Vortex excitations in the insulating state of an oxide interface

In two-dimensional (2D) superconductors, an insulating state can be induced either by increasing disorder or by applying a magnetic field H. Many scenarios have been put forth to explain the superconductor to insulator transition (SIT). One of the main difficulties in discerning between the various scenarios is to elucidate the nature of the emergent insulating state. This complicatedness stems from the lack of a continuous mapping of the superconducting to insulating phase diagram in a single sample. Here we use the 2D electron liquid formed at the interface between (111) SrTiO3 and LaAlO3 to study the SIT as a function of electrostatic gate and magnetic field. This crystalline interface exhibits very prominent features: (a) a very large and anisotropic magnetoresistance (MR) peak emerging from the superconducting state and persisting even when the sample is totally insulating, (b) hysteresis in the MR peak, and (c) linear MR for low perpendicular magnetic fields. We identify a new magnetic field scale, where the superconducting fluctuations are muted, and find a length scale ξins for the vortex fluctuation in the insulating state. Our findings suggest that vortex excitations and Cooper pair localization are responsible for the observed SIT. Surprisingly, these excitations exist even when the sample is tuned into the zero-field insulating state.