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 comparison 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 negative contribution from the DOS fluctuations is almost field independent. Furthermore, 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 that the existence of the Maki-Thompson contribution to W evidences an s-wave symmetry component for the pairing in YBa2Cu3O7-delta.