Set of supplementary notes.

158 CHAPTER 10. FERMI LIQUID THEORY
10.4.4 Quasiparticles detected in de Haas-van Alphen experiments
Direct evidence for the existence of a heavy Fermi liquid state has come from the observation of
quantum oscillations (see Lent term part) in a number of heavy fermion materials. Fig. 10.11
shows one of the clearest examples, UPt 3 . The volume enclosed by the Fermi surface is a very
stringent criterion, which can be used to decide between the heavy Fermi liquid scenario – in
which the f-electrons contribute to the Fermi surface – and local moment models. Moreover,
the temperature dependence of the signal observed in quantum oscillation measurements can
be used to determine the effective mass of the quasiparticles. This can be compared against the
measured heat capacity. Where these comparisons were possible, such as in UPt 3 , the measured
heat capacity was consistent with what would be expected from the measured effective masses.
10.4.5 Heavy fermions, summary:
• Many intermetallic compounds containing elements with partially filled 4f (Ce, Yb) or 5f
(U) orbitals show heavy fermion behaviour.
• Their low temperature properties are consistent with Fermi liquid theory, if we assume
very high effective carrier masses.
• What is the role of electrons in partially filled f-orbitals Do they behave like local
moments or like conduction electrons Where quantum oscillation studies have been
successful, they indicate that at low temperature the electrons contribute to the Fermi
surface, like in a normal metal. At high temperature, on the other hand, they behave like
local moments.
• Because g(E F ) is so high in these materials, they tend to order magnetically or even
become superconducting. There are many different ordered low temperature states in
these metals, some simple, some very exotic.
• There is an increasing number of materials (e.g. YbRh 2 Si 2 ), which do not follow Fermi
liquid theory at low T . Are they ’non-Fermi’ liquids Do the f-electrons remain as local
moments down to absolute 0 in this case, not contributing to the Fermi surface This is
being investigated at the moment.