Bragg spectroscopy of 1D fermions with attractive interactions

Spin-1/2 fermions are expected to pair in one dimension for any strength of attractive interactions. Evidence of pairing of fermionic neutral atoms in quasi-1D traps has previously been obtained using RF spectroscopy to directly probe the binding energy of the pairs [1] but can also be obtained by performing Bragg spectroscopy to measure the speeds of the (charge) density waves and the spin density waves. These speeds are equal in the ideal gas, but for weak attractive interactions, the spin-mode velocity increases while the charge-mode velocity decreases. This is the opposite of the classic hierarchy observed in Tomonaga-Luttinger liquids with repulsive interactions [2]. For strong attractive interactions, the speed of the charge-mode halves, indicating tightly bound pairs, and the spin-mode becomes gapped. We prepare spin-balanced gases of ⁶Li in quasi-1D traps, formed by a 2D optical lattice, and perform Bragg spectroscopy, probing either the charge or spin mode in the regime of weak attractive interactions. We observe an inversion of the classic spin-charge velocity hierarchy in this regime, confirming expectations from exact Bethe ansatz solutions for the homogeneous gas at zero temperature. This observation, near a zero-crossing in the 3D s-wave scattering length on the BEC-side of a Feshbach resonance, indicates the formation of confinement-induced background dimers [3], which are distinct from the previously observed confinement-induced Feshbach dimers. We also observe charge-mode Bragg spectra in the strongly attractive regime consistent with tightly bound Feshbach dimers.

[1] H. Moritz, T. Stöferle, K. Günter, M. Köhl, and T. Esslinger, Phys. Rev. Lett. 94, 210401 (2005).

[2] R. Senaratne, D. Cavazos-Cavazos, S. Wang, F. He, Y.-T. Chang, A. Kafle, H. Pu, X.-W. Guan, R. G. Hulet, Science 376, 6599 (2022).

[3] T. Kristensen and L. Pricoupenko, Phys. Rev. A 91, 042703 (2015).