Luick, Niclas

Date:   Tuesday, January 26, 2021
Time:   14:30
Place:   scheduled Zoom meeting
Host:    Jean-Philippe Brantut

Josephson oscillations and superfluidity in a strongly correlated 2D Fermi gas

Niclas Luick
University of Hamburg, Germany

Strongly correlated 2D systems can give rise to superconductivity with high critical temperatures, but the origin for such unconventional superconductivity is still under debate. Ultracold 2D Fermi gases have emerged as clean and controllable model systems to study superfluidity in the presence of strong correlations and reduced dimensionality, but so far superfluidity has not been directly observed in these systems. In this talk, I present our observation of phase coherence and superfluidity in an ultracold 2D Fermi gas.

We observe phase coherence by creating a tunnel junction in a homogeneous 2D Fermi gas and detecting Josephson oscillations between the weakly coupled reservoirs. We measure the frequency of such Josephson oscillations as a function of the phase difference across the junction and find excellent agreement with a sinusoidal current phase relation.

In a separate set of measurements, we observe superfluidity in a 2D Fermi gas by moving a periodic potential through the system and detecting no dissipation below a critical velocity. We measure the critical velocity as a function of interaction strength and find that the gas is superfluid throughout the entire BEC-BCS crossover. In particular, we observe that in the bosonic limit the critical velocity is found at low lattice wave vectors corresponding to phononic excitations, while on the BCS side the lowest-lying excitations are found at k = 2k_F determined by pair breaking excitations.