microwave properties.