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Condensed matter physics studies materials made of a huge number of interacting elementary constituents. One speaks about "condensed" matter because typically in these materials, the average distance between elementary constituents is of the order of their own dimension: matter is "dense". For example, in a simple liquid the average distance between atoms is about the size of an atom. The global properties of a system differ in general largely from the individual behaviour of its elementary constituents: one speaks of collective behaviour. A well-known example is that of sound propagating in a crystal. A crystal is made of many atoms distributed on a regular lattice. When the crystal is at low temperature, the atoms are almost still. However, the motion of elementary excitations of the crystal do not resemble that of a free atom. They are acoustic (sound) waves involving a large number of atoms in the crystal. The associated quanta are the famous phonons.

Researchers in the THEO group are interested in diverse condensed matter systems. They seek to provide qualitative (and sometimes quantitative) explanations to phenomena observed in experiments and to predict new ones. Their aim is to understand the collective behaviour of these materials and to propose general concepts that apply to many such systems.

Main subject of interest:

  • "Strongly correlated electrons" (Mott transition, quantum Hall effects, high Tc superconductors, heavy fermions and Kondo effect, etc.)
  • "Quantum mesoscopic physics" (quantum coherence effects, weakly disordered metals, etc.)
  • "Ultracold atomic gases" (Bose-Einstein condensates, ultracold fermions, superfluidity, BCS-BEC crossover, etc.)
  • "Quantum magnetism. New phases induced either by frustration, or quasi-periodicity, or both"
  • "Clusters, surfaces and nanostructures"
  • "Complex orders" (quasicrystals, e.g.)
  • "Soft condensed matter and the interface between physics and biology" (polymers, e.g.)
  • "Molecular and low dimensional conductors" (charge/spin density waves, Luttinger liquids, etc.)


Fractional quantum Hall effect:
second generation of composite fermions.

Noise excess of an impurity in the middle of a one-dimensional quantum wire at zero temperature.


Two impurities in a cold one-dimensional atomic gas can lead to a Casimir effect.

Frustrated magnetic chain (spin 1/2) in YCuO2.5.

Research topics:
Techniques and materials:
 
- Strongly correlated fermions
- Quantum Hall effects
- Organic superconductors
- Ultracold atomic gases
- Heavy fermions and Kondo effect
- Frustrated quantum magnetism
- High Tc superconductors
- Luttinger liquids
- Glassy polymers
- Clusters and surfaces
- Low dimension conductors
- Mesoscopic quantum physics
- Disordered electronic systems
- Charge/spin density waves
 
Techniques:
- Field theory in the many body problem:
Green’s functions and functional integrals
- Keldysh’s formalism for Green’s functions
- Bosonization and Luttinger liquids
- Numerical methods (DMFT, Monte-Carlo, etc.)

Materials:
- Superconducting cuprates
- Organic conductors
- Fullerenes
- Cobaltites
- Oxides with remarkable properties
- Two-dimensional electron gas in AsGa
- Ultracold atomic (alcaline) vapors