63Cu(2) Nuclear Spin-Spin Relaxation in YBa2Cu3O6.98, YBa2(63Cu)3O7-δ, and YBa2Cu4O8

We have measured the temperature dependence of the transverse relaxation time of 63 Cu(2) nuclear spin in YBa 2 Cu 3 O 6.98 , YBa 2 ( 63 Cu) 3 O 7 , and YBa 2 Cu 4 O 8 . After subtraction of the T 1 process, the decay curve of the spin-echo envelope for YBa 2 Cu 4 O 8 in a strong H 1 followed a single Gaussian form from 280 K to 4.2 K, while for YBa 2 Cu 3 O 6.98 it follows only above T c . The absolute value and the temperature dependence of the decay rate 1/ 63 T G could be understood by the combined contributions of the direct dipole-dipole and the temperature-dependent indirect nuclear spin-spin coupling via χ( q ). From the temperature dependence of 1/ 63 T G for YBa 2 Cu 4 O 8 below T c , we considered that χ( Q ) even below T c remains enhanced. In a viewpoint of the 63 Cu(2) nuclear spin-spin relaxation experiments, so far as YBa 2 Cu 4 O 8 is concerned, the high- T c superconductivity seems to exhibit the d -wave pairing.