Evidence for an Anderson transition in granular Sn films

Conductivity and density-of-states measurements have been performed on granular Sn films with room-temperature conductivities σ(300 K) between 1 and ${10}^4$ ${\Omega }^{\mathrm{-}1}$ ${\mathrm{cm}}^{\mathrm{-}1}$. Our conductivity measurements indicate that superconductivity as well as normal metallic behavior disappear at the metal-insulator transition (${\sigma }_c$≃40 ${\Omega }^{\mathrm{-}1}$ ${\mathrm{cm}}^{\mathrm{-}1}$). The anomalous magnetoconductivity in the metallic phase is consistent with the scaling picture for the Anderson transition. The temperature dependence of the conductivity is more complex, probably due to the influence of the granular film structure. The importance of the enhanced Coulomb repulsion near the Anderson transition is directly reflected by the appearance of a square-root anomaly in the density of states around the Fermi level. The granularity of the Sn films causes this anomaly to be considerably smaller than for homogeneous amorphous metals.