Pseudogap in YBa2Cu3O7-δ from NMR in high magnetic fields

We report on ¹⁷O(2,3) and ⁶³Cu(2) spin-lattice relaxation rates and the ¹⁷O(2,3) spin-spin relaxation rate in different magnetic fields in YBa₂Cu₃O₇ near T_c. Together these measurements enable us to test the magnetic-field dependence of the pseudogap effect on the spin susceptibility in different regions of the Brillouin zone using the known form factors for different nuclei as filters. Thus, we study the momentum dispersion of the pseudogap behavior. We find that near the antiferromagnetic wave vector the pseudogap is insensitive to magnetic fields up to 15 T. In the remaining region, away from the (π,π) point, the pseudogap shows a magnetic-field dependence at fields less than 10 T. The first result is indicative of the opening of a spin pseudogap that suppresses antiferromagnetic correlations below a temperature T^*; whereas, the second result shows the effect of pairing fluctuations on the spin susceptibility as a precursory effect of superconductivity.