Fundamental and harmonic AC responses to different annealing rates of Bi 2Sr 2CaCu 2O 8 single crystals

Abstract

Three Bi 2Sr 2CaCu 2O 8 single crystals with different annealing processes have been measured in terms of complex AC susceptibilities χ n = χ ′ n −i χ n ′′ ( n=1,3) to reveal the effect of flux-pinning strength. The third harmonic susceptibility changes dramatically when the pinning strengths are different due to different annealing processes, although the fundamental susceptibilities are quite similar. The experimental data are explained by the calculations based on finite thickness type-II superconductors in the presence of a geometric barrier. It has been found that the temperature dependence of the third harmonic susceptibility of the crystals follows the model predictions. When the sample is slowly cooled, the geometric barrier plays a significant role in the third harmonic AC susceptibility. Its effect is suppressed when the single crystals are cooled rapidly from high temperature.