Nuclear spin--lattice relaxation in pure and impure indium. II. Superconducting state

Nuclear spin-lattice relaxation times T₁(T) have been measured in superconducting indium and the InGa, InCd, InTl, InHg, and InPb 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 $\frac{T_c}{T}$, T₁ 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 〈a²〉=0.005±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 T₁ in aluminum alloys. We find 〈a²〉=0.003±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.