Peak effect of critical current in a Bi-2212 superconducting single crystal

Force-displacement characteristics of flux lines were measured using the Campbell method in the vicinity of the peak effect for an overdoped Bi-2212 single crystal. It was revealed that there are two quasi-stable states with different critical current densities, suggesting a phase transition of the first order. It was also found that the interaction distance increased discontinuously around the peak field, while the Labusch parameter showed no anomaly. This shows that the flux-pinning mechanism does not change across the peak field and that the peak effect is caused by the change in the property of flux lines. It is considered that the dimensional crossover of flux lines brings about the first-order transition and such characteristics. The elastic correlation length of flux lines along the c-axis was of the order of 10 μm and agreed with the theoretical prediction. This shows that the flux lines are not in the complete two-dimensional state even above the peak field. (author)