Observation of spin-charge separation in a 1D ⁶Li Fermi gas

Interacting fermions that are confined to 1D can only support collective excitations and are thus governed by the Tomonaga-Luttinger liquid (TLL) theory, in which collective excitations decouple into charge and spin modes instead of particle-like excitations. Previously, we characterized the dynamic response for the low energy charge excitations of a 1D spin-1/2 gas of atomic fermions using Bragg spectroscopy, and found good agreement with TLL theory [1]. Spontaneous emission induced by the near-detuned Bragg probe, however, prevented the observation of spin excitations. In this work, we use the relatively narrow 2S-3P transition in ⁶Li in order to minimize the rate of spontaneous emission. A pseudospin-1/2 system is realized with the lowest- and third-to-lowest, |1>-|3>, hyperfine sublevels of ⁶Li. The atoms are loaded into a 2D optical lattice, which creates an array of quasi-1D tubes. We tune the inter-species interactions via a magnetic Feshbach resonance and use Bragg spectroscopy with a momentum transfer of 0.2 k_F to obtain the low-energy excitation spectra for both modes. We compare the measured dynamical structure factor with the Tomonaga-Luttinger liquid theory, thus realizing the first direct measurement of the speeds of the spin and charge excitations with tunable interactions.

[1] T. Yang et al., PRL 121, 103001 (2018)