We report measurements of the 9Be spin-lattice relaxation rate (1/T1) in the superconducting state of U0.999Gd0.001Be13 for temperatures below 0.2 K and magnetic fields in the range 1-15 kOe. Above 8 kOe the relaxation is governed by spin diffusion to the normal regions in the vortex cores of the flux lattice. For lower fields 1/T1 is dominated by longitudinal fluctuations of the Gd3+ moment. The relaxation data are analyzed on the basis of the usual model of nuclear spin diffusion to rapidly relaxing regions (paramagnetic centers or vortex cores), modified to include the low temperature regime. Our best fit to the data indicates a Gd3+ spin relaxation time of approximately (2.8 × 10-9 sK3) T-3. This temperature dependence is the same as that reported earlier for the intrinsic nuclear spin-lattice relaxation rate in the superconducting state of undoped UBe13, and is consistent with lines of zeros of the superconducting energy gap. © 1988.

Be9 NMR and relaxation have been used to probe heavy-fermion superconductivity in U1-xThxBe13, x=0 and 0.033. The spin-lattice relaxation rate 1T1 varies approximately as T3 well below the transition temperature. This is consistent with a class of anisotropic pairing models for which the gap vanishes along lines on the Fermi surface. NMR spectra give no indication of magnetic, structural, or charge-density ordering at a second transition for x=0.033. © 1984 The American Physical Society.

We report the use of the 9Be nuclear magnetic resonance to study heavy-fermion superconductivity in the U1-xThxBe13 alloy system, x = 0 and 0.033. The nuclear spin-lattice relaxation rate 1/T1, which yields information on thermal excitations in the superconducting state, is found to vary more slowly the temperature than expected for a conventional BCS superconductor with a nonzero energy gap Δ. This indicates an enhanced density of excitations for low energies E ≪ Δ. At intermediate temperatures 1/T1T3≃const. for UBe13, which is consistent with highly anisotropic pairing; the specific temperature dependence suggests lines of zeros of Δ on the Fermi surface. The relaxation data do not agree qualitatively with theories of superconducting pair breaking due to paramagnetic impurities. At low temperatures (T < 0.2 K) 1 T1T is approximately constant in UBe13, with a value which decreases with decreasing applied field. This behavior is probably not due to direct relaxation by paramagnetic impurities, for which the temperature dependence would be in the opposite sense. For x= 0.033 1/T1 varies less rapidly than T3. The additional relaxation may be due to filling in the gap by Th-induced pair breaking. The 9Be spectra give no indication of additional broadening due to vortex-lattice inhomogeneity or (for x = 0033) of magnetic ordering at a second transition temperature Tc2 below the superconducting transition (at Tc1). Our results are ompared to NMR studies of CeCu2Si2, and to other experiments in heavy-fermion superconductors. © 1985.

We report the extension of measurements of the 9Be spin-lattice relaxation rate in UBe13 from 300 up to 1000 K. They reflect the fluctuating local magnetic field at the site of the 9Be nuclei. Above about 100 K, there appears a new contribution in addition to the rate observed at lower temperatures. The high temperature contribution is modeled in terms of the thermal excitation of low-lying states of the U 3+ ion along lines used by Felton et al. to explain their measurements of the specific heat. Our results indicate a splitting of 200±20 K between the ground and excited states of the system, and a lifetime of 8×10-14 s for the excited states (width =95 K).

The dependence of the 9Be spin-lattice relaxation rate 1/T1 on magnetic field has been measured in the heavy-fermion superconductor UBe13 at temperatures well below Tc. A crossover between relaxation via spin diffusion to mixed-state vortex cores (H > 6 kOe) and to paramagnetic impurities (H < 6 kOe) is inferred. © 1987.

9Be NMR spectra have been studied in the heavy-fermion compound UBe13 as functions of spectrometer frequency, crystal orientation, and temperature. BeI-site isotropic Knight shift and BeII-site anisotropic shifts, corresponding hyperfine fields, and quadrupolar tensors have been determined. For T {less-than or approximate} 10 K a typical heavy-fermion Knight shift anomaly is observed. © 1987.