Condensed Matter Seminar, Thursday September 16, 2010

Gamow Tower 11th floor commons room, 12:00pm


Breakdown of diffusion, equilibration rates and negative absolute temperatures for ultracold atoms in optical lattices

Stephan Mandt, University of Cologne


The last years have seen dramatic progress in the control of quantum gases in optical lattices. It has become possible to simulate models of strongly interacting
quantum particles for which the Hubbard model is probably the most important example. A major advantage of these systems compared to real solids is the
possibility to change all relevant parameters in real-time by e.g. varying laser intensities or magnetic fields. This allows to study out-of-equilibrium dynamics and
transport properties.

Here we investigate the breakdown of diffusion in the transport of fermionic quantum particles on a lattice described by a homogeneous Hubbard model, based
on an experiment by the group of Immanuel Bloch. We observe a crossover from diffusive behaviour in the center of the cloud to a ballistic motion of atoms in its
outer regions. This crossover manifests itself in a striking change of the cloud's shape: While it remains round in the diffusive regime, it obtains a square shape
when the motion becomes ballistic. Surprisingly, the system exhibits a strong feedback from the ballistic on the diffusive regions characterized by a universal loss
rate of particles obeying singular diffusion equations. In addition, the observed dynamics is independent of the sign of the interaction, highlighting a novel, dynamic
symmetry of the Hubbard model.

In the second part we suggest and model a dynamical process in which a fermionic gas with a negative absolute temperature can be created, and analyze the time scales needed to equilibrate to T < 0.

References:
1. U. Schneider, L.Hackermüller, J. P. Ronzheimer, S. Will, S. Braun, T. Best, I. Bloch, E. Demler, S. Mandt, D. Rasch, A. Rosch: Breakdown of diffusion: From collisional hydrodynamics to a continuous quantum walk in a homogeneous Hubbard model, arXiv:1005.3545v1
2. A. Rapp, S. Mandt and A. Rosch: Equilibration rates and negative absolute temperatures for ultracold atoms in optical lattices, arXiv:1008.0468