Transient photoimpedance response of YBa2Cu3O7-delta epitaxial films in the mixed state

This paper reports on an investigation of transient photoimpedance response to radiation, in other words, real-time variations of the impedance induce d by femtosecond optical pulses in superconducting films transferred to the mixed state by an external magnetic field applied parallel to the c-axis. When the films were in a state characterized by the absence of dc resistivi ty, the response amplitude increased with the magnetic field faster than ex pected owing to the contribution of magnetic vortices to the impedance of a superconductor with s-wave pairing of electrons. It turned out that the ef fect is due to a growth in the effective density of quasiparticle states in the mixed state of a d-wave superconductor. In the absence of magnetic fie ld, however, the response amplitude was higher at lower temperatures, which contradicts the predictions of both models. Possible reasons for this feat ure in the response as a function of temperature are discussed. (C) 1999 Am erican Institute of Physics. [S1063-7761(99)01909-5].