EFFECT OF SUPERCONDUCTING FLUCTUATIONS ON CU-63 NQR-NMR RELAXATION INUBA2CU3O7

The report deals with the effects of superconducting fluctuations (SF) on the Cu-63 NQR-NMR relaxation rate in YBCO, as studied from a caref ul comparison of measurements carried out in the absence and in the pr esence of a field parallel to the c-axis in an oriented powder sample. The application of a magnetic field is observed to cause the reductio n of the relaxation rate W in a range of about 10 K above T-c. This ef fect is related to the suppression by the magnetic field of the phase- sensitive positive Maki-Thompson contribution which allows one to evid ence the decrease in W due to the reduction of the one-electron densit y of state. It is argued how the fluctuation effects on W can be used to discuss the pairing state symmetry, at variance with the insensitiv ity of the transport measurements. It is pointed out how the existence of the Maki-Thompson contribution to W supports an s-wave symmetry co mponent for the pairing in YBCO.