Optical Conductivity Group

Research areas

Lattice dynamics and magneto-electric couping in multiferroic materials

A multiferroic material shows simulatneously at least two ferroic (or antiferroic) orders (ferroelectric, ferromagnetic, ferroelastic or ferrotoroidal). Materials showing coexisting electric and magnetic ordering have raised a consideralble intest in the recent years. Coupling of these two properties could lead to memory devices having magnetic reading and electrical writing. Ferroelectricitiy is closely related to lattice distortions and structural changes in a crystal. As these changes are related to the appearance of a spontaneous electrical polarization, the phonons controlling these atomic displacements are polar and, in general, visible by infrared spectroscopy. In our research we look at the phonon spectra and track the changes induced by the electrical and magnetic ordering of the system. Our ultimate goal is the understanding of the microscopic mechanism controlling the magneto-electric coupling. More coming soon ...

The electron-boson interaction spectral density in superconductors

[Figure from Farnworth and Timusk, Phys. Rev. B 14, 5119 (1976)] In the BCS theory for superconductors, electrons bind into Cooper pairs by exchanging a boson. To explain the isotopic effect in conventional supercondutors, BCS took a phonon as the biding particle. However, any other boson can act as the superconducting glue. The Eliashberg theory extend BCS theory by utilizing the full bosonic spectral function [ α²F(ω)] instead of average values taken by BCS. In conventional superconductors, this function was obtained by inversion of tunneling data. However,it can also be obtained from the optical conductivity. In our research we try to link the "optical" electron-boson spectral density to other spectroscopic techniques aiming to understand more about the superconducting glue. More coming soon ...

The f-sum rule on hole doped cuprate superconductors

There is a growing consensus that the comprehension of high temperature superconductivity passes through the understanding of the various exotic non superconducting phases. Which physical processes underlie the pseudogap phase? What is the role of electronic correlations in creating a non Fermi liquid behavior in cuprates? How close to a true Fermi liquid do we get when we increase doping? What is the nature of the glue that keeps the carriers bound in Cooper pairs? These are only a few questions that the optical conductivity f-sum rule can shed light on. More coming soon ...

Partial gaps, effective doping and the phase diagram of n-type cuprates

Only a few cuprate superconductors are electron-doped. The overall lines of their phase diagram are similar to the hole coped compounds. The details are however strikingly different. T_c's are much lower than the ones observed in hole doped materials. In addition, a competition between a very broad anti-ferromagnetic phase and the superconducting dome seems to dominate the physics of these systems. And disorder effects cannot be discarded. We use infrared-visible spectroscopy to probe the normal state properties and how do they evolve into the superconducting phase. Our measurements addresses the quantum criticallity of the materials and effects of phase competition and/or supperposition. More coming soon ...

Metal insulator transition in functional materials

[Figure: A.V. Pronin] The metal-insulator transition is a vast problem that covers a multitude of systems and materials (semiconductors, granular metals, colossal magnetoresistance manganites, superconducting cuprates, nickelates, hydrides, transparent conducting oxides and many more). In many of these systems, the metallic phase comes from either a band insulator or a Mott insulator. In our current work we look into both kinds of materails: transparent conducting oxides and reduced SrTiO₃ are exemples of the former whereas nickelates and (possibly) electrides are exemples of the latter. More coming soon ...

Non equilibrium superconductivity - Disturbing superconductors with light

How do Cooper pairs recombine in a superconductor? Once a non-equilibrium state is created by sending photons to break pairs, the question becomes a subtle fight between unpaired electrons willing to recombine and low-energy phonons willing to break pairs. At the end of the day, the phonons let go and allow the system to relax. We studied this dynamics in several classical superconductors as well as the "two-superconductors-in-one" MgB₂. In the latter, the non equilibrium spectroscopy shows the two bands chit-chatting. More...