Session G1: Scale-Invariance in Two-Dimensional Quantum Gases

Two-dimensional Fermi liquid with attractive interactions and near scale-invariant dynamics

We realize and study an attractively interacting two-dimensional Fermi liquid. Using momentum-resolved photoemission spectroscopy, we measure the self-energy, determine the contact parameter of the short-range interaction potential, and find their dependence on the interaction strength. Further, we investigate collective excitations of a harmonically trapped two-dimensional Fermi gas from the collisionless (zero sound) to the hydrodynamic (first sound) regime. The breathing mode, which is sensitive to the equation of state, is observed with an undamped amplitude at a frequency 2 times the dipole mode frequency for a large range of interaction strengths and different temperatures. This provides evidence for a dynamical SO(2,1) scaling symmetry of the two-dimensional Fermi gas. Moreover, we investigate the quadrupole mode to measure the shear viscosity of the two-dimensional gas and study its temperature dependence.   

Apparent Low-Energy Scale Invariance in Two-Dimensional Fermi Gases

Strongly-interacting systems in two dimensions have occupied a central position in the study of quantum materials. From high temperature superconductors to the Hall effect in two-dimensional electron gases, strong quantum and thermal fluctuations conspire to make this an extremely rich yet poorly-understood regime to work in. Several remarkable and surprising recent experiments in ultracold atomic gases show us that there are puzzles to be understood even in the simplest nontrivial two-dimensional system: a dilute quantum gas with strong s-wave interactions. Amongst these is an experiment that finds an undamped breathing mode oscillating at twice the trap frequency over a wide range of parameters [1], behaviour nominally associated with scale invariance, even though scale invariance is strictly broken in this system by a finite s-wave scattering length. This apparent scale symmetry is all the more remarkable given that the mean-field BCS theory for the 2D gas predicts an exact low-energy scale invariance, relevant to the low-energy breathing mode, meaning that only quantum and thermal fluctuations can break this low-energy scale symmetry [2]. Understanding why the symmetry breaking is so weak may give insight into how to make reliable theoretical predictions in systems with strong fluctuation effects, where there is no obvious small parameter from which a perturbation expansion can be formulated.\\[4pt] [1] E. Vogt, M. Feld, B. Frohlich, D. Pertot, M. Koschorreck, and M. Kohl, Phys. Rev. Lett. 108, 070404 (2012).\\[0pt] [2] E. Taylor and M. Randeria, Phys. Rev. Lett. 109, 135301 (2012).   

Viscosity and damping of collective modes in two-dimensional and scale-invariant Fermi gases

We study the consequences of exact and approximate scale invariance in dilute Fermi gases, with a particular emphasis on fluid dynamics and transport properties. We review recent attempts to extract the shear and bulk viscosity of the dilute Fermi gas in two and three dimensions from the damping of collective excitations. We compare the results to predictions from kinetic theory.   

Superfluid behavior of a 2D Bose gas

Two dimensional Bose gases cannot undergo a conventional phase transition associated with the breaking of a continuous symmetry. Nevertheless they may exhibit a phase transition to a state with quasi-long range order via the Berezinskii--Kosterlitz--Thouless (BKT) mechanism. Even if they cannot undergo Bose-Einstein condensation at non-zero temperature they become superfluid above a critical phase space density. The quasi-long-range coherence and the microscopic nature of the BKT transition have been recently explored with ultracold atomic gases. Here we report the direct observation of superfluidity in terms of frictionless flow. We probe the superfluidity of a 2D trapped Bose gas by stirring a moving obstacle formed by a repulsive potential created by micrometre-sized laser beam. We move this obstacle on circles of different radii at constant velocity. Depending on the degree of degeneracy of the gas at the obstacle location we find a dramatic variation of the response of the fluid. We demonstrate a superfluid behavior where the local phase space density is high enough and a normal one otherwise. Using the local density approximation we compare our findings with the expected results for a uniform gas. Finally we present the heating rates measured for normal or superfluid behavior.