Microwave spectroscopy of two-dimensional superconductivity at LaAlO₃/SrTiO₃(111) interfaces

The emergent two-dimensional electron system (2DES) formed at the interface between LaAlO₃ (LAO) and SrTiO₃ (STO) insulating oxides has been a subject of great attention in condensed matter physics during the last decade. Recently, (111)-oriented LAO/STO interfaces have been shown to exhibit an electronic correlation driven reconstruction of its band structure and a 2D superconducting (SC) ground state, both tunable by electrostatic field-effect.

Here, in order to study superconductivity at the LAO/STO(111) interface, we have designed embedded SC coplanar waveguide resonators, whose resonance frequency can be tuned by electrostatic gating through a concomitant change of the 2DES kinetic inductance. We systematically map the superfluid density as a function of electron doping and temperature, allowing us to report the SC phase diagram of this 2D system in a detection scheme that goes beyond traditional resistive measurements. Our study reveals the dilute nature of the superconducting condensate, and a temperature dependence of the superconducting gap at odd with a clean BCS scenario. Our work highlights the potential of such an approach to the fundamental study of superconductiviy of quantum materials.