Superconducting fluctuations andCu63NQR-NMR relaxation inYBa2Cu3O7−δ: Effect of magnetic field and a test for the pairing-state symmetry

Evidence is presented of superconducting fluctuations in the ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ NQR-NMR relaxation rate in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$, 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 $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$.