Effect of superconducting fluctuations on Cu-63 NQR-NMR relaxation in UBa2Cu3O7

The report deals with the effects of superconducting fluctuations (SF) on the Cu-63 NQR-NMR relaxation rate in YBCO, as studied from a careful comparison of measurements carried out in the absence and in the presence of a field parallel to the c-axis in an oriented powder sample. The application of a magnetic field is observed to cause the 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 which allows one to evidence the decrease in W due to the reduction of the one-electron density of state. It is argued how the fluctuation effects on W can be used to discuss the pairing state symmetry, at variance with the insensitivity of the transport measurements. It is pointed out how the existence of the Maki-Thompson contribution to W supports an s-wave symmetry component for the pairing in YBCO.