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

Abstract

We report extensive ${}^{11}\mathrm{B}$ and ${}^{9}\mathrm{Be}$ nuclear magnetic resonance (NMR) spin-lattice relaxation time ${(T}_{1})$ and Knight shift measurements in the heavy-fermion superconductor ${\mathrm{UBe}}_{13\ensuremath{-}x}{\mathrm{B}}_{x}(x=0.030,0.067).$ 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 ${}^{11}\mathrm{B}$ T dependence of ${T}_{1}$ displayed several energy scales. In a narrow T range, between 1 and 2 K, ${1/T}_{1}T$ was roughly constant. At lower temperatures and in a magnetic field exceeding the upper critical field, ${1/T}_{1}T$ exhibited a monotonic decrease down to approximately 0.18 K. Above roughly 4 K, ${1/T}_{1}$ exhibited a weak T dependence until \ensuremath{\sim}60 K. At higher temperatures, additional relaxation mechanisms were present. The ${1/T}_{1}$ values for both ${}^{9}\mathrm{Be}$ and ${}^{11}\mathrm{B}$ for all B concentrations were indistinguishable above \ensuremath{\sim}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 ${1/T}_{1}$ magnitude and T dependence as undoped ${\mathrm{UBe}}_{13}.$ From the linear plot of the Knight shift ${(K}_{\mathrm{B}})$ versus the static magnetic susceptibility (\ensuremath{\chi}), the ${}^{11}\mathrm{B}$ hyperfine field coupling constant was calculated to be $\ensuremath{-}361.1\mathrm{Oe}/{\ensuremath{\mu}}_{B}.$ An approximate analysis of the U moment electronic fluctuation rate (\ensuremath{\Gamma}) was performed using the measured values of ${1/T}_{1},{K}_{\mathrm{B}},$ and \ensuremath{\chi}. We observed a $\ensuremath{\Gamma}\ensuremath{\propto}\sqrt{T}$ dependence between 8 and 70 K, which is consistent with the existence of isolated local moment fluctuations. For $T>70\mathrm{K},$ the \ensuremath{\Gamma} analysis breaks down due to the presence of crystal field excitations. Measurements of ${1/T}_{1}$ indicated a moderate decrease in \ensuremath{\Gamma} with the addition of boron at low temperatures, where $\ensuremath{\Gamma}(x=0.030)/\ensuremath{\Gamma}(x=0.067)\ensuremath{\approx}2.$ At lower temperatures, \ensuremath{\Gamma} approached a constant value of the order of 1 meV at $T\ensuremath{\approx}2\mathrm{K}.$ In the superconducting state, the T dependence of ${1/T}_{1}$ 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 ${}^{9}\mathrm{Be}$ to ${}^{11}\mathrm{B}{1/T}_{1}$ increased with decreasing T below ${T}_{c}.$ This indicates additional contributions to the ${}^{9}\mathrm{Be}{1/T}_{1}$ at low temperatures that we attribute to spin diffusion processes that have negligible impact on the dilute ${}^{11}\mathrm{B}.$ Below ${T}_{c},$ the ${}^{11}\mathrm{B}$ Knight shift was measured to be -0.08 (\ifmmode\pm\else\textpm\fi{}0.01)% and was independent of both x and T.