## Non-Fermi Liquids

'Metals without electrons' seems like a paradox, for the very notion of a metal, we learn, relies on electrons becoming delocalized from their atomic hosts to form a conduction sea. Yet including the effects of quantum mechanics and inter-particle interactions one often finds that the 'particles' which characterize a strongly correlated quantum fluid can be very different from the initial ingredients in the problem.

In bringing atoms together to form a crystalline solid, for example, one finds that the low energy properties are best described in terms of a gas of weakly interacting Bose particles – phonons – which are the quanta of the lattice vibrations.

It should therefore be a surprise that we can very often think of
the electrons in a metal simply as a non interacting gas of
electrons. The reason this works so well was first formulated by
Landau in the 1950s in his **Fermi-liquid** theory. He
showed that really this picture is describing electron-like
'quasiparticles' which share many of the properties of electrons but
are in fact complex many-body approximations to eigenstates of the
system. These approximate eigenstates become an increasingly better
approximation as the temperature (or other relevant energy-scale) is
reduced.

Recent years have seen tremendous advances in material science
which has resulted in the discovery of many new types of complex
metals. Among them include the various oxide metals such as
high
temperature cuprate superconductors, the ruthenates, the manganate CMR
materials, as well as f electron alloys such as UBe_{13}
picture above. Many of these seem to lie outside Landau's Fermi liquid
paradigm: the electron-like quasiparticle does not appear to describe
their properties. It seems that we need a new picture of the
correlated quantum fluid to describe these new materials. Our research
is focused towards this endeavour.

For some general information on current topics in physics of strongly correlated electron systems, see

- Quantum Matter - the challenge to theory

An invited talk given at the Institute of Physics Einstein Centenial Meeting (Physics - a century after Einstein) - New states of correlated electrons

The Future Conference.

Current research work by the group in this area includes the theory of quantum critical points, and spin-charge separation in Luttinger liquids.