Optical Properties of Ultrathin Films: Evidence for a Dielectric Anomaly at the Insulator-to-Metal Transition

Optical properties of ultrathin layers ($<50\AA$) of Au and Pb quench condensed on amorphous germanium ($\alpha \mathrm{\text{−}}\mathrm{G}\mathrm{e}$) have been measured in situ at 10 K. The development of these films from an insulating state to a metallic state is traced as a function of the film thickness as well as the sheet resistance, $R_{\square }$. Of particular interest is the regime of $R_{\square }$ near $3000\Omega$ where there is an anomaly in the optical transmission. This anomaly is due to a singularity in the dielectric function when the system undergoes an electronic percolation or insulator-to-metal ($I/M$) transition.