Nuclear Resonance Studies of the Superconducting State of Yttrium BARIUM(2) COPPER(3) OXYGEN(7)

The author reports measurements of nuclear spin -lattice relaxation rates W_1 for the planar ^{63}Cu(2) and ^{17}O(2,3) sites in rm YBa_2Cu_3O_7 (T _{c} = 93K). We have observed a dependence of the rates in the superconducting state on the strength of the applied magnetic field H _0, with strong fields causing an increase in the measured rates. This effect was much greater for fields along the crystalline c axis than for fields applied in the a-b plane and is presumed to result from fluxoids since the relaxation rates were found to depend more or less linearly on the field strength. We therefore used weak magnetic fields in order to obtain the true superconducting -state behavior of the relaxation rates. Our measurements of the copper rates ^{63}rm W_{1a} (for vec H_0|vec c) and ^{63}W _{1c} (for vec H_0parallelvec c) in weak magnetic fields revealed an unexpected result for T < T_{rm c} : the ratio ^{63}W_{1a}/^{63}W_ {1c}, which was essentially independent of temperature in the normal state, decreased upon entering the superconducting state with a minimum for T ~ 80 K but then increased as the temperature was decreased below 80 K. Also, our weak-field data for ^{63}W_{1c } and ^{17}W_{1c} showed that these rates varied approximately as T^3 at low temperature, which is the expected temperature dependence for an orbital d-wave pairing state. In addition, unlike previous measurements, the ratio of copper-to-oxygen rates ^{63}W_{1c }/^{17}W_{1c} was found to be dependent on temperature in the superconducting state; the ratio was constant for 77 K < T < 100 K with a value of approximately 22 but decreased as the temperature decreased, to a value of 7.5 at 17 K. Several theoretical calculations employing a generalized BCS approach have been able to explain all of the NMR data from the superconducting state by assuming an orbital d-wave pairing state whereas an orbital s-wave pairing state with an isotropic gap failed to account for the data.