^<b>63</b>Cu(2) Nuclear Spin-Spin Relaxation in YBa_<b>2</b>Cu_<b>3</b>O_<b>6.98</b>, YBa_<b>2</b>(^<b>63</b>Cu)_<b>3</b>O_{<b>7-<i>δ</i></b>}, and YBa_<b>2</b>Cu_<b>4</b>O_<b>8</b>

Abstract

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.