Torsional oscillator and synchrotron x-ray experiments on solid 4He in aerogel

TL;DR

This study combines x-ray diffraction and torsional oscillator experiments to investigate solid helium-4 in aerogel, revealing a highly polycrystalline structure with a surprisingly small supersolid fraction similar to pure crystals.

Contribution

It provides new insights into the structure and supersolid behavior of helium-4 in aerogel, showing minimal supersolidity despite high disorder.

Findings

Solid 4He in aerogel is polycrystalline with ~100 nm crystallites.

Supersolid fraction is approximately 3x10^-4 at low temperatures.

Onset of supersolidity occurs around 100 mK.

Abstract

X-ray diffraction experiments show that solid 4He grown in aerogel is highly polycrystalline, with a hcp crystal structure (as in bulk) and a crystallite size of approximately 100 nm. In contrast to the expectation that the highly disordered solid will have a large supersolid fraction, torsional oscillator measurements show a behavior that is strikingly similar to high purity crystals grown from the superfluid phase. The low temperature supersolid fraction is only ~3x10-4 and the onset temperature is ~ 100 mK.