Excitations of an interacting 1D Fermi gas

We have investigated the excitations of a two-component $^6$Li Fermi gas confined to one dimension. We measure the dynamic structure factor (DSF) for both repulsive and attractive interactions, using Bragg spectroscopy. The spin-density and charge-density waves may be excited selectively. For repulsive interactions, the charge density wave propagates faster than the spin-wave, consistent with spin-charge separation [1]. This separation is inverted for attractive interactions, where the spin-wave velocity exceeds the charge-density wave due to a spin gap [2,3]. While the charge mode remains universally gapless and insensitive to spin imbalance, the spin sector is highly sensitive to polarization. Specifically, a small spin polarization strongly suppresses the observable spin gap, causing spin excitations to appear gapless in the measured DSF because of the dominance of unpaired atoms. The presence of paired molecules for attractive interactions is further confirmed by RF spectroscopy, showing an atom-molecule mixture and reduced spin correlations as attraction increases.

 

We extend these studies by introducing an axial standing wave to realize the 1D Fermi-Hubbard model in the weak lattice limit. Preliminary data reveal a second peak in both the spin and charge (DSF) spectra, and stronger rates of correlation decay with increasing lattice depth.

[1] R. Senaratne et al, Science \textbf{376}, 1305-1308 (2022)

[2] A. Luther and V. Emery, Phys. Rev. Lett. \textbf{33}, 589 (1974)

[3] A. Kafle et al, arXiv:2512.08866v1