Nanomechanical damping via electron-assisted relaxation of two-level systems

TL;DR

This study investigates how electron-assisted relaxation of two-level systems affects nanomechanical damping and noise at millikelvin temperatures, revealing the role of conduction electrons in dissipation mechanisms.

Contribution

It introduces a model incorporating electron and phonon interactions to explain damping and noise in nanomechanical devices, extending beyond standard tunneling models.

Findings

Excess damping observed in the normal state after magnetic switching.

Model with electron and phonon relaxation explains the data.

Increased 1/f noise in the normal state indicates electron impact.

Abstract

We report on measurements of dissipation and frequency noise at millikelvin temperatures of nanomechanical devices covered with aluminum. A clear excess damping is observed after switching the metallic layer from superconducting to the normal state with a magnetic field. Beyond the standard model of internal tunneling systems coupled to the phonon bath, here we consider the relaxation to the conduction electrons together with the nature of the mechanical dispersion laws for stressed/unstressed devices. With these key ingredients, a model describing the relaxation of two-level systems inside the structure due to interactions with electrons and phonons with well separated timescales captures the data. In addition, we measure an excess 1/f-type frequency noise in the normal state, which further emphasizes the impact of conduction electrons.