Pressure-temperature phase diagram of an antiferromagnetic . Under an external parameter (here pressure) the magnetic order (AF) is suppressed and a Landau Fermi liquid regime (FL) is reached under pressure. Close to the critical pressure (pc) unconventional superconductivity appears. At low temperature the systems is dominated by strong quantum fluctuations.

We are interested in quantum phase transitions in heavy fermion compounds. These are intermetallic compounds with an incomplete filled electronic f shell, like Ce, Yb of U compounds showing strongly renormalized Fermi liquid properties at low temperature. This renormalization manifests itself in an extremely enhanced effective mass observed in specific heat or quantum oscillation experiments. The ground state of these systems is very sensitive to external pressure, thus the systems can be easily tuned to an antiferromagnetic or ferromagnetic magnetic instability. In the very low temperature limit the physical properties are determined by quantum fluctuations, which can give rise to new exotic ground states, e.g. the observation of superconductivity close to the point where the magnetic order vanishes.

Today two different theoretical approaches exist to understand such an quantum phase transition. The first one is based on a spin-fluctuation approach. At the critical point critical spin fluctuations diverge giving rise to a second order phase transition. Another approach, often called 'local' quantum criticality, supposes that the Kondo effect breaks down at the critical pressure as the f electrons are not coupled only to the conduction electrons, but also among themselves. In this picture abrupt modifications of the Fermi surface are expected.

Our research concentrates on the understanding of the quantum critical regime, and of the characterization of new phases appearing in this regime.

Further reading:

Knebel G, Aoki D, Braithwaite D, Brison JP, Lapertot G, Raymond S, Salce B and Flouquet J
Quantum criticality, antiferromagnetism and superconductivity.

Aoki D, Bourdarot F, Flouquet J, Hassinger E, Knebel G, Raymond S and Villaume A
Hidden Order State in URu₂Si2₂.

Aoki D, Flouquet J, Knebe Gl, Miyake A, Hassinger E, Mineev V, Huxley A, Levy F and Sheikin I
Ferromagnetic superconductivity.