NMR study of U(Be,B)13 in the normal and superconducting states

Abstract

We report extensive 11B and 9Be nuclear magnetic resonance (NMR) spin-lattice relaxation time (T1) and Knight shift measurements in the heavy-fermion superconductor UBe13−xBx(x=0.030,0.067). Data were acquired over a broad temperature (T) range spanning 0.096–300 K in both the normal and superconducting states. In the normal state, the 11B T dependence of T1 displayed several energy scales. In a narrow T range, between 1 and 2 K, 1/T1T was roughly constant. At lower temperatures and in a magnetic field exceeding the upper critical field, 1/T1T exhibited a monotonic decrease down to approximately 0.18 K. Above roughly 4 K, 1/T1 exhibited a weak T dependence until ∼60 K. At higher temperatures, additional relaxation mechanisms were present. The 1/T1 values for both 9Be and 11B for all B concentrations were indistinguishable above ∼4 K; however, at temperatures of the order of 1 K, the x=0.067 sample had values roughly twice that of the x=0.030 sample for either nuclei. The x=0.030 sample had essentially the same 1/T1 magnitude and T dependence as undoped UBe13. From the linear plot of the Knight shift (KB) versus the static magnetic susceptibility (χ), the 11B hyperfine field coupling constant was calculated to be −361.1Oe/μB. An approximate analysis of the U moment electronic fluctuation rate (Γ) was performed using the measured values of 1/T1,KB, and χ. We observed a Γ∝T dependence between 8 and 70 K, which is consistent with the existence of isolated local moment fluctuations. For T>70K, the Γ analysis breaks down due to the presence of crystal field excitations. Measurements of 1/T1 indicated a moderate decrease in Γ with the addition of boron at low temperatures, where Γ(x=0.030)/Γ(x=0.067)≈2. At lower temperatures, Γ approached a constant value of the order of 1 meV at T≈2K. In the superconducting state, the T dependence of 1/T1 exhibited an absence of power-law behavior and a strong B concentration dependence for both nuclei. This behavior is consistent with gapless superconductivity induced by the B impurities. The ratio of the 9Be to 11B1/T1 increased with decreasing T below Tc. This indicates additional contributions to the 9Be1/T1 at low temperatures that we attribute to spin diffusion processes that have negligible impact on the dilute 11B. Below Tc, the 11B Knight shift was measured to be -0.08 (±0.01)% and was independent of both x and T.

Source-reported materials — not catalogue approval

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