Anisotropy and magnetic field dependence of the planar copper NMR spin-lattice relaxation rate in YBa2Cu4O8

Abstract

We have measured the temperature and magnetic field dependence of the ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spin-lattice relaxation rate W and its anisotropy r at plain Cu(2) sites in normal and superconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$. Below ${\mathit{T}}_{\mathit{c}}$ we observed that an applied magnetic field B∥c enhances W=${\mathit{W}}_{\mathit{c}}$, whereas B⊥c suppresses W=${\mathit{W}}_{\mathit{a}\mathit{b}}$. Such a behavior seems to rule out the spin diffusion to the fluxoid cores and the fluxoid motion as being responsible for the effect. It indicates more an unexpected field-related breaking of the spin-rotation invariance in the superconducting state. The anisotropy r defined as the ratio ${\mathit{W}}_{\mathit{a}\mathit{b}}$/${\mathit{W}}_{\mathit{c}}$ is almost field and temperature independent in the normal state but develops a nonmonotonic temperature dependence below ${\mathit{T}}_{\mathit{c}}$ with a flat minimum at 45 K in B=5.17 T and a much more pronounced minimum at 55 K in B=0.58 T. A qualitatively similar behavior of r has been reported previously for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. Comparing r in both compounds, we note one essential difference at low B. Namely, the slope dr/dT just below ${\mathit{T}}_{\mathit{c}}$ is large for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ but almost zero for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$.