Investigation of ac screening response in the mixed state of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ single crystals

We have measured the ac screening response and resistance of superconducting ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ (BSCCO) crystals versus frequency, temperature, and applied dc field. Within the examined field and temperature range, we find the systematic behavior of the observed dissipation peak to be consistent with electromagnetic skin size effects rather than a phase transition. However, a clear contradiction is found between the observed screening response and the response time implied by thermal activation models on the basis of the measured resistance, which raises important questions for the theoretical treatment of the vortex dynamics. We note that a partial resolution of this paradox points to a distribution of pinning energies with the sample.