Effects of Strong Magnetic Fields on Pairing Fluctuations in High Temperature Superconductors

We present the theory for the effects of superconducting pairing fluctuations on the nuclear spin-lattice relaxation rate, 1/T_1, and the NMR Knight shift for layered superconductors in high magnetic fields. These results can be used to clarify the origin of the pseudogap in high-T_c cuprates, which has been attributed to spin fluctuations as well as pairing fluctuations. We present theoretical results for s-wave and d-wave pairing fluctuations and show that recent experiments in optimally doped YBa_2Cu_3O_{7-d} are described by d-wave pairing fluctuations. In addition, we show that the orthorhombic distortion in YBa_2Cu_3O_{7-d} accounts for an experimentally observed discrepancy between 1/T_1 obtained by NQR and NMR at low field. We propose an NMR experiment to distinguish a fluctuating s-wave order parameter from a fluctuating strongly anisotropic order parameter, which may be applied to the system Nd_{2-x}Ce_xCuO_{4-d} and possibly other layered superconductors.