Criteria for two-dimensional kinematics in an interacting Fermi gas

Ultracold Fermi gases subject to tight transverse confinement offer a highly controllable setting to study the two-dimensional (2D) BCS to Berezinskii-Kosterlitz-Thouless superfluid crossover. Achieving the 2D regime requires confining particles to their transverse ground state which presents challenges in interacting systems. Here, we establish the conditions for an interacting Fermi gas to display 2D kinematics. Transverse excitations are detected by measuring the transverse expansion rate which displays a sudden increase when the atom number exceeds a critical value N2D signifying a density driven departure from 2D kinematics. For weak interactions N2D is set by the aspect ratio of the trap. Close to a Feshbach resonance, however, the stronger interactions reduce N2D and excitations appear at lower density.