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

We report extensive B-11 and Be-9 nuclear magnetic resonance (NMR) spin-lat tice relaxation time (T-1) and Knight shift measurements in the heavy-fermi on superconductor UBe13-xBx (x= 0.030, 0.067). Data were acquired over a br oad temperature (T) range spanning 0.096-300 K in both the normal and super conducting states. In the normal slate, the B-11 T dependence of T-1 displa yed several energy scales. In a narrow T range, between 1 and 2 K, 1/T1T wa s roughly constant. At lower temperatures and in a magnetic field exceeding the upper critical field, 1/T1T exhibited a monotonic decrease down to app roximately 0.18 K. Above roughly 4 K, 1/T-1 exhibited a weak T dependence u ntil similar to 60 K. At higher temperatures, additional relaxation mechani sms were present. The 1/T-1 values for both Be-9 and B-11 for all B concent rations were indistinguishable above similar to 4 K; however, at temperatur es of the order of 1 K, the x= 0.067 sample had values roughly twice that o f the x=0.030 sample for either nuclei. The x=0.030 sample had essentially the same 1/T-1 magnitude and T dependence as undoped UBe13. From the linear plot of the Knight shift (K-B) versus the static magnetic susceptibility ( chi), the B-11 hyperfine field coupling constant was calculated to be -361. 1 Oe/mu(B). An approximate analysis of the U moment electronic fluctuation rate (T) was performed using the measured values of 1/T-1, K-B, and chi. We observed a Gamma proportional to root T dependence between 8 and 70 K, whi ch is consistent with the existence of isolated local moment fluctuations. For T> 70 K, the Gamma analysis breaks down due to the presence of crystal field excitations. Measurements of 1/T1 indicated a moderate decrease in Ga mma with the addition of boron at low temperatures, where T(x= 0.030)lr(x= 0.067)approximate to 2. At lower temperatures, Gamma approached a constant value of the order of 1 meV at T approximate to 2 K. In the superconducting state, the T dependence of 1/T-1 exhibited an absence of power-law behavio r 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 B-11 1/T-1 increased with decreasing T below T-c. This indicates additional contributions to the 9Be 1/T-1 at low temperatures th at we attribute to spin diffusion processes that have negligible impact on the dilute B-11. Below T-c, the B-11 Knight shift was measured to be -0.08 (+/-0.01)% and was independent of both x and T. [S0163-1829(99)03602-4].