Microwave radiometry of a quantum-critical, hybrid Josephson array

TL;DR

This study introduces a microwave radiometry technique to investigate a hybrid Josephson array across different regimes, revealing unique thermal and non-equilibrium behaviors near quantum criticality.

Contribution

The paper presents a novel microwave radiometry method for in-situ analysis of Josephson arrays, enabling detailed study of thermal and quantum-critical phenomena.

Findings

Anomalous metal is hotter than other regimes.

Anomalous-metallic regime is more prone to heating.

Nonlinear radiative noise scaling observed at quantum criticality.

Abstract

Arrays of Josephson junctions can be tuned through anomalous metallic, quantum-critical, and insulating regimes. We introduce a new experimental probe, capturing microwave radiation across all three regimes, using a two-dimensional array of superconductor-semiconductor hybrid Josephson junctions as a model system. Our approach allows in-situ calibration of the sample's circuit parameters and provides isolation from measurement back-action effects. We measure the radiation temperature of the anomalous metal, and find that it is hotter than both the quantum-critical and insulating regimes. We further show that the anomalous-metallic regime is more susceptible to additional heating than other regimes, explaining its emergence in otherwise thermalized systems. Turning to the quantum-critical regime, we discover nonlinear scaling of radiative noise with applied bias, consistent with theoretical predictions of universal non-equilibrium behavior at quantum critical points.