### 3.F.3. Liquid *He*^{3}-*He*^{4} Mixtures

In a mixture of *He*^{3}-*He*^{4}
, one expects to find two types of phase transitions, namely, the superfluid
and the binary. This is shown in the
phase diagram in Fig.3.19 with *x*_{3}
indicating the molar fraction of *He*^{3}.

The *l*-line, extending from (
T=2.19K
,
x
3
=0
) to the tricritical point at (
T=0.87K
,
x
3
=0.67
), denotes the 2^{nd} order superfluid
phase transition.

The tricritical point is also the critical
point of the 1^{st} order binary phase transition. Thus, below
T=0.87K
,
the mixture can segregate into coexisting *He*^{3}
rich and *He*^{4} rich
regions. This can be observed using NMR
technique.

The tricritical point is so called because
it is actually the intersect of 3 lines of 2^{nd} order transitions.
[see R. B. Griffith, Phys. Rev. Lett. 24, 715 (1970)]