Probing superconducting fluctuations in high-temperature superconductors by local AC susceptibility measurements in a wide frequency range

AC-susceptibility curves for YBCO-123 (with T-C of 90 and 60 K) and BSCCO-2212 single crystals were measured between 50 Hz and 4 MHz using the magnetooptic technique. The effect of frequency appeared to be different for different superconductors. Thus for 90 K YBCO crystals increasing the ac field frequency results in narrowing of the transition curves, while the transition onset remains unchanged. However, the transition onset temperature for 60 K YBCO and BSCCO crystals was found to increase by up to 10 K as the frequency increases. We explain this unpredicted behavior in terms of fluctuations of the superconducting order parameter and discuss Which type of fluctuations may be responsible for the observed effect. (C) 2004 Elsevier B.V. All rights reserved.