### 3.F.1. Liquid *He*^{4}

The phase diagram of *He*^{4} 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
T→0
.

2.
There are 2 liquid phases
separated by a *λ*-line near
T≃2K
.
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
T→0
,
in accordance with the 3^{rd} law.