SUPERCONDUCTING FLUCTUATIONS AND CU-63 NQR-NMR RELAXATION IN YBA2CU3O7-DELTA - EFFECT OF MAGNETIC-FIELD AND A TEST FOR THE PAIRING-STATE SYMMETRY

Evidence is presented of superconducting fluctuations in thr Cu-63 NQR -NMR relaxation rate in YBa2Cu3O7-delta as obtained from a careful com parison of measurements carried out in the absence and in the presence of a field parallel to the c axis. It is shown that the field causes a reduction of the relaxation rate W in a range of about 10 K above T- c. This effect is related to the suppression by the magnetic field of the phase-sensitive positive Maki-Thompson contribution while the nega tive contribution from the DOS fluctuations is almost field independen t. Furthermore, it is argued how the fluctuation effects on W can be u sed to discuss the pairing state symmetry, at variance with the insens itivity of the transport measurements. It is pointed out that the exis tence of the Maki-Thompson contribution to W evidences an s-wave symme try component for the pairing in YBa2Cu3O7-delta.