Excitations of amorphous solid helium

TL;DR

This study uses neutron scattering to compare the dynamic structure factor of amorphous solid helium in nanopores with bulk solid helium, finding no evidence of superflow or unique amorphous modes, and observing melting behavior at higher temperatures.

Contribution

It provides the first detailed comparison of $S(Q,)$ between amorphous and polycrystalline solid helium confined in nanopores, highlighting their similarities and the absence of superfluid signatures.

Findings

No superflow or unique amorphous modes detected.

Amorphous and polycrystalline solids show similar $S(Q,)$.

Solid helium melts to a normal liquid with characteristic broad $S(Q,)$ at higher temperatures.

Abstract

We present neutron scattering measurements of the dynamic structure factor, $S(Q,\omega)$, of amorphous solid helium confined in 47 $\AA$ pore diameter MCM-41 at pressure 48.6 bar. At low temperature, $T$ = 0.05 K, we observe $S(Q,\omega)$ of the confined quantum amorphous solid plus the bulk polycrystalline solid between the MCM-41 powder grains. No liquid-like phonon-roton modes, other sharply defined modes at low energy ($\omega<$ 1.0 meV) or modes unique to a quantum amorphous solid that might suggest superflow are observed. Rather the $S(Q,\omega)$ of confined amorphous and bulk polycrystalline solid appear to be very similar. At higher temperature ($T>$ 1 K), the amorphous solid in the MCM-41 pores melts to a liquid which has a broad $S(Q,\omega)$ peaked near $\omega \simeq$ 0 characteristic of normal liquid $^4$He under pressure. Expressions for the $S(Q,\omega)$ of amorphous and polycrystalline solid helium are presented and compared. In previous measurements of liquid $^4$He confined in MCM-41 at lower pressure the intensity in the liquid roton mode decreases with increasing pressure until the roton vanishes at the solidification pressure (38 bars), consistent with no roton in the solid observed here.