NONEQUILIBRIUM MICROWAVE RESPONSE AND KOSTERLITZ-THOULESS TRANSITION IN YBCO GRANULAR FILMS

The characteristic of microwave radiation (λ＝8mm) response of YBCO granular films was reported. Experimental results show that the response behavior can be described with nonequilibrium response effect, instead of thermal effect model. Near below the transition temperature T∞, nonequilibrium microwave response signal decays gradually and disappears with the sample resistance becoming zero. It was also found that the response signal is very sensitive to a weak magnetic field. The sample I-V curve was studied carefully, and the Kosterlitz-Thouless transition model can be used to describe the two-dimensional transport characteristic of the YBCO granular films. Based on the analysis and experimental results, we believe that the mechanism of nonequilibrium microwave response may be related with the breaking of the votex-antivotex pair in high Tc superconductor.