Hysteresis measurement of anomalous microwave surface resistance in superconducting thin films

The anomalous decrease in microwave surface resistance, R-s, of superconduc ting YBa2Cu3O7-d (YBCO) thin films in the presence of a low de magnetic fie ld is studied using a microstrip resonator technique. We have done a de fie ld 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 l evel far below the low critical field, H-c1,H-strip of the superconducting microstrip and that the vortex (Abrikosov) penetration would eliminate the anomalous effect observed at low field. This implies that the anomalous eff ect does not originate from vortices.