Comparing fermionic superfluids in two and three dimensions

Effects of quantum fluctuations on the low-energy collective modes of two-dimensional superfluid Fermi gases from the BCS to the Bose Limit

“…Furthermore, we demonstrate that phase and modulus fluctuations of the superfluid order parameter become increasingly more coupled with growing interaction strength, leading to a change in curvature of the collective mode dispersion from concave in the BCS to convex in the Bose regime. A comparison of our results to very recent experiments [30] shows excellent agreement.…”

“…Superfluids in 2D are inherently different from their 3D counterparts, due to the importance of fluctuations [14,15] leading to a Berezinskii-Kosterlitz-Thouless (BKT) transition [16,17]. In the context of ultracold atoms, the desire to study 2D Fermi superfluids is driven not only by connections to high-temperature superconductors [18][19][20][21], but also by the high degree of experimental control that allows the measurement of the equation of state [22][23][24], the observation of the BKT transition [25,26], and the examination of collective modes [27][28][29][30].…”

“…Ultracold fermions with tunable interactions in nearly 2D configurations were studied using harmonic traps and optical lattices [31][32][33][34]. With the very recent advent of box potentials, it is now possible to study experimentally homogeneous 2D fermions [28,30,35]. Inspired by recent measurements of collective excitations via Braggspectroscopy [30,36,37], we investigate the low-energy collective modes of 2D Fermi superfluids in box poten-tials.…”

“…With the very recent advent of box potentials, it is now possible to study experimentally homogeneous 2D fermions [28,30,35]. Inspired by recent measurements of collective excitations via Braggspectroscopy [30,36,37], we investigate the low-energy collective modes of 2D Fermi superfluids in box poten-tials. We find that mean field (saddle point) results in 2D [38] produce incorrect values of the chemical potential and lead to the erroneous conclusion that the speed of sound is a constant throughout the BCS to Bose evolution [39][40][41].…”

“…Although numerical solutions are useful for comparison to recent experiments [28,30], analytical insight is essential to understand the underlying physics. To reveal the interplay between modulus and phase fluctuations, we show next that their coupling increases dramatically as the system evolves from the BCS to the Bose regime.…”