Spin-polarized 3He : liquid gas equilibrium

We study the effects of nuclear polarization M on the liquid-gas equilibrium phase diagram of 3He, in situations where the saturating vapour pressure increases or decreases as a function of M, and where a metamagnetic instability may occur in the liquid phase. Numerical predictions are given within the frame of a phenomenological model with parameters fitted to reproduce accurately the saturating vapour pressure of the unpolarized liquid as well as its magnetic susceptibility. At temperatures of the order of 0.5 K situations are found where interesting phenomena such as liquid overpolarization and metamagnetic plateaus occur. J. Physique 48 (1987) 1337-1350 AOÛT 1987, F Classification Physics Abstracts 67.50 64.70F There is currently an active interest in phase transitions in spin-polarized 3He. Several groups [1, 2] have recently obtained interesting results on the solid-liquid transition, using the polarization method proposed by Castaing and Nozi6res [3]. The results might indicate the existence of a metamagnetic transition (or near metamagnetic transition), as suggested by B. Castaing [4] and by K. Bedell and C. Sanchez-Castro [5]. Such a transition could also have an important effect on the superfluid properties of the liquid [6]. In the present article, we study another transition which takes place at lower pressures, the liquid-gas transition. Gaseous 3He can be strongly polarized by laser optical pumping [7], but also cooled down and condensed to form liquid 3He t , which should allow studying the effects of the nuclear polarization on the saturating vapour pressure. These effects are non-trivial [8] : for example, the nuclear polarizations of the two phases at equilibrium can be significantly different, because particle indistinguishability effects are more important in the (denser) liquid phase than in (dilute) gas phase. Liquefaction experiments of this type are in progress at the Ecole Normale [9]. Experimental data would give access to physical quantities which depend on many body effects and are difficult to calculate, such as the change of the binding energy of the liquid under the effect of spin polarization. Moreover, they might also reveal the existence of a metamagnetic transition in the liquid phase, as we see in more detail below. To study these problems, in the present article we use a simplified model already employed for investigating the thermodynamical properties of asymmetrical nuclear matter [10]. In a nucleus, the asymmetry (difference between the number of neutrons and protons) plays the role of spin polarization (difference between the number of spin up and spin down particles) in 3He system ; asymmetrical nuclear matter occurs in several important problems in astrophysics [11]. The model is based on the use of a Skyrme interaction [12] in the framework of a meanArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019870048080133700