Ever since the discovery of the phenomenon superconductivity, the flow of electrical current without any resistive losses, scientists have been intrigued by the question what causes it. One of the unquestionable principles of superconductivity is, that electrons pair up to form Cooper pairs. Since pairs of electrons are effectively bosons, they can -and do- condense in a superfluid state.

The big question is however why these pairs would form in the first place since, after all, electrons in vacuum rather move away from each other due to the Coulomb repulsion. The most common type of superconducting pairing is believed to be a phonon-mediated electron-electron interaction. Even in that case the physics of the superconducting state can be rather unconventional, in particular if the density of charge carriers becomes so small, that electron-motion is effectively adiabatic as compared to the lattice-vibrations to which they are coupled. For example, this is the case in doped SrTiO3.

In many high-Tc superconductors one observes superconductivity in close proximity to magnetic order. In these cases it is suspected that not phonons, but a purely electronic mechanism is responsible for superconductivity. In those materials it is of crucial importance to explore the correlations between the electrons directly. It is also of interest to monitor how the Coulomb energy, the exchange energy, or the kinetic energy are affected when a material undergoes a phase transition from normal metal to superconducting. Experimentally accessible quantities, such as correlation functions, Coulomb energy, kinetic energy, and exchange energy, can be measured by using various different kinds of spectroscopic probes, including high precision low temperature optical spectroscopy; these probes allow to investigate how experimental quantities are affected when the system goes superconducting. Our experiments explore aforementioned energy changes when tuning the material from normal to superconducting, and elucidate the nature of the superconducting state.


Square modulus of an order parameter for a 2D superconductor with dx2-y2 symmetry,  in reciprocal space of momenta (kx,ky). [D. Valentinis]
Optical sheet conductance of Hg1201: the energy gap, related to the superconducting order parameter, is observed in the optical spectrum as a decrease of optical conductivity in the region around 100 meV, below the critical temperature. [S. Mirzaei et al. (2012)]

Réalisation: Sur Mesure concept