Observation of spin and charge excitations in a strongly interacting 1D Fermi Gas

One of the key predictions of the Tomonaga-Luttinger liquid (TLL) theory of interacting fermions in 1D is the decoupling of the spin and charge degrees of freedom. We measure the difference in the propagation speeds for the density and the spin modes in a TLL as a function of the strength of repulsive interactions. 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 k = 0.2 k_F to measure the low-energy excitation spectra for both modes. Using the narrow-linewidth 2S-3P transition in combination with the chosen state mixture minimizes spontaneous emission while exciting the spin-mode with Bragg beams tuned between the resonance frequencies of the two states. We compare the measured dynamical structure factor with the TLL theory, thus realizing the first observation of spin-charge separation with tunable interaction strength.