Spin-charge separation and pairing in a 1D Fermi gas

We investigate the behavior of spin-1/2 fermions in one dimension for tunable repulsive and attractive interactions. We confine ⁶Li atoms to a 2D optical lattice, which forms an array of quasi-one-dimensional waveguides. For repulsive interactions, we realize a Tomonaga-Luttinger liquid (TLL), and observe the low-energy collective excitations and spin-charge separation using Bragg spectroscopy [1]. The velocity of the charge wave increases, while the velocity of the spin wave decreases with increasing interaction strength. The measured Bragg spectra for the two modes compare well with the dynamic structure factors computed using Bethe ansatz and TLL theory [1]. Pairing is expected for attractive interactions. We have observed a pairing gap using RF spectroscopy, showing evidence of molecule formation. The molecular-state binding energy reaches threshold at a zero-crossing of the 3D scattering length on the low-field side of the Feshbach resonance, which exists due to the negative background scattering length of this collision channel. This is the first observation of a so-called confinement induced background dimer, which has no accessible analog in 3D. We map the binding energy as functions of 3D scattering length and confinement strength, and find the molecules to be long lived, perhaps due to their highly-elongated, anisotropic nature.

[1] Ruwan Senaratne, Danyel Cavazos-Cavazos, Sheng Wang, Feng He, Ya-Ting Chang, Aashish Kafle, Han Pu, Xi-Wen Guan, Randall G. Hulet  arXiv:2111.11545 [cond-mat.quant-gas]