Existence of an Independent Phonon Bath in a Quantum Device

TL;DR

This paper provides experimental evidence that phonons in a mesoscopic quantum device can be thermally decoupled from the substrate phonon bath at sub-Kelvin temperatures, challenging previous assumptions.

Contribution

It demonstrates the existence of an independent phonon bath in a quantum device through direct measurements and a simple heat balance model.

Findings

Phonons in the device are thermally decoupled from the substrate.

A heat balance model explains the experimental data.

Phonon decoupling persists at sub-Kelvin temperatures.

Abstract

At low temperatures, the thermal wavelength of acoustic phonons in a metallic thin film on a substrate can widely exceed the film thickness. It is thus generally believed that a mesoscopic device operating at low temperature does not carry an individual phonon population. In this work, we provide direct experimental evidence for the thermal decoupling of phonons in a mesoscopic quantum device from its substrate phonon heat bath at a sub-Kelvin temperature. A simple heat balance model assuming an independent phonon bath following the usual electron-phonon and Kapitza coupling laws can account for all experimental observations.