Brownian motion in superfluid $^4$He

TL;DR

This paper investigates the thermal fluctuations of quasiparticles in superfluid helium-4 by studying the Brownian motion of a microsphere, revealing the significant role of rotons at higher temperatures.

Contribution

It introduces a method to analyze Brownian motion in superfluid helium-4, highlighting the importance of rotons in thermal fluctuations at elevated temperatures.

Findings

Resonant mode of fluctuational motion can be resolved with current laser technology.

Roton contribution becomes dominant above 0.76 K.

Temperature dependence of friction and random force strength analyzed.

Abstract

We propose to study the Brownian motion of a classical microsphere submerged in superfluid $^4$He using the recent laser technology as a direct investigation of the thermal fluctuations of quasiparticles in the quantum fluid. By calculating the temperature dependence of both the friction coefficient and the strength of the random force, we show that the resonant mode of the fluctuational motion can be fully resolved by the present technology. Contrary to the previous work, it is found that the roton contribution is not negligible, and it even becomes dominant when the temperature is above 0.76\,K.