FIELD-INDUCED TRANSITION OF THE AC SUSCEPTIBILITY IN HIGH-T-C SUPERCONDUCTORS

We have investigated the angle and field dependence of the ac suscepti bility in Bi2Sr2CaCu2O8 Single crystals at low external fields. The ac field was applied perpendicular to the CuO2 planes. The first and thi rd harmonic of the ac susceptibility exhibit a remarkably sharp transi tion when the de field component perpendicular to the CuO2 planes cros ses a threshold field H-th. H-th is strongly temperature dependent but is independent of the parallel field component. We prove that these s tructures are related to the irreversibility line and propose a simple model which excellently explains the measurement data.