Dynamic response of microwave absorption triggered by magnetic modulation in high anisotropy Bi2Sr2CaCu2Ox crystals

We study magnetic field modulated microwave absorption in Bi2Sr2CaCu2Ox sin gle crystals. In addition to the well-known signal corresponding to the hel d derivative of the thermally-activated flux-flow resistivity, which appear s close to T-c and depends linearly on the modulation level, we observe a n ew signal with a nonlinear (NL) dependence on the magnitude of the modulati on field. This new signal (i) extends to low temperatures, (ii) appears onl y above some threshold modulation level, (iii) strongly depends on the magn itude and direction of the d.c. magnetic field, and (iv) appears only in hi ghly anisotropic superconductors. It cannot be simulated from the microwave dissipation measurements of Enriquez et al, where an almost identical expe rimental set up was used but without magnetic field modulation, Hence, the NL signal is due to dynamic response triggered by the magnetic modulation. We suggest that the NL signal arises from the removal of vortices out of th eir shallow pinning potential well by the modulation magnetic field. (C) 20 01 Elsevier Science B.V. All rights reserved.