Magnetooptic Measurements Of The Local Ac Susceptibility In High-Temperature Superconductors In A Wide Frequency Range

Using magnetooptic technique, local ac-susceptibility measurements were made in YBa(2)Cu(3)O(7-delta) (YBCO) and Bi(2)Sr(2)CaCu(2)O(X) (BSCCO) single crystals. A striking difference has been observed between 90-K YBCO crystals on the one hand and 60-K YBCO and BSCCO samples on the other hand. In 90-K samples the onset of the ac transition is frequency independent, and the transition width decreases with increasing frequency, which can be explained well in the frame of the flux creep model. However, in BSCCO and 60-K YBCO samples the transition onset shifts to higher temperatures as the ac field frequency grows. An explanation of the observed effect in terms of phase fluctuations of the superconducting order parameter is proposed.