Dynamical structure factors of a two-dimensional Fermi superfluid within random phase approximation

Abstract

Based on random phase approximation, we numerically calculate dynamical structure factors of a balanced two-dimensional (2D) Fermi superfluid, and discuss their energy, momentum and interaction strength dependence in the 2D BEC–BCS crossover. At a small transferred momentum, a stable Higgs mode is observed in the unitary 2D Fermi superfluid gas where the particle–hole symmetry is not satisfied. Stronger interaction strength will make the visibility of the dispersion of Higgs mode harder to be observed. We also discuss the dimension effect and find that the signal of the Higgs mode in two dimension is more obvious than that in 3D case. At a large transferred momentum regime, stronger interaction strength will induce the weight of the molecules excitation increasing, while in verse the atomic one decreasing, which shows the pairing information of Fermi superfluid. The theoretical results qualitatively agree with the corresponding quantum Monte Carlo data.

Following the Higgs mode across the BCS-BEC crossover in two dimensions

“…We note in passing that our result that the peak in Im χ H (ω + iδ, q) in 2D exists for all µ > 0 (in contrast to 3D, where the peak only exists for µ > 0.8267∆), agrees with a previous numerical study, which found that the Higgs mode is more visible in the dynamical structure factor in 2D compared to 3D [45]. We also note that as µ → 0, the boundary frequency ω 2 = 2∆ + q 2 2m µ ∆ approaches 2∆, i.e.…”

“…This is consistent with the hidden nature of the Higgs mode. The disappearance of the observable Higgs peak in the BEC regime, where µ < 0, agrees with previous theoretical results in 3D [36,47,48], experimental results for 3D cold-atom systems [33], and numerical studies in 2D [45]. As in the case of positive µ, the ABG mode is visible below the two-particle continuum.…”