Using an original modulation technique, the longitudinal (spin-lattice) relaxation of the impurity ${\mathrm{Gd}}^{3+}$ ions has been investigated in the superconducting state of ${\mathrm{Y}}_{0.99}{\mathrm{Gd}}_{0.01}{\mathrm{Ba}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ magnetically aligned powders. The results agree well with the linewidth analysis performed on a single crystal. At low temperatures $(T<\mathrm{}60\mathrm{}\mathrm{K})$ the relaxation rate $T_1^{-1}$ is found to be proportional to $T^3,$ suggesting d-wave superconducting pairing. The data are consistent with previous Knight shift ${(}^{\mathrm{Gd}}K)$ measurements that revealed a linear temperature dependence of the uniform spin susceptibility. The Korringa relation between $T_1$ and ${}^{\mathrm{Gd}}K$ is confirmed indicating the validity of the Fermi-liquid model.

• Received 26 January 2000

DOI:https://doi.org/10.1103/PhysRevB.61.R14944