Observation of confinement-induced background dimers in a 1D Fermi Gas*

Ultracold atoms confined to optical lattices enable systematic studies of quantum systems in reduced dimensions. Dimensionality effects are particularly prominent in their ability to modify the two-body bound and scattering states. In free space, for example, Feshbach dimers exist only on the positive scattering length side of the Feshbach resonance, while in harmonically-confined, one-dimensional (1D) systems weakly bound molecular states have been observed on both sides of the corresponding confinement-induced resonance. Moreover, a combination of quasi-1D confinement and a negative, s-wave background scattering length is predicted to give rise to a weakly-bound molecular state with a wavefunction that extends well beyond the interparticle spacing, known as a confinement-induced background (CIB) dimer [1]. Contrary to the formation of weakly-bound Feshbach dimers, the threshold for CIB dimer formation occurs far from the Feshbach resonance. Rather, it takes place at the zero-crossing of the scattering length, where the effective 1D scattering length has a pole. Here, we report the first observation of CIB dimers in a 1D Fermi gas. We realize a pseudospin-1/2 system with the lowest- and third-to-lowest, |1〉-|3〉, hyperfine sublevels of ⁶Li. The atoms are loaded into a 2D optical lattice, thus creating an array of quasi-1D atomic waveguides.  We characterize the binding energies and the population ratio between dimers and unpaired atoms in the gas using radio-frequency spectroscopy for different confinement strengths. Although the binding energy of the CIB shallow bound state is comparable to the Fermi energy, we observe them to exhibit long (> 1s) lifetimes, perhaps because of their highly-extended anisotropic wavefunction.  We will also discuss their many-body properties in 1D.

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