Nuclear electric quadrupole measurements of ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spin-lattice relaxation rates (${\mathit{R}}_1$) in zero applied magnetic field and nuclear magnetic resonance ${\mathit{R}}_1$ in 8.2 T are reported for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$. A direct effect of the magnetic field is seen for the first time both above and below ${\mathit{T}}_{\mathit{c}}$. Above ${\mathit{T}}_{\mathit{c}}$ the effect is ascribed to the partial suppression of antiferromagnetic (AF) spin fluctuations. For T<${\mathit{T}}_{\mathit{c}}$ the effect goes in the opposite sense and may be related to the spin-temperature diffusion for the core of the fluxons or to the enhancement of AF spin fluctuations persisting in the superconducting phase.

• Received 20 May 1991

DOI:https://doi.org/10.1103/PhysRevLett.68.698