2-ORDER-PARAMETER PHENOMENOLOGICAL THEORY OF THE PHASE-DIAGRAM OF LIQUID-HELIUM

A two order-parameter phenomenological theory is proposed to explain s ome essential features of the phase diagram of liquid He-4. Among othe r things, the theory (i) distinguishes the liquid phase from the gas p hase in the P-T plane, (ii) yields a lambda line in the liquid phase w ith a negative slope, and (iii) gives a coexistence line between an or dered liquid phase and a normal gas phase which is a continuation belo w T-lambda of the normal Liquid-gas coexistence line but with a smalle r curvature. These results are in general agreement with the experimen tal situation in liquid He-4. The coupled nonlinear equations of the t heory are also solved numerically to gauge the accuracy of analytical results.