Realizing Fulde-Ferrell Superfluids via a Dark-State Control of Feshbach Resonances

Finite-momentum pairing superfluidity, or the so-called Fulde-Ferrell-Larkin-Ovchinikov (FFLO) state, has been studied and pursued for over half a century in different fields of physics, including condensed matter physics, nuclear physics and most recently ultracold atoms. Yet, its existence remains elusive. In ultracold atomic Fermi gases, the conventional scenario of spin-population imbalance leads to a rather narrow window for FFLO states, which makes them extremely difficult to observe.

In recent theoretical work, CQOS theorists, together with Assistant Professor Lianyi He from Tsinghua University in Beijing, proposed that a typical form of FFLO states - the long-sought Fulde-Ferrell superfluid - can be realized in ultracold two-component Fermi gases of 40K or 6Li atoms by optically tuning their magnetic Feshbach resonances via the creation of a closed-channel dark state with a Doppler-shifted Stark effect. In this scheme, two counter-propagating optical fields are applied to couple two molecular states in the closed channel to an excited molecular state, leading to a significant violation of Galilean invariance in the dark-state regime and hence to the possibility of Fulde-Ferrell superfluidity. The resultant Fulde-Ferrell superfluid has remarkable properties, such as an anisotropic single-particle dispersion relation, suppressed superfluid density at zero temperature, anisotropic sound velocity, and rotonic collective mode. The latter two features can be experimentally probed using Bragg spectroscopy, providing a smoking-gun proof of Fulde-Ferrell superfluidity.

This work has been published in Physical Review Letters.

Link to paper:
Lainyi He, Hui Hu, and Xia-Ji Liu, Phys. Rev. Lett. 120, 045302 (2018).