Anomalous behavior of nuclear spin-lattice relaxation rates in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ below ${\mathit{T}}_{\mathit{c}}$

The authors have measured the change in the anisotropy of the NMR spin-lattice relaxation rates for ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$(2), ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$(2,3), and ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ between the normal and superconducting states of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. These results cannot be explained by a straightforward extension to T<${\mathit{T}}_{\mathit{c}}$ of current theories, such as the theory of Millis, Monien, and Pines of normal-state NMR relaxation.