| Electrical transport properties were measured in ultrathin films of Ag, Bi, Pb, Pd and Ga deposited onto liquid-helium cooled substrates. The properties of these films depend strongly on thickness. For all materials, the thinnest films are insulating, with conduction by variable range hopping. The temperature dependence of resistance is given by R(T) = R0exp(T0/T) x with x = 0.75 +/- 0.05, for a wide range of films of the first four materials, not explained by any of the traditional models. We argue that this anomalous hopping exponent is a general property of very strongly disordered systems, consistent with a collective hopping mechanism, rather than the result of an improper fit or crossover behavior. As the thickness is increased, these four materials enter a weakly localized regime where conductance depends logarithmically on temperature. Bismuth and Pb, deposited on amorphous Ge, exhibit a direct insulator to superconductor transition as thickness is further increased. Silver and Pd never become superconducting. Gallium films deposited directly onto glazed alumina substrates exhibit local superconductivity, with a dip in R(T) at the bulk transition temperature of amorphous Ga, before becoming globally superconducting with increased thickness. These films have a large range of temperature independent resistance at the lowest temperatures. We report nonlinear current-voltage characteristics in both high and low resistance regimes, pointing out the qualitative electrical duality between these two regimes. We suggest an interpretation in terms of the depinning of a Cooper pair charge density wave or crystal in the high resistance films, and the depinning of a vortex lattice in the low resistance films, and argue that this collective picture is a more likely description of the observed effects than single junction effects. Below a voltage threshold in the high resistance films, conductance may be the result of quasiparticles created by thermal activation and quantum fluctuations. Below a current threshold in the low resistance films, resistance may be the result of quantum tunneling of vortices. Spanning these two regimes, a Bose metal phase is predicted, possibly stabilized by dissipation. The resistance of Ga films is fit by the predicted dependence on normal state sheet resistance of one Bose metal model. |
