Size Effect of Nuclear Spin Relaxation Time in Superconducting Aluminum

Particles of aluminum having diameters less than 2000 Å 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° and 1.3°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°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 $\ln T_1$ versus $\frac{T_c}{T}$ indicates a gap considerably smaller than the bulk value, but the limited temperature range covered makes such conclusions dubious.