LOW-TEMPERATURE NUCLEAR-SPIN-LATTICE RELAXATION PROCESS IN THE CU(2) SITES OF YBA2CU3O6.98

The NQR spin-lattice relaxation time T, in the planar Cu(2) sites has been carefully remeasured for both the isotopes, Cu-63 and Cu-65, in the typical high-T(c) oxide superconductor YBa2Cu3O6.98. A substantial rf-field amplitude H-1 dependence of the decay of nuclear magnetization has been observed at 4.2 K. Namely, a short relaxation process becomes more pronounced for the weaker H-1 and the lower temperatures. It is nevertheless assured that the relaxation process is totally governed by the magnetic process in the superconducting state, as well as the normal state. This is because we have confirmed that the isotopic ratio of the relaxation between Cu-63 and Cu-65 is found to be very close to the square of the nuclear gyromagnetic ratio rather than the nuclear quadrupole ratio even at the lowest temperature investigated, 4.2 K.