Model formalism of liquid 3He-B at equilibrium

Abstract

The approximate formal treatment of the nuclear spin system of normal liquid /sup 3/He given some time ago is extended to the ordered /sup 3/He phase. The formalism leads to the prediction of normal thermal behavior of /sup 3/He-B at lower pressures and at temperatures approaching its phase-boundary temperatures. In contrast to the disordered normal liquid phase, which is thermally anomalous, the entropy of the /sup 3/He-B decreases on isothermal compression, or its isobaric volume expansion coefficient is positive. The equilibrium thermal behavior of ordered /sup 3/He-B is thus qualitatively different from that of disordered liquid /sup 3/He. Experimental control of these aspects of the liquid /sup 3/He phase transformation is lacking at the present time. Both early and new /sup 3/He-B paramagnetic susceptibility data, extended recently over a wide reduced-temperature range, disclose a fundamental competition between the spontaneous ordering mechanism responsible for the existence of /sup 3/He-B and the specific ordering process imposed upon this phase on application of an external constant and uniform magnetic field. As a consequence, magnetized /sup 3/He-B will be shown to increase its entropy on isothermal magnetization and to cool on adiabatic magnetization. The magnetocaloric effect is, however, only moderate. The competition of themore » ordering process leads to the delay or possibly even to the suppression of the formation of the ordered phase, a state of affairs foreseen in our earlier work. At low or moderate magnetic field strengths, the zero-field phase-boundary temperatures are shown to shift toward lower temperatures while, simultaneously, the order of the phase change decreases, from second order, in the absence of the field, to first order. Although of model-theoretic character, involving limitations of various types, the rich physical content of /sup 3/He-B at equilibrium clearly emerges in the present work.« less