Nonequilibrium photoresponse of YBa2Cu3O7-x granular films to 8 mm microwave radiation

Nonequilibrium photoresponse behavior has been investigated for YBa2Cu3O7-x (YBCO) granular films to 8 mm microwave radiation under various bias curre nts and magnetic fields. The measurements reveal that the nonequilibrium ph otoresponse mode occurs only in the tail region of the resistance transitio n curve R(T) from the normal to the superconducting state, where transporta tion behavior of the granular superconducting film is found to be character ized by the Kosterlitz-Thouless (KT) phase transition model. Based on the K T model, the photoresponse mechanism has been interpreted in terms of the d epinning process of the unbinding vortices, which are generated from the de coupling process of the vortex-antivortex pairs by current, and are held at the intrinsic pinning sites of the granular high-T-c superconducting films at low temperature. Under the co-action of the bias current and the incide nt microwave photons, these unbinding vortices will be driven out of the pi nning center, creating viscous motion in the Josephson junction array syste m. An analytical result of the unbinding vortices density n(T,I) induced by applied current has been worked out based on the model of two-dimensional Josephson junction arrays that is employed as a model system for the YBCO g ranular films. The distribution of the n(T,I) is found to be analogous to t hat of the photoresponse measured in the temperature region of 2/3T(KT)<T < T-KT. Additionally, the measurements reveal that the magnitude of the photo response is linearly increased with an increase of the incident microwave p ower. These results imply that the nonequilibrium photoresponse induced by microwave irradiation may be intrinsically related to the decoupling proces s of the vortex-antivortex pairs, as well as to the depinning dynamics of t he unbinding vortices in the granular high-T-c superconducting films. (C) 2 001 American Institute of Physics.