Dresden 2011 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 37: MLT: Quantum Liquids, Bose-Einstein Condensates, Ultra-cold Atoms, ...

TT 37.2: Talk

Wednesday, March 16, 2011, 14:15–14:30, HSZ 105

Breakdown of diffusion and negative absolute temperatures for ultracold atoms in optical lattices — •Stephan Mandt, Akos Rapp, David Rasch, and Achim Rosch — Institute of Theoretical Physics, University of Cologne, Germany

The last years have seen dramatic progress in the control of quantum gases in optical lattices, which allows to study out-of-equilibrium dynamics and transport properties in strongly correlated systems. We investigate the breakdown of diffusion in the transport of fermionic atoms on a lattice described by a homogeneous Hubbard model, based on an experiment by the group of Immanuel Bloch on an expanding cloud. We observe a crossover from diffusive behavior in the center of the cloud to a ballistic motion of atoms in its outer regions: While the cloud’s shape 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 the second part we suggest and model a dynamical process in which an atomic gas with a negative absolute temperature can be created by inverting the sign of the harmonic trap. Due to the finite band-width of the lattice, which gives both an upper and a lower bound to the kinetic energy, the system still gets trapped even by the inverted potential, but equilibrates to a negative absolute temperature. We analyze the time scales needed to equilibrate to T < 0 and study the amount of heat generated by this non-equilibrium process.