Far-Infrared Absorption in Ultrafine Al Particles

Abstract

The first far-infrared measurements on metallic particles with diameters 10 nm are reported. The absorptivity is well approximated by ${\ensuremath{\alpha}}_{\mathrm{exp}}\ensuremath{\approx}{C}_{\mathrm{exp}}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\nu}}}^{2}$, where $3\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\nu}}150$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The results are compared with the Gor'kov-Eliashberg (GE) theory of particles with quantized electronic levels as well as with the classical Drude theory. When a size distribution is included, ${\ensuremath{\alpha}}_{\mathrm{GE}}={C}_{\mathrm{GE}}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\nu}}}^{2}$ irrespective of level statistics. However, ${C}_{\mathrm{exp}}\ensuremath{\gg}{C}_{\mathrm{GE}}$ and the observed dependence on the diameter is directly contrary to predictions of the GE theory. Neither can the data be reconciled with the classical theory.