The authors have used ' Y NMR to measure the internal magnetic field of a sample of YBa:Cu30& z (T, =90 K) in the superconducting state to correct for the effects of the Meissner shielding currents. They used this information to separate the magnetic-shift tensor into orbital (chemical-shift) and spin (Knight-shift) contributions. They find that the temperature dependence of the chain Cu Knight shift fits the classic Yosida function of weak-coupling, orbital-s-state, spinsinglet BCS theory. For the planes, the Knight shift also requires a spin singlet, but with a strongcoupling Yosida function. The best fit is for an orbital s state, but an orbital d state is also possible.They find a zero-temperature gap b(0) of 1.76k& T, for the chains. For the planes, 6(0) can range from 1.9k8T, to 3.lk~T, .

We have measured the resistivity tensor in one nearly twin-free crystal and one twin-free crystal of YBa2Cu307 z. The first crystal was grown with a large twin-free region. We removed the twins in the second crystal by applying a uniaxial stress. Using a modified Montgomery technique, we measured the electrical resistivities in all three crystal directions and found them to be linear in temperature above the superconducting transition temperature T, . Our results for p, and pb at room temperature are among the lowest that have been reported in the literature, indicating that our samples are of high quality. The ratio p, /pb is independent of temperature between 150 and 275 K, and its value, 2.2+0.2, indicates that the Cu-0 chains contribute 60% of the current when the electric field is parallel to them, with the rest of the current being contributed by the Cu-0 planes. This result, at zero frequency, is in good agreement with the recent conductivity at infrared frequencies obtained by Schlesinger et al.

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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...

The authors report measurements of the planar 63 Cu nuclear spin-lattice relaxation rates 63 H / u / ( 63 W| r ) for the static field oriented along the a (c) axis in the superconducting state of YBa2Cu307. These measurements were made on a single sample as a function of temperature and magnetic field. The weak-field results are well explained by several recent calculations in terms of a BCS spin-singlet, orbital-d-wave pairing state. They also find that at low temperatures bi W\ c varies linearly with the applied magnetic field, suggesting that at high field it is dominated by fluxoid cores. PACS numbers: 74.70.Vy, 74.30.Gn, 76.60.Es, 76.60.Gv Nuclear spin-lattice relaxation time measurements in superconductors have provided information about the nature of the superconducting state [l]. They have been employed extensively in the study of Cu02-based hightemperature superconductors [2,3]. Recently we reported [4] that measurements below T v of the relaxation rates of the planar 63 Cu, ,7 0, and 89 Y nuclei in YBa 2 Cu 3 0 7 had puzzling properties, especially when viewed in the framework of the antiferromagnetic Fermi liquid (AFL) theory [5], which is so successful in describing the NMR properties of the normal state of high-temperature superconductors. In this Letter we report significant new experimental results, and compare them with recent calculations of Bulut and Scalapino [6], Lu [7], and Lu and Pines [8], which successfully account for the results. These results suggest that YBa2Cu37 is a superconductor of the BCS variety with spin-singlet pairing and nodes in the gap (as in t/-wave pairing).In the normal state the nuclear spin-lattice relaxation rates n W\ a are anisotropic [r/=63, 17, or 89 corresponding to the planar 63 Cu(2), l7 0(2,3), or 89 Y nuclei, where a=a or c specifies the orientation of the static applied field]. For the Cu(2) in the normal state, b3 W ]a / b3 W u . = 3.4-3.7 (depending on the sample) and is independent of temperature and magnetic field strength [4,9]. In our earlier paper we reported that below T c the ratio dropped substantially and was temperature dependent, a result which was hard to understand within the AFL framework [4]. We studied the ratio at 8.31 T and also, at 77 K, in a weak field (0.45 T). The latter experiment proved that the change in b3 W\J^3W\ c just below T c was not an artifact resulting from the presence of the static magnetic field. However, our data for b3 W\ c . measured at zero field [nuclear quadrupole resonance (NQR)] in one sample and at 8.31 T in another sample had slightly different temperature dependences, deviating from each other progressively as one went to lower temperatures. It was not clear whether this was a sample dependence or a field dependence.In our present work, we have measured relaxation rates in strong, intermediate, weak, and zero fields in a single sample. We find that as one lowers T through T c in weak fields, the anisotropy ratio b3 W\ a / b3 W\ c first drops (as we showed previously) [4] then rises above the normal-state v...