We report, for magnetic fields of 0, 8.8, and 14.8 T, measurements of the temperature dependent 63 Cu NMR spin lattice relaxation rate for near optimally doped YBa 2 Cu 3 O 72d , near and above T 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. [S0031-9007(98)08138-1] PACS numbers: 74.25.Nf, 74.72.Bk, 76.60.EsA dominant feature of optimally and underdoped cuprates is the appearance of a pseudogap in the normal state excitation spectrum. The microscopic mechanism which is responsible remains a mystery. A number of scenarios for explaining the pseudogap have been proposed (see Ref.[1] for a recent review). However, no calculations of the consequences of a large applied field for the pseudogap have been published. The high magnetic field behavior of the pseudogap provides additional experimental characterization of the pseudogap which is crucial for differentiating between various pictures.We report very high accuracy measurements of the magnetic field dependence of the 63 Cu spin lattice relaxation rate in near optimally doped YBa 2 Cu 3 O 72d . Our measurements demonstrate, in sharp contrast with previous work [2-6], that there is no magnetic field dependence to 63 ͑T 1 T͒ 21 in YBa 2 Cu 3 O 72d . This result has three important ramifications. Although the magnetic fields we apply shift T c down by as much as 8 K, the onset of pseudogap effects does not shift down in temperature. Hence the pseudogap is unrelated to superconducting fluctuations, even in near optimally doped YBa 2 Cu 3 O 72d where the gap behavior appears just above T c . The onset of the pseudogap is very rapid, clearly demonstrating that its magnitude is temperature dependent, opening very rapidly near 110 K. Finally, the absence of any field effect indicates a relatively large energy scale for the gap mechanism. If dynamical pairing correlations or preformed pairs are involved, the length scales must be very short.The 63 Cu NMR spin lattice relaxation rate reveals the spin part of pseudogap behavior, the "spin gap." In underdoped YBa 2 Cu 3 O 6.6 , 63 ͑T 1 T͒ 21 famously exhibits a broad maximum in the vicinity of room temperature and then decreases as T approaches T c from above. In optimally doped YBa 2 Cu 3 O 72d (data shown in Fig.

The authors have measured the change in the anisotropy of the NMR spin-lattice relaxation rates for 63 Cu(2), 17 7. These results cannot be explained by a straightforward extension to T < T c of current theories, such as the theory of Millis, Monien, and Pines of normal-state NMR relaxation.PACS numbers: 74.70.Vy, 74.30.Gn, 76.60.Es, 76.60.Gv NMR measurements have shown that spin fluctuations in high-temperature superconductors have strong antiferromagnetic correlations with a characteristic correlation length £ of several lattice constants. £ varies with temperature in most of the normal state but has been assumed to be independent of temperature throughout the superconducting state. We have measured the anisotropy of the NMR spin-lattice relaxation rate W\ in the normal and superconducting states of YBa2Cu3C>7 (7V =93 K) for the 63 Cu and 17 0 nuclei in the planes [the Cu(2) and 0(2,3) sites] and for the 89 Y nuclei. The 63 Cu anisotropy decreases dramatically upon entering the superconducting state. When combined with our measurements of the 17 0 and 89 Y anisotropy at 100 and 77 K, we see that the change in the copper relaxationrate ratio appears to require a modification of theories such as that by Millis, Monien, and Pines 1 (MMP) in which the temperature dependence of the NMR ratios comes only from the temperature dependence of the (assumed isotropic) antiferromagnetic correlation length. We describe four possible modifications.The data presented below involve six samples. All samples but one exhibit 100% shielding fractions, and T c (H 0 =0) was 93 K as measured by the change in coil inductance of a nuclear quadrupole resonance (NQR) coil and SQUID measurements. 2 Sample 1 was composed of -50 single crystals whose c and alb axes were optically aligned. Samples 2 and 3 were unaligned powder samples. Samples 4, 5, and 6 were aligned powder samples sealed in Stycast 1266 epoxy. Sample 6 was isotopically enriched ( I7 0) and exhibited a smaller shielding fraction (80%); however, all static and dynamic NMR properties agreed with previous measurements. 3,4 Since 63 Cu and ,7 0 have spins of /=y and f, respectively, their spin-lattice relaxation curves are multiexponential. 5 We report the rate W\ a (a=x,y,z) utilized by MMP which is y of the single-exponential relaxation rate which would occur in a strong magnetic field in the absence of electric quadrupole splittings. It is the temperature-dependent anisotropy of the quantities W\ a that we report. W\ a and W\t for Cu(2) and Y are not experimentally distinguishable. However, three different oxygen resonances are distinguishable. Following Takigawa et al., 3 we define xl W\ a and xl W\b to be the rates when the field is in the CuC>2 planes parallel and perpendicular, respectively, to the Cu-O-Cu bond axis.The values of W\ a as measured by NMR for the Cu(2) site in single-crystal sample 1 and aligned-powder sample 4 are shown in Fig. 1, demonstrating that the measured W\ a \ are sam...

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