Excitations of amorphous solid helium

Abstract

We present neutron scattering measurements of the dynamic structure factor, S(Q, ω), of amorphous solid helium confined in 47Å pore diameter MCM-41 at pressure 48.6 bar. At low temperature, T = 0.05 K, we observe S(Q, ω) 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 (ω < 1.0 meV) or modes unique to a quantum amorphous solid that might suggest superflow are observed. Rather the S(Q, ω) 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, ω) peaked near ω ≃ 0 characteristic of normal liquid 4 He under pressure. Expressions for the S(Q, ω) 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.

Saga of Superfluid Solids

“…Neutron scattering experiments have also been used for studying the dynamic structure factor of amorphous solid helium. No liquid-like phonon-roton modes or other sharply defined modes at low energy or modes unique to a quantum amorphous solid that might suggest superflow are observed [117]. The maximal studied mode energy was lower than 1 meV, i.e., ω(k) < 10 K.…”

Saga of Superfluid Solids

“…Neutron scattering experiments have also been used for studying the dynamic structure factor of amorphous solid helium. No liquid-like phonon-roton modes or other sharply defined modes at low energy or modes unique to a quantum amorphous solid that might suggest superflow are observed [117]. The maximal studied mode energy was lower than 1 meV, that is ω(k) < 10 K.…”

Static and dynamic structure factor in solid 4 He: Absence of a glassy phase

“…-To differentiate between elastic and inelastic diffuse scattering we then performed further experiments using the cold neutron time-focusing time-of-flight spectrometer IN6. Pure elastic signal arising from amorphous solid 4 He confined in porous media was recently observed using this technique [40]. A monochromatic neutron beam is selected using a composite pyrolytic graphic monochromator, and neutron flight timing is synchronized using a Fermi chopper.…”