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

Abstract

Transmission x-ray diffraction experiments have been carried out on solid helium grown in porous Vycor glass. Measurements were made at temperatures near 0.7 K and at pressures up to 162 bars. The crystalline phases of solid helium in Vycor are found to differ significantly from the bulk. At pressures from 70 bars through 98 bars the helium is polycrystalline and displays a single size broadened scattering peak. Above 98 bars the peak splits into three close peaks. No higher order peaks are seen at any pressure, indicating significant reduction in intensity due to disorder. A broad peak is present at all pressures, which may indicate the presence of amorphous solid. We tentatively identify the low-pressure phase as bcc and the high-pressure phase as coexistence between bcc and hcp. Size broadening indicates an average grain size of approximately 5 nm, comparable to the Vycor pore size.

4He glass phase: A model for liquid elements

“…A similar amorphous S(Q) was also observed in 34 Å Gelsil [8]. The presence of an amorphous phase has been confirmed at higher pressures using porous Vycor glass [9]. In this work we consider that the supercooled liquid far below the melting temperature T m is condensed in a glass phase accompanied by an exothermic latent heat associated with a first-order transition.…”

“…This gives V = 60.3 mm 3 . The molar volume V m under pressure is interpolated from known values [3] using (9):…”

Topological Quantum Phases ofHe4Confined to Nanoporous Materials

“…in Ref. [33] at high pressure. It features three main peaks in the momentum region ∼ 2.0 − 2.2 Å−1 : one strong and two satellite peaks reminiscent of the three main Bragg peaks of hcp solid.…”

“…In the recent experiment [33], 4 He in Vycor was found to be in the bcc phase at P < 98 bar and T ≈ 0.5-0.7 K, whereas the transformation to the hcp solid takes place at higher pressure. The structure factor for CS and CSS (averaged over all orientations) found in our simulations and shown in Fig.…”

Prediction of Second Melting Temperatures Already Observed in Pure Elements by Molecular Dynamics Simulations

“…The enthalpy coefficients ∆ε lg0 of Phase 3 being equal to zero at T m , the glass transition occurs at T g during cooling through a first-order transition and an exothermic latent heat equal to ∆ε lg (θ g ) [ 72 ]. Liquid 4 He in a confined space under pressure is amorphous and undergoes a transition during heating which looks like a first-order transition [ 73 , 74 , 75 ].…”