Infrared Studies of the Onset of Conductivity in Ultrathin Pb Films

In this paper we report the first experimental measurement of the low-temperature, normal-state infrared conductivity of ultrathin quenched-condensed Pb films. For dc sheet resistances such that $\omega \tau \ll 1$ the ac conductance increases with frequency but is in disagreement with the predictions of weak localization. We attribute this behavior to the effects of an inhomogeneous granular structure in these films when they are first formed, which is manifested at the very small probing scale of infrared measurements. Our data are consistent with predictions of two-dimensional percolation theory.