Discontinuity on the ac susceptibility curves of Y-Ba-Cu-O superconductors

The ac susceptibility versus temperature of very pure Y 1.01 Ba 1.95 Cu 2.97 O x , in the presence of huge thermal gradients, has been studied. Five high quality samples are used and no fine structures, only step discontinuities for two of the five samples (presenting thermal cycling and hysteresis) have been observed. A simple model, including the necessary conditions (the existence of at least two components with different specific heat in the sample and, at least for one of them, the validity of the local heating model) for the appearance of the discontinuity is presented.