Séminaire Théorie

Mardi 7 Avril 2015 à 14h00.

Disordered quantum matter: a theoretical and experimental challenge

''Disordered quantum systems have become a central theme of modern research in condensed matter, after the seminal prediction of P. W. Anderson (1958) that disorder leads to localisation of matter waves, and hence can completely inhibit transport phenomena in the absence of interaction. Ever since Anderson’s prediction, a central challenging question concerns the interplay between disorder and interactions, and in particular the possibility that interactions lead to a transition from a localised to a conducting phase. In the context of bosonic particles (relevant to systems as diverse as He-4 in porous media, disordered superconductors, cold atomic gases, etc.) the conducting phase is actually a superfluid one, and the transition in question is one of the most challenging examples of quantum phase transitions, still poorly understood after more than two decades of intense investigations. In this talk I will discuss how a particular class of magnetic materials - called Bose-Einstein magnets - can provide controlled realisations of disordered quantum fluids of bosons when subject to doping. This opens the path to a deep understanding of the localised phase of interacting bosonic particles (corresponding to a paramagnetic phase persisting down to zero temperature), as well as of the transition to the superfluid phase (which corresponds to the onset of long-range magnetic order). I will review recent progress in this endeavour, coming in particular from extensive numerical simulations, as well as from experiments on a model material (doped dichloro-tetrakis-thiourea Nickel — DTN).''