Observation of superfluidity in a strongly correlated two-dimensional Fermi gas

Abstract

Understanding how strongly correlated two-dimensional (2D) systems can give rise to unconventional superconductivity with high critical temperatures is one of the major unsolved problems in condensed matter physics. Ultracold 2D Fermi gases have emerged as clean and controllable model systems to study the interplay of strong correlations and reduced dimensionality, but direct evidence of superfluidity in these systems has been missing. We demonstrate superfluidity in an ultracold 2D Fermi gas by moving a periodic potential through the system and observing no dissipation below a critical velocity vc. We measure vc as a function of interaction strength and find a maximum in the crossover regime between bosonic and fermionic superfluidity. Our measurements enable systematic studies of the influence of reduced dimensionality on fermionic superfluidity.

Comparing fermionic superfluids in two and three dimensions

“…By ensuring that all interaction ramps are adiabatic, we keep the system at an approximately constant entropy per particle for all experiments. The entropy in our system corresponds to a temperature of T = 0.04(1) T F at an interaction strength of ln(k F a 2D ) = −2.9, which according to [54] is well below the critical entropy for superfluidity in the crossover regime.…”

“…For the preparation of our homogeneous 2D Fermi gases, we use the experimental setup and procedure described in [20,54]. In brief, we use first laser-and then evaporative cooling to prepare ultracold gases of approximately 6000 fermionic 6 Li atoms in a balanced mixture of the two lowest-energy hyperfine states.…”

“…The influence of the third dimension also becomes relevant when there is a significant population of higher energy levels in the strongly confined direction. In our systems, the temperature is negligible compared to the level spacing (T < 0.1 ω z ) [54], and we therefore neglect the influence of thermal excitations in the strongly confined direction. The second relevant quantity is the reduced chemical potential μ.…”

Superfluidity of a laser-stirred Bose-Einstein condensate

“…Other stirring experiments include Refs. [22][23][24][25][26]. Laser stirring is also employed to generate controlled vortex distributions [27][28][29].…”

$\mathcal{PT}$-Symmetry Enhanced Berezinskii-Kosterlitz-Thouless Superfluidity

“…BKT transitions have been experimentally studied in a wide range of systems, including liquid helium films [6], 2D magnets [7-10], superconducting thin films [11][12][13][14], and 2D atomic hydrogen [15], where only macroscopic properties of the systems can be measured. In recent years, ultracold atomic superfluids have emerged as a versatile playground for the studies of BKT physics [16][17][18][19][20][21][22][23][24][25][26][27][28][29] with access to the underlying microscopic phenomena such as the visualization of the proliferation of free vortices. Experimental signatures of BKT physics have been observed in harmonically trapped , and the recent realization of ultracold superfluids in box potentials [30][31][32][33][34] offers a uniform platform for exploring BKT transitions.…”