Peak effect of critical current densities ofBi2Sr2CaCu2O8+

For Bi2Sr2CaCu2O8+δ (Bi-2212) high-Tc superconducting whiskers, we investigate the dependence of critical current densities Jc on magnetic fields along the crystal c axis at elevated temperatures around 70 K by electrical transport measurements, where a dc current is applied along the longest dimension, i.e., the a axis, of the whisker. The voltage-current (V-I) characteristics are varied nonmonotonically by the applied magnetic field H. It is found that Jc increases with the field in the low-field regime near the lower critical field Hc1 and then decreases with increasing field. The peak of Jc(H) occurs below several tens of oersteds, which is remarkably low by comparison with the conventional peak effect in Bi-2212 bulk single crystals. In addition to the enhancement of Jc, the field-induced zero-voltage state is confirmed to be a function of the magnetic field. The present peak effect is a phenomenon involving dilute vortices confined by surface barriers in a finite narrow width, which is of the order of the translational correlation length with respect to the triangular vortex lattice. The low-field-induced peak effect for a quasi-one-dimensional geometry with surface barriers is discussed