Implication of vortex dimensional crossover on the second magnetization peak in Bi2Sr2CaCu2O8 single crystals

We applied weak static and dynamic perturbations to the vortex state in Bi2Sr2CaCu2O8 single crystals to investigate the underlying mechanism of the second magnetization peak. Low-density columnar defects were introduced into the sample as a static perturbation and small additional transverse ac fields were applied as a dynamic perturbation during the magnetization measurement. A reduction of the irreversible magnetization at the second peak was observed after introducing low-density columnar defects, which is unusual regarding the commonly observed magnetic enhancement due to columnar defects. Applying ac transverse field also reduced the magnetization due to easier flux entry and exit by dithering the vortices, but the amount of reduction was much higher above the second peak with the appearance of flux jump. We find that both results are well explained by a vortex dimensional crossover near the second magnetization peak.