Magnetic Field Independence of the Spin Gap in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

We report, for magnetic fields of 0, 8.8, and 14.8thinspthinspT, measurements of the temperature dependent {sup 63}Cu NMR spin lattice relaxation rate for near optimally doped YBa{sub 2}Cu {sub 3}O{sub 7{minus}{delta}} , near and above T{sub c} . In sharp contrast with previous work we find no magnetic field dependence. We discuss experimental issues arising in measurements of this required precision and implications of the experiment regarding issues including the spin gap or pseudogap. {copyright} {ital 1998} {ital The American Physical Society}