Quantum simulations with strongly interacting two dimensional Fermi Gases
Dr. Marco Koschorreck (Cavendish Laboratory, Cambdridge / Universität Bonn)

Two-dimensional Fermi gases play a pivotal role in quantum many-body physics. The restriction of particle motion to a plane profoundly increases the role of fluctuations and leads to qualitatively new effects in the interparticle interaction. In the solid state context, strongly interacting twodimensional Fermi gases are found in the cuprates, the two-dimensional electron gas in nanostructures, and in thin 3He films. With the advent of ultra-cold atomic Fermi gases and the ability to confine them to two-dimensional configurations, research has revived because tunable and ultra-clean samples have become available. The direct experimental access to microscopic parameters, such as the particle interaction, promotes ultra-cold two-dimensional Fermi gases as quantum simulators of fundamental many-body effects. First, we will discuss the observation of a many-body pairing gap above the superfluid transition temperature in a harmonically trapped, two-dimensional atomic Fermi gas in the regime of strong coupling. Our observations mark a significant step in the emulation of layered two-dimensional strongly correlated super-conductors using ultra-cold atomic gases. Second, the dynamics of a single spin impurity is studied. A spin-up impurity dressed by a bath of spin-down particles, constitutes the Fermi polaron problem. This is the extreme, but conceptually simple, limit of two important quantum many-body problems: the BEC-BCS crossover with spinimbalance for attractive interactions and Stoners itinerant ferromagnetism for repulsive interactions. Third, we study spin dynamics both in the weakly and strongly interacting regime. In particular for strong interactions, at unitarity, spin transport is dictated by diffusion and the spin diffusivity is expected to reach a universal, quantum-limited value on the order of the reduced Planck constant divided by the particle mass. Using the spin-echo technique, for strong interactions, we measure a transverse spin diffusion constant of 1/100 hbar/m. For weak interactions, we observe a collective transverse spin-wave mode that exhibits mode softening when approaching the strongly interacting regime.