Cu63nuclear transverse relaxation rate and spin correlations inYBa2Cu3O6.63

The ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear transverse relaxation rate in the normal state of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.33}$ (${\mathit{T}}_{\mathit{c}}$=62 K) is reported here. The Gaussian relaxation rate 1/${\mathit{T}}_{2\mathit{G}}$ at the planar Cu sites increases with decreasing temperature from 300 down to 100 K, below which it becomes independent of temperature. This behavior is quite different from the nuclear spin-lattice relaxation rate 1/(${\mathit{T}}_1$T), which shows a peak near 150 K, and indicates that antiferromagnetic spin correlations in the ${\mathrm{CuO}}_2$ planes continue to grow in the temperature range where the low-frequency spin fluctuations are depressed by a spin gap.