NMR study of U„Be, B...13 in the normal and superconducting states

We report extensive B and Be nuclear magnetic resonance ~NMR! spin-lattice relaxation time (T1) and Knight shift measurements in the heavy-fermion superconductor UBe132xBx (x50.030, 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 B 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 Be and B for all B concentrations were indistinguishable above ;4 K; however, at temperatures of the order of 1 K, the x50.067 sample had values roughly twice that of the x50.030 sample for either nuclei. The x50.030 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 ~x!, the B hyperfine field coupling constant was calculated to be 2361.1 Oe/mB . An approximate analysis of the U moment electronic fluctuation rate ~G! was performed using the measured values of 1/T1 , KB , and x. We observed a G}AT dependence between 8 and 70 K, which is consistent with the existence of isolated local moment fluctuations. For T.70 K, the G analysis breaks down due to the presence of crystal field excitations. Measurements of 1/T1 indicated a moderate decrease in G with the addition of boron at low temperatures, where G(x50.030)/G(x50.067)'2. At lower temperatures, G approached a constant value of the order of 1 meV at T'2 K. 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 Be to B 1/T1 increased with decreasing T below Tc . This indicates additional contributions to the Be 1/T1 at low temperatures that we attribute to spin diffusion processes that have negligible impact on the dilute B. Below Tc , the B Knight shift was measured to be 20.08 ~60.01!% and was independent of both x and T. @S0163-1829~99!03602-4#