ANISOTROPY AND MAGNETIC-FIELD DEPENDENCE OF THE PLANAR COPPER NMR SPIN-LATTICE RELAXATION RATE IN YBA2CU4O8

We have measured the temperature and magnetic field dependence of the Cu-63 nuclear spin-lattice relaxation rate W and its anisotropy r at plain Cu(2) sites in normal and superconducting YBa2Cu4O8. Below T(c) we observed that an applied magnetic field B parallel-to c enhances W = W(c), whereas B perpendicular-to c suppresses W = W(ab). 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 W(ab)/W(c) is almost field and temperature independent in the normal state but develops a nonmonotonic temperature dependence below T(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 YBa2Cu3O7. Comparing r in both compounds, we note one essential difference at low B. Namely, the slope dr/dT just below T(c) is large for YBa2Cu3O7 but almost zero for YBa2Cu4O8.