Phase Separation of LiquidHe3-He

Abstract

We study the phase separation of helium mixtures inside thin channels in contact with a bulk 'He bath at zero temperature.It is found that the substrate potential favors mixing near the walls. While in bulk, the transition takes place between a He-rich mixture with 6.6% of He and pure 'He; in thin channels, the He-rich phase is richer in He and the He-rich phase contains a nonzero fraction of 4He. We anticipate that both phases can be superfluid, so that the superAuid line is detached from the coexistence region. The behavior of helium mixtures in porous aerogel is discussed in relation to the present findings.A mixture of liquid 3He and 4He is a fascinating binary fluid in which superfluid transitions and phase separations can be studied in a variety of situations. Recent experiments by Chan and collaborators [1]on helium mixtures in porous aerogel have lead to surprising new features which are not well understood. In the case of aerogel with 0.98 porosity, it is suggested that the phase diagram of 3He-4He mixtures is modified with respect to the bulk one. A new phase diagram is proposed for this system, in which the coexistence region is detached from the superfluid transition line, giving rise to an intriguing 3He-rich superfluid mixture. Experimental evidence for this phase appears to persist down to zero temperature, with a 4He concentration as large as 20%. In contrast, 4He atoms are known to be completely excluded from bulk liquid 3He.There are two ways by which the aerogel may affect the helium phase diagram, namely disorder and confinement.Disorder is produced by the random interconnections of the delicate silica strands, about 30 A in diameter, constituting the gel, leaving open spaces of various size in which the liquid is confined In the p. resent Letter, we will not discuss the effect of disorder and focus our interest essentially on the effect of confinement on the phase separation at zero temperature. In this exploratory study, we use a simple geometry and consider helium mixtures inside thin channels of various size. In aerogel with 0.98 open volume fraction the size of the large pores are of the order of a few hundred angstroms. One thus expects in those pores a bulk behavior. Since we are interested mainly in the 3He-rich phases, there is no 4He in the bulk, so that the 3He chemical potential p, 3 is fixed to its bulk value, i.e. , -2.49 K. In other words-! and this is an essential aspect of the present workwe look for mixtures in thin channels in thermodynamical equilibirum with bulk 3He. Our main result is that both the size of the channel and the strength of the substrate potential drive the phase separation. The implications are twofold: (i) In contradiction to a common belief based on comparing the zero point motion of a 3He and a He atom, the liquid in the vicinity of the substrate is not pure 4He, but a 3He-He mixture with comparable concentrations of both isotopes. (ii) With decreasing channel thickness, the phase separation involves at the transition a He-rich phase which gets richer in...

Quasi-one-dimensional parahydrogen in nanopores

“…27, except those for Glass, which were obtained starting from those proposed in Ref. 28 for Helium atoms near a glass substrate, using Lorentz-Berthelot mixing rule to adjust them for p-H 2 . Our focus here is mosty on metallic alkali substrates which are known to be weak adsorbers, for the purpose of identifying physical conditions in which the equilibrium phase of the system is quasi-1D, i.e., with p-H 2 molecules lined up along the axis of the channel.…”

Enhanced superfluid response of parahydrogen in nanoscale confinement

“…The second choice, namely D = 100 K and a = 2.05Å, is roughly in the ballpark of what one would expect for p-H 2 molecules near a silica substrate [20]; thus we shall henceforth refer to the scenario described by this parameter set as glass. It should be stressed, however, that in neither case do we aim at reproducing accurately any realistic interaction (which requires more elaborate functional forms anyway), as that turns out not to matter much in the end, as we show below.…”

Ordering of Helium Mixtures in Porous Gold

“…Three separate experiments using different techniques found only a single superfluid transition phase boundary at high X 3 and the phase boundary originating from low X 3 and low T appears to fade away at intermediate X 3 without attaching to the superfluid transition boundary [5][6][7]. Recent theoretical [8][9][10] and experimental [3,7,11] efforts have shed substantial understanding on the mixture in aerogel systems. However, there is as yet no clear microscopic picture of the coexisting phases in the lighter aerogel, and there is no satisfactory explanation of the findings in 87% aerogel.…”