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

Abstract

We present results on neutron scattering in solid 4 He in the range of parameters where supersolidity is observed. The measurements address, among other questions, the viability of one possible mechanism of supersolidity: via a metastable amorphous phase. We have attempted to observe a glassy phase by neutron scattering. We have found that it is impossible to do this by total scattering, as it would be common in a classical solid, due to an extremely large inelastic diffuse signal related to the anomalously strong zero-point motion of helium atoms. This raises a general question on the interpretation of such scattering as the signature of an amorphous phase. Results from energy-resolved elastic scattering are heavily influenced by multiple scattering of neutrons which may be the major contribution to the measured elastic signal, but allow to put the limit on the concentration of an amorphous phase to 5% in a polycrystal with millimeter-size crystallites and to 2% in a single crystal. The values of NCRIf, expected from these limits should be much lower, although exact values depend strongly on a particular model of glass-related supersolidity.

Pressure-dependent phase transformation of solid helium confined within a nanoporous material

“…23 This explanation has become less compelling since NCRI now appears to be accounted for in terms of sheer modulus stiffening. 24 While earlier experiments have claimed to observe amorphous helium in porous materials 13 these results have been called into question by the recent neutron scattering results of Mukharsky et al 14 They report inelastic neutron scattering measurements which imply that all the observed diffuse scattering is dynamic, and hence does not result from an amorphous solid. Further they find that the diffuse scattering of the crystalline phase is nearly identical to that from the liquid phase.…”

“…6,7 The freezing pressure is sensitive to confinement in small pores. 8,9 In porous Vycor glass, with a typical pore diameter of 6-7 nm, the low-temperature freezing pressure can be raised by [12][13][14][15] bars. This is believed to be due to the fact that the crystalline solid does not wet the amorphous layer that forms on the pore walls, and that crystallites form by homogeneous nucleation.…”