Local spectroscopies across the superconductor-insulator transition

Abstract

We explore the manifestation of quantum fluctuations across the superconductor-insulator transition (SIT) in three different local measurements: the two-particle density of states P(r,ω), compressibility κ(r), and diamagnetic susceptibility χ(r) revealed through their local maps. The map of χ(r) displays growing fluctuations upon increasing temperature as well as upon tuning the quantum tuning parameter g, as expected, but remarkably, these fluctuations persist well below Tc as the SIT is approached, indicating the quantum nature of these fluctuations. The map of κ(r) paints a similar picture when tuned via g, but in contrast to χ(r), we find a fundamental difference in its evolution with temperature, providing a complementary local probe to χ(r). P(r,ω), obtained using maximum entropy analytic continuation of Monte Carlo simulations on 2D quantum Josephson junction arrays, shows strongly diminished zero-energy spectral weight in nearly-insulating islands, correlating with regions of suppressed κ(r). We discuss the experimental implications of our results for scanning Josephson spectroscopy, compressibility, and scanning SQUID measurements in the context of quantum fluctuations.

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