DC magnetic field induced anomalous microwave response in YBCO thin films: temperature dependence

Using a microstrip, resonator technique, the magnetic field dependence of the surface impedance Z(s) = R-s + jX(s), where R-s is the surface resistance and X-s the surface reactance, of superconducting YBa2Cu3O7-d thin films has been measured at a frequency of 1.1 GHz and at different temperatures. The anomalous microwave response, where R-s decreases in the presence of low dc magnetic fields, has been observed for all the samples under test at different temperatures. The anomalous microwave effect is found to be stronger at higher temperatures. The threshold field H*, at which the surface resistance show a minimum, decreases with temperature and it is governed by the competition between the anomalous effect at low fields and the vortex mechanism at high fields. (C) 2001 Elsevier Science B.V. All rights reserved.