Dynamics of a $^4$He quantum crystal in the superfluid liquid

TL;DR

This study investigates how helium crystals fall in superfluid helium at different temperatures, revealing the influence of the normal component and surface mobility on their motion and associated pressure changes.

Contribution

It provides experimental insights into the dynamics of helium crystals in superfluid helium, highlighting the role of surface mobility and the normal component in their motion.

Findings

Higher fall rate at 1.25 K indicates normal component influence.

Surface mobility affects pressure changes during crystal fall.

Crystal growth occurs during flow in superfluid helium.

Abstract

The motion of helium crystals has been experimentally studied when the crystals fall in the superfluid liquid owing to gravity at temperatures above the roughening transitions where the whole crystal surface is in the atomically rough state. The rate of crystal fall at T = 1.25 K is higher than at T = 1.54 K. This is proof of the essential role of the normal component of superfluid helium in the deceleration of crystal motion. The pressure measurements in the container have shown the effect of surface kinetics on the motions of the crystal and its size. The fall of crystals with the low surface mobility at T = 1.54 K does not change the pressure significantly. The high surface mobility at T = 1.25 K results in decreasing the pressure in the container in the course of the fall of a crystal. The pressure drop exceeds the difference in the hydrostatic pressure between the initial and final positions of the crystal. After the stop, the pressure in the container relaxes to the difference mentioned above. This fact demonstrates an additional growth of the crystal in the flow of a superfluid liquid.