The fate of the superfluid density near the superconductor-insulator transition

Superconducting films of amorphous Indium Oxide (a:InO) undergo a transition to insulation with increasing disorder, which is due to the localization of pre-formed Cooper pairs. The continuous decrease in critical temperature as critical disorder is approached suggests an equally continuous suppression of superfluid density. In this talk I will discuss a systematic study of the superfluid density measured via plasmon dispersion spectroscopy of microwave resonators made of a:InO, combined with DC resistivity measurements, as a function of disorder. We observed that the superfluid stiffness defines the superconducting critical temperature over a wide range of disorder, highlighting the dominant role of phase fluctuations. Furthermore, we found that the superfluid density remains surprisingly finite at the critical disorder, indicating an unexpected first-order nature of the disorder-driven quantum phase transition to insulator.