Far-infrared absorption in small metallic particles

Far infrared transmission measurements were made on powder samples of Cu, Al, Sn, and Pb. The diameter of the particles ranged from 65 to 350. All of the samples showed absorption which was very small at low frequencies and which increased more or less linearly as the frequency increased. There was no change in the absorption as the temperature was varied from 1. 2 to 20 K. For the smaller particles absorption curves calculated from the modified theory of Gor'kov and Eliashberg give a good description of the data, although clear choices between different energy-level ensembles cannot be made. Our measurements provide direct evidence that, for superconducting particles of diameter 100 and mean energy-level spacing T₂, the energy gap structure in the density of states is either much attenuated or is completely absent.