Gapless superconductivity in the low-frequency electrodynamic response of two-dimensional granular In/InO$_x$ composites

TL;DR

This study investigates the electrodynamic response of two-dimensional granular In/InO$_x$ composites, revealing gapless superconductivity and quantum critical behavior near a superconductor-metal transition through complex ac conductance measurements.

Contribution

It demonstrates how annealing tunes inter-grain coupling, suppresses the anomalous metallic phase, and uncovers gapless superconductivity and quantum criticality in granular superconductors.

Findings

Elimination of the anomalous metallic phase with increased annealing.

Observation of a $T o0$ dissipative response coexisting with superfluid density.

Identification of quantum critical behavior near the superconductor-metal transition.

Abstract

We measured the full complex ac conductance of two-dimensional granular In/InO$_x$ composites using the mutual inductance technique to explore the transition from a "failed-superconductor-turned anomalous metal" to a robust superconductor. In this system, room-temperature annealing was adopted to tune the InO$_x$-mediated coupling between In grains, allowing for the observation of both a "true" superconductor-to-insulator transition and the emergence of an intervening anomalous metallic state. In this paper, we show that further annealing increases the inter-grain coupling, which eliminates the anomalous metallic phase, but at the same time prevent the emergence of strong Bose-dominated insulating phase. The complex ac conductance revealed a $T\to0$ saturating dissipative response in a finite magnetic field, coexisting with a robust superfluid density. The anomalous power-law spectra for the dissipative response appear to indicate quantum critical behavior proximate to a quantum superconductor to anomalous-metal transition as probed in the kilo-Hertz range, and point to signatures of gapless superconductivity in our granular superconducting system.