Evidence for superconducting vortices in the insulating regime of the LaAlO3–SrTiO3 interface electron system
E. Fillis-Tsirakis, C. Richter, J. Mannhart, and H. Boschker
A superconducting phase with an extremely low carrier density of order 1013/cm2 is present at LaAlO3–SrTiO3 interfaces. If depleted from charge carriers by means of a gate field, this superconducting phase undergoes a transition into an insulating state that is, however, characterized by a gap in the spectral density of states. Measuring and analyzing the critical field of this gap, we provide evidence that macroscopically phase-coherent Cooper pairs are present in the insulating state. This state is characterized by fluctuating vortex-antivortex pairs, and not by individual, immobile Cooper pairs.