The phase diagram of He4 is shown in Fig.3.15 and should be compared with that of a typical classical fluid shown in Fig.3.4. Salient points of interest are
1. The solid phase appears only for pressures above 25atm even as .
2. There are 2 liquid phases separated by a λ-line near . A λ-line is a line of λ-points, so called because the graph of C vs T near a λ-point looks like the letter λ. (see Fig.3.16)
3. The high temperature liquid phase is called the He I or normal phase. Its behavior is similar to a classical fluid.
5. He II exhibits frictionless flow and can leak through cracks impermeable to He gas. Hence the name superfluid. [cf. superconductor]
6. The He I-II transition is continuous with broken gauge symmetry. The order parameter is the macroscopic superfluid wave function.
7. The slopes of g-l and s-l coexistence curves goes to zero as , in accordance with the 3rd law.