Phonon cooling of nanomechanical beams with tunnel junctions

TL;DR

This paper reports the first demonstration of electronic cooling of 1D phonon modes in suspended nanowires using NIS tunnel junctions, achieving simultaneous electron and phonon cooling to sub-50 mK temperatures.

Contribution

It introduces a novel method for cooling phonons in nanomechanical beams via tunnel junctions and compares its effectiveness with non-suspended devices.

Findings

Achieved cooling of phonons and electrons to 42 mK.

Enhanced cooling performance in suspended nanowires.

Thermal transport limited by ballistic phonon scattering.

Abstract

We demonstrate electronic cooling of 1D phonon modes in suspended nanowires for the first time, using normal metal--insulator--superconductor (NIS) tunnel junctions. Simultaneous cooling of both electrons and phonons to a common temperature was achieved. In comparison with non-suspended devices, better cooling performance is achieved in the whole operating range of bath temperatures between 0.1-0.7 K. The observed low-temperature thermal transport characteristics are consistent with scattering of ballistic phonons at the nanowire-bulk contact as being the mechanism limiting thermal transport. At the lowest bath temperature of the experiment $\sim$ 100 mK, both phonons and electrons in the beam were cooled down to 42 mK, which is below the refrigerator bath temperature.