On the Role of Out-of-Equilibrium Phonons in Gated Superconducting Switches

TL;DR

This study demonstrates that high-energy electron currents, not electric fields, suppress superconductivity in nanowires through non-thermal phonon generation, highlighting a phonon-mediated mechanism distinct from heating effects.

Contribution

The paper provides experimental evidence that high-energy electron injection, rather than electric fields, suppresses superconductivity via non-thermal phonons, clarifying the underlying mechanism.

Findings

Superconductivity suppression depends on high-energy electron current, not electric field.

Injected electrons decay into phonons affecting the nanowire.

Non-thermal phonon distribution differs from Joule heating effects.

Abstract

Recent experiments suggest the possibility to tune superconductivity in metallic nanowires by application of modest gate voltages. It is largely debated whether the effect is due to an electric field at the superconductor surface or small currents of high-energy electrons. We shed light on this matter by studying the suppression of superconductivity in sample geometries where the roles of electric field and electron-current flow can be clearly separated. Our results show that suppression of superconductivity does not depend on the presence or absence of an electric field at the surface of the nanowire, but requires a current of high-energy electrons. The suppression is most efficient when electrons are injected into the nanowire, but similar results are obtained also when electrons are passed between two remote electrodes at a distance $d$ to the nanowire (with $d$ in excess of $1~\mathrm{\mu m}$). In the latter case, high-energy electrons decay into phonons which propagate through the substrate and affect superconductivity in the nanowire by generating quasiparticles. We show that this process involves a non-thermal phonon distribution, with marked differences from the loss of superconductivity due to Joule heating near the nanowire or an increase in the bath temperature.