Thermally activated depinning in polycrystalline Bi-based high-Tcsuperconductors

With the vibrating-reed technique we have studied the coupling of the flux-line-lattice (FLL) to the atomic lattice between 4.2 and 100 K at constant applied fields 0.014T≤${\mathrm{B}}_{\mathrm{a}}$≤4 T in three different Bi-based polycrystalline samples. Depinning of the FLL is observed at temperatures above ${\mathrm{T}}_{\mathrm{D}}$ where a peak occurs in the damping of the vibrating sample. With increasing temperature, the FLL dissipation changes at ${\mathrm{T}}_{\mathrm{D}}$ from a hysteretic to a viscous regime. The depinning temperature ${\mathrm{T}}_{\mathrm{D}}$<${\mathrm{T}}_{\mathrm{c}}$(${\mathrm{B}}_{\mathrm{a}}$) follows a logarithmic magnetic field dependence which is similar to the dependence of the depinning temperature ${\mathrm{T}}^{\mathrm{*}}$ obtained from measurements of the electrical resistivity due to thermally activated flux motion.