# Probing superfluid $^4\mathrm{He}$ with high-frequency nanomechanical resonators down to $\mathrm{mK}$ temperatures

## TL;DR

This study uses high-quality nanomechanical resonators to investigate superfluid helium-4 across a wide temperature range, revealing different damping mechanisms from viscosity to ballistic quasiparticles and acoustic emission.

## Contribution

It demonstrates the capability of a single nanobeam device to probe multiple superfluid regimes over six orders of magnitude in damping, down to millikelvin temperatures.

## Key findings

- Damping dominated by normal fluid viscosity above 1 K
- Ballistic quasiparticles influence damping between 0.3-0.8 K
- Damping saturates at low temperatures due to losses or acoustic emission

## Abstract

Superfluids, such as superfluid $^3\mathrm{He}$ and $^4\mathrm{He}$, exhibit a broad range of quantum phenomena and excitations which are unique to these systems. Nanoscale mechanical resonators are sensitive and versatile force detectors with the ability to operate over many orders of magnitude in damping. Using nanomechanical-doubly clamped beams of extremely high quality factors ($Q>10^6$), we probe superfluid $^4\mathrm{He}$ from the superfluid transition temperature down to $\mathrm{mK}$ temperatures at frequencies up to $11.6 \, \mathrm{MHz}$. Our studies show that nanobeam damping is dominated by hydrodynamic viscosity of the normal component of $^4\mathrm{He}$ above $1\,\mathrm{K}$. In the temperature range $0.3-0.8\,\mathrm{K}$, the ballistic quasiparticles (phonons and rotons) determine the beams' behavior. At lower temperatures, damping saturates and is determined either by magnetomotive losses or acoustic emission into helium. It is remarkable that all these distinct regimes can be extracted with just a single device, despite damping changing over six orders of magnitude.