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 $T_0$ 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 $T_0$ extrapolates to zero at $H_0$, 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.