First-order quantum breakdown of superconductivity in an amorphous superconductor

Abstract

Continuous quantum phase transitions are widely assumed and frequently observed in various systems of quantum particles or spins. Their characteristic trait is a second-order, gradual suppression of the order parameter as the quantum critical point is approached. The localization of Cooper pairs in disordered superconductors and the resulting breakdown of superconductivity have long stood as a prototypical example. Here we show a departure from this paradigm, in which a discontinuous first-order quantum phase transition is tuned by disorder. We measure the plasmon spectrum in superconducting microwave resonators on amorphous superconducting films of indium oxide to provide evidence for a marked jump in both the zero-temperature superfluid stiffness and the transition temperature at the critical disorder. This discontinuous transition sheds light on the role of repulsive interactions between Cooper pairs and the subsequent competition between superconductivity and insulating Cooper-pair glass. Furthermore, we show that the critical temperature of the films no longer relates to the pairing amplitude but aligns with the superfluid stiffness, consistent with the pseudogap regime of preformed Cooper pairs. Our findings raise fundamental new questions about the role of disorder in quantum phase transitions and carry implications for superinductances in quantum circuits.