Resistive transitions in ultrathin superconducting films: Possible evidence for quantum tunneling of vortices

Unusual temperature dependence of electrical resistance has been observed in ultrathin superconducting films of Pb, Al, and Bi. In small magnetic fields, at low temperatures, sheet resistances varied with temperature as R≊${\mathit{R}}_0$ exp(T/${\mathit{T}}_0$), where ${\mathit{T}}_0$ and ${\mathit{R}}_0$ are constants. This result is not expected in conventional models of vortex motion involving thermal activation. On the other hand, it may be explained in a quantum tunneling picture.