Evidence for a quantum-vortex-liquid regime in ultrathin superconducting films

We report the observation of activated dissipation in ultrathin superconducting films near the superconductor-to-insulator transition (SIT) in magnetic fields. The activation energy To scales with the mean field transition temperature, film thickness, and logarithm of the applied field, in qualitative agreement with the activation of defects in a vortex solid. We find To extrapolates to zero at Ho, a field well below the upper critical field implying the existence of a quantum-vortex-liquid regime. We discuss the implications of this result for the magnetic field tuned SIT in this system.