**April 06 2015**

**13.04. - 14.04.2015: Visit of Laurent de Forges de Parny **

Laurent de Forges de Parny (ENS Lyon) is visiting us and will give a talk in our group seminar about: "Multiple Transitions in a Coupled Atom-Molecule Mixture".

Abstract:

Since the pioneering experiments in 2002 by M. Greiner et al., ultracold bosons in optical lattices are ideal test bed for many condensed matter models. As you know, the Bose-Einstein condensation of pairs of particles is central in the theory of superconductivity and the control of such a mechanism is still a big challenge. Feshbach resonances - namely resonances between an unbound two-body state (atomic state) and a bound (molecular) state - allow the conversion of two unbound atoms into a molecule and vice versa.

My numerical studies focus on a 2D mixture of interacting bosonic atoms and molecules with conversion between these two species. This system involves many interesting phases and phase transitions:

- At zero temperature, the system exhibits molecular and atomic-molecular condensates and an insulating phase stabilized by the conversions - a "Feshbach insulator". First order, 3D XY and 3D Ising transitions appear.

- At finite temperature, this system also reveals interesting transitions: when increasing the temperature, the system evolves from an atomic-molecular superfluid to a molecular superfluid and then to a normal Bose liquid. The multiple transitions involved (2D Ising and KT transitions) are very well described by a coupled XY model and scaling analysis.