Phonon bottleneck in graphene-based Josephson junctions at millikelvin temperatures

TL;DR

This paper investigates electron overheating and phonon interactions in graphene-based Josephson junctions at millikelvin temperatures, revealing a phonon bottleneck effect that influences superconducting transitions.

Contribution

It demonstrates that electron overheating causes hysteresis in Josephson junctions and identifies a phonon bottleneck limiting electron cooling at very low temperatures.

Findings

Hysteresis is due to electron overheating.

Retrapping current corresponds to an elevated critical temperature.

Electron-phonon interactions follow a $P\propto T^3$ relationship.

Abstract

We examine the nature of the transitions between the normal and the superconducting branches of superconductor-graphene-superconductor Josephson junctions. We attribute the hysteresis between the switching (superconducting to normal) and retrapping (normal to superconducting) transitions to electron overheating. In particular, we demonstrate that the retrapping current corresponds to the critical current at an elevated temperature, where the heating is caused by the retrapping current itself. The superconducting gap in the leads suppresses the hot electron outflow, allowing us to further study electron thermalization by phonons at low temperatures ($T \lesssim 1$K). The relationship between the applied power and the electron temperature was found to be $P\propto T^3$, which we argue is consistent with cooling due to electron-phonon interactions.