We report NMR measurements of the nuclear relaxation rate at all copper and oxygen sites in magnetically aligned powder samples of YBa2Cu3O7. There is no peak in the oxygen relaxation rate below Tc at any oxygen site supporting the possibility that d-wave pairs are formed in the superconducting state. Comparison of the oxygen and copper rates in the planes reveals a characteristic temperature greater than Tc. © 1989.

We report NMR measurements of the nuclear relaxation rate at all copper and oxygen sites in magnetically aligned powder samples of YBa2Cu3O7. Comparison of the oxygen and copper relaxation reveals a characteristic temperature greater than Tc. The copper relaxation rate is enhanced by antiferromagnetic copper spin fluctuations which are undiminished in the superconducting state. The absence of a coherence peak indicates that the superconductive pairing is not of the conventional BCS type. © 1989 The American Physical Society.

We report measurements of the temperature dependence of the Bi209 nuclear quadrupole resonance frequency νQ, the Knight shift K, and the spin-lattice relaxation rate 1/T1 in the small-gap semiconductor Ce3Bi4Pt3 between 1.8 and 300 K. Corresponding measurements also are reported for the nonmagnetic metallic isomorph La3Bi4Pt3. The νQ data in the Ce compound show a characteristic departure from metallic-to-insulating behavior when the sample is cooled below TM=80 K, the temperature of the susceptibility maximum, attributable to a loss of low-frequency vibrational modes in the insulating state. The Knight shift has both isotropic and axial components; this anisotropy originates from the presence of Ce via a transferred hyperfine coupling between Ce 4f and conduction electrons. An s-f exchange constant 0.4 eV is found, consistent with hybridization in other rare-earth intermetallic compounds. A change in the scaling between the susceptibility and both the isotropic and axial Knight shifts at temperature TM provides evidence that hybridization between the Ce 4f orbitals and the conduction electrons is responsible for the gap structure. The temperature dependence of the 1/T1 data is consistent with a model electronic density of states possessing a temperature-independent gap δ of 180 K and a bandwidth of the order of 1600 K. The temperature dependence of 1/T1 can also be fit well with a temperature-dependent gap with δ(0) also 180 K. © 1994 The American Physical Society.

We have measured the copper nuclear relaxation rate in GdBa2Cu3O7 in zero applied field using nuclear quadrupole resonance. Fluctuations in the 4f moment associated with the gadolinium contribute strongly to the copper relaxation rate, and this contribution will depend strongly on the copper-gadolinium separation. This separation differs considerably for the two copper sites. Comparison of the relaxation rates shows that the copper signal at the higher frequency (32 MHz) originates from the Cu(2) site, which is located closer to the gadolinium than is the Cu(1) site. © 1988 The American Physical Society.

The Cu63 Knight shift in YBa2Cu3O7 has been measured at both Cu(1) (chain) and Cu(2) (plane) sites, using a uniaxially aligned powder. The isotropic part of the spin Knight shift (Ks) was found to be positive for both sites, suggesting large transferred hyperfine fields from oxygen p states. The Knight shifts at both sites show significant change below Tc, indicating important roles of both the planes and chains in the superconductivity. Despite the uncertainty in the diamagnetic screening, the axial part of Ks at Cu(2) sites is found to show linear-T behavior at low temperatures. © 1989 The American Physical Society.

It is demonstrated that weak boron doping of UBe13 drastically alters the temperature dependence of the specific heat just above and also below the superconducting transition. A strong enhancement of the anomaly at Tc and distinct deviations from mean-field behavior are the most significant features. The influence of small amounts of boron replacing Be in thorium-doped UBe13 is shown to depend on the concentration of Th impurities. Excessive specific heat well below Tc in these compounds indicates considerable variation of the electronic spectrum even at very low temperatures. © 1991 The American Physical Society.

O17 nuclear-magnetic-resonance spectra are obtained, using uniaxially aligned powder of YBa2Cu3O7, and resonance lines from the four inequivalent oxygen sites are clearly distinguished. Principal components of the electric field gradient (EFG) and the shift tensors are obtained in the normal state. Assigning the direction of the largest EFG components of Cu-O bond axes, the anisotropy of the shift suggests that the spin density at the plane O(2,3) and the bridging O(4) sites resides on the p orbitals. The domonant contribution to the spin susceptibility comes from Cu d states. © 1989 The American Physical Society.

The microscopic magnetic properties of the CuO2 planes in YBa2Cu3O6.63 (Tc=62 K) have been investigated in Cu and O NMR experiments. Unlike the fully oxygenated Y-Ba-Cu-O7 (Tc 90 K), the various components of the Cu and O Knight-shift tensors show strong but identical temperature dependences in the normal state. This supports the picture that there is only one spin component in the CuO2 planes. The spin susceptibility deduced from Knight-shift results shows significant reduction with decreasing temperature in the normal state. The temperature dependences of the nuclear-spin-relaxation rates (1/T1) are very different for the Cu and the O sites. 1/(T1T) at the O sites is nearly proportional to the spin susceptibility. 1/(T1T) at the Cu sites shows a broad peak around 150 K. We discuss these relaxation behaviors based on a model of the dynamical spin susceptibility proposed by Millis, Monien, and Pines. © 1991 The American Physical Society.

We present our 17O NMR experimental results in a magnetically aligned powder sample of YBa2Cu3O7-δ (Tc = 93 K). The sign of the anisotropic Knight shift shows that the spin density resides mainly on the pσ orbitals at the planar O(2,3) and bridging O(4) sites. About 30 % of the total spin susceptibility is attributed to the oxygen 2pσ states. The Knight shift at the O(2,3) sites decrease more rapidly than that at the chain O(1) sites, indicating a larger gap in the planes. The nuclear relaxation rate at the O(2,3) sites shows linear-T (Korriga) behavior above Tc, in contrast to the much weaker temperature dependence at the planar Cu(2) sites. However, these two rates show identical temperature dependence below about 110 K, indicating that an important change in the spin dynamics takes place above Tc. © 1989.

Bismuth NMR Knight shift and spin lattice relaxation rate 1/T1 data are reported between 35 and 325 K in the low-carrier heavy fermion system YbBiPt. The Knight shift is strongly temperature dependent and negative. Its temperature dependence tracks the bulk susceptibility with a hyperfine coupling constant Ahf = -7.88 kOe/μB. At low temperatures 1/T1 exhibits a dramatic increase, such that the average 4f spin correlation time τf shows a crossover behavior at about 75 K. The rate 1 τf is proportional to temperature above 75 K, consistent with non-interacting 4f local moments which are relaxed via Korringa-type scattering with the conduction electrons. We discuss the behavior below 75 K in terms of crystal-field effects or a strongly temperature dependent contribution from non-zero q regime of the dynamical susceptibility. © 1995.