Size Effect of Nuclear Spin Relaxation Time in Superconducting Aluminum

Abstract

Particles of aluminum having diameters less than 2000 \AA{} were produced by evaporation in an argon atmosphere. The nuclear spin relaxation time was measured for three such samples in the normal and superconducting states between 0.36\ifmmode^\circ\else\textdegree\fi{} and 1.3\ifmmode^\circ\else\textdegree\fi{}K, for applied fields up to 400 G. In the normal state, and near ${T}_{c}$, the relaxation time is nearly the same as for bulk aluminum. Well below ${T}_{c}$ the zero-field relaxation time is shorter than that of bulk aluminum, being less than one-fifth the bulk value at 0.4\ifmmode^\circ\else\textdegree\fi{}K for two of the samples studied. At low temperatures there is also a field dependence which is characteristic of the superconducting state; ${T}_{1}$ increases with increasing field. At all fields, and low temperatures, the slope of a plot of $\mathrm{ln}{T}_{1}$ versus $\frac{{T}_{c}}{T}$ indicates a gap considerably smaller than the bulk value, but the limited temperature range covered makes such conclusions dubious.