The anomalous decrease in microwave surface resistance, R_{ s}, of superconducting YBa_{ 2}Cu_{ 3}O_{ 7-d} (YBCO) thin films in the presence of a low dc magnetic field is studied using a microstrip resonator technique. We have done a dc field hysteresis measurement of R_{ s} to study the effects of vortex penetration on the anomalous effect. It is shown that the anomaly happens at a field level far below the low critical field, H_{ c1,strip}, of the superconducting microstrip and vortex (Abrikosov) penetration would eliminate the anomalous effect observed at low field. This implies that the anomalous effect is not contributed by vortices.
Categories: cond-mat.supr-con