3.F.1. Liquid He4

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 T0 .

2.         There are 2 liquid phases separated by a λ-line near T2K .  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.

4.         The low temperature liquid phase is called the He II or superfluid phase.  It’s the first discovered example of a quantum 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 T0 , in accordance with the 3rd law.