Global ac susceptibility of low pinning high-T c crystals near T c

Abstract

We have studied the hysteretic loss and the shielding capability of Bi2Sr2CaCu2O8and YBa2Cu4O8 crystals as a function of ac field (μ 0 H ≤ 1mT), frequency, and temperature at small dc fields (μ0 H a < 1 mT) near the critical temperature T c . We show that the global ac field dependence of the susceptibility can be described in terms of a geometrical barrier model in agreement with local permeability results. At temperatures very near T c , however, we observe a crossover to diffusive behavior. The frequency dependence of the temperature of the dissipation maximum at finite dc fields shows a thermally activated behavior.