Crossover from 2D to 3D in a Weakly Interacting Fermi Gas

Abstract

We have studied the transition from two to three dimensions in a low temperature weakly interacting 6Li Fermi gas. Below a critical atom number N(2D) only the lowest transverse vibrational state of a highly anisotropic oblate trapping potential is occupied and the gas is two dimensional. Above N(2D) the Fermi gas enters the quasi-2D regime where shell structure associated with the filling of individual transverse oscillator states is apparent. This dimensional crossover is demonstrated through measurements of the cloud size and aspect ratio versus atom number.

Observation of Pair Condensation in the Quasi-2D BEC-BCS Crossover

“…This measurement is performed similar to the technique described in [18] and it relies on the same principles as the previously used band-mapping technique [48]. We release the sample from the SWT, let it expand for a time-of-flight of 3 ms, and measure its vertical extension.…”

“…Recently, efforts have been made to combine reduced dimensionality with the tunability of interactions and to experimentally explore ultracold 2D Fermi gases [16][17][18][19][20][21]. However, the phase transition to a condensed phase has not yet been observed.…”

“…Reducing the dimensionality [9] led to the observation of a Berezinskii-Kosterlitz-Thouless (BKT) type phase transition to a superfluid phase in weakly interacting 2D Bose gases [10,11]. Tuning the strength of interactions in a three-dimensional two-component Fermi gas made it possible to explore the crossover between a molecular Bose-Einstein Condensate (BEC) and a BCS superfluid [12][13][14][15].Recently, efforts have been made to combine reduced dimensionality with the tunability of interactions and to experimentally explore ultracold 2D Fermi gases [16][17][18][19][20][21]. However, the phase transition to a condensed phase has not yet been observed.…”

“…This leads to a pancake-shaped cloud with an aspect ratio of ωz ωr ≈ 310 (see Fig.1) and an axial harmonic oscillator length z = /mω z ≈ 551 nm with the reduced Planck's constant , the atom mass m, and the axial trapping frequency ω z . We ensure that there is no significant population of axially excited states by measuring the axial momentum distribution of the gas [18,27]. Assuming that the internal structure of pairs, i.e.…”

Bound States in a Quasi-Two-Dimensional Fermi Gas

“…Such 2D geometries have recently been realised in ultracold atomic Fermi gases [19][20][21][22][23], where the fermions are confined to 2D with an effective harmonic potential. In addition to allowing one to explore the 2D-3D crossover, the harmonic confinement can strongly modify the scattering properties of atoms via confinement-induced resonances [6,24,25].…”