Effect of Crystal Growth Rate on Liquid-like Droplets Formation in the hcp Solid Helium

TL;DR

This study investigates how different crystal growth rates affect the formation and size of liquid-like droplets in solid helium using NMR, revealing that faster growth rates produce more and larger droplets, which diminish with slower growth and annealing.

Contribution

It demonstrates the direct influence of crystal growth rate on liquid-like droplet formation and size in solid helium, using NMR measurements to quantify these effects.

Findings

Faster growth rates lead to more liquid-like inclusions.

Slower growth results in smaller droplets and fewer inclusions.

Liquid-like droplets diminish after annealing near the melting curve.

Abstract

The samples of hcp solid helium (1% $^3$He in $^4$He) are studied by NMR technique. The samples are grown by the blocking capillary method at different growth rates (about 8, 2, and 0.08 mK/s). The NMR technique is used for phase identification by measurements of the diffusion coefficient $D$ and the spin-spin relaxation time $T_2$ at temperatures of $1.3-2.0$ K and pressures of 34-36 bar. Along with $D$ and $T_2$ for the hcp phase, we simultaneously observe the $D$ and $T_2$ typical for a liquid at growth rates 8 and 2 mK/s. It means that liquid-like inclusions are quenched from the melting curve during fast crystallization of the samples. It is also shown that the slower growth rate corresponds to a smaller size of liquid-like droplets. It results from lower spatially restricted values of $D$ and, finally, absence of these inclusions at the longest crystallization times. The diffusion coefficient measured for liquid-like droplets is also decreasing during the NMR experiment at constant temperature which indicates the reduction of the size of these droplets. Liquid-like droplets are shown to disappear after sample annealing near the melting curve.