Magnetic field independence of Cu(2) NMR spin-lattice relaxation rate in the normal state of optimally doped YBa2Cu3O7-δ

We report NMR measurements of ⁶³Cu spin-lattice relaxation rates, ⁶³(T₁T)^-1, for magnetic fields of 0, 4, 8.8, and 14.8 T. The measurements were performed on four different samples within the family of near optimally doped YBa₂Cu₃O_7-δ and for the range of temperatures above and below T_c. For each sample we consistently find that, in the normal state, ⁶³Cu spin-lattice relaxation rate is magnetic field independent and that field dependence develops only below T_c where ⁶³(T₁T)^-1 curves ``fan out.'' We describe in detail experimental methods enabling the high precision required in such measurements.