Ac-susceptibility measurements have been used to investigate the irreversibility line (IL) of a Bi2Sr2CaCu2O8 single crystal. The activation energy, U-0, determined from the frequency dependence of the ac-susceptibility, can be related to the IL if one employs the model of thermally activated flux creep. This correlation is clearly demonstrated by the observation of a kink in the IL and the existence of two regions in the U-0(B) curve. These results show that for the magnetic field strengths considered in this work, the IL is a depinning line arising from a thermally activated process
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