Nuclear Spin-Lattice Relaxation in Pure and Impure Indium. II. Superconducting State

Abstract

Nuclear spin-lattice relaxation times $T_1\mathrm{}\left(T\right)\mathrm{}$ have been measured in superconducting indium and the $In\mathrm{Ga}$, $In\mathrm{Cd}$, $In\mathrm{Tl}$, $In\mathrm{Hg}$, and $In\mathrm{Pb}$ dilute-alloy systems, using a transient nuclear-quadrupole-resonance spectrometer. Local sample heating was identified and its effects minimized. No effects attributable to trapped flux were observed. For fixed $\frac{T_c}{T}$, $T_1$ decreases with increasing impurity concentration, which indicates that the anisotropy of the energy gap decreases with decreasing mean free path $l$. Although the effects of a mean-square anisotropy $〈a^2〉=0.005\mathrm{}\pm 0.001$ are essentially averaged out for $l\lesssim {\xi }_0$ (the pair dimension), the data indicate additional gap broadening in the more concentrated alloys. Lifetime effects associated with thermal-phonon scattering of the quasiparticles are insufficient to explain the data, and we tentatively attribute the additional temperature- and solute-independent broadening to a 2% rms energy-gap inhomogeneity. Similar behavior is noted in previous measurements to $T_1$ in aluminum alloys. We find $〈a^2〉=0.003\mathrm{}\pm 0.001$ for pure aluminum and suggest that an energy-gap inhomogeneity of about 1% rms determines the density of excited states in concentrated aluminum alloys.