Spin-Orbit Coupled Fermi Gases across a Feshbach Resonance

Abstract

In this letter we study both ground state properties and the superfluid transition temperature of a spin-1/2 Fermi gas across a Feshbach resonance with a synthetic spin-orbit coupling, using meanfield theory and exact solution of two-body problem. We show that a strong spin-orbit coupling can significantly enhance the pairing gap for 1/(kFas) 0 due to increased density-of-state. Strong spinorbit coupling also significantly enhances the superfluid transition temperature when 1/(kFas) 0, while suppresses it slightly when 1/(kFas) 0. The universal interaction energy and pair size at resonance are also discussed.During the last few years, studies of ultracold Fermi gases across a Feshbach resonance (FR) have brought a lot of excitements to physics [1]. On a separate development, recent experimental breakthrough on synthetic gauge field has open up a lot of new opportunities to cold atom physics [2,3]. One application of this technique is to engineer an effective spin-orbit coupling (SOC) in cold atom system [4]. Very recently, a pioneer experiment in NIST has already achieved a restricted class of spin-orbit coupled BEC of 87 Rb atoms [3]. Theoretically, even the mean-field study of boson condensate with SOC has revealed many interesting physics [5][6][7]. For fermions, a concrete scheme has also been proposed for generating SOC in 40 K atom in the regime where a magnetic FR is available [8], and the experiment of implementing this proposal is now going on in the laboratory. However, a theoretical study of spin-orbit coupled Fermi gases across a FR is still lacking.In the absence of SOC, a Fermi gas across a FR possesses three key physical properties: i) across a FR the system undergoes a crossover from a BCS type fermion superfluid to a BEC of molecules; ii) at the FR, it is a strongly interacting system and exhibits many universal behaviors; iii) nearby a FR, the transition temperature of fermion superfluid T c /T F is the highest one among all fermion superfluids (or superconductors). The question is that how these three properties evolve in the presence of SOC. For i), since now the pair wave-function has more complicated structure with both singlet and triplet components, and exhibits p-wave character in the helicity bases, one needs to investigate whether it is still a crossover or there is a phase transition in between. Even if it is still a crossover, how SOC affects it. For ii), since the strength of SOC introduces another length scale λ/k F , the universal constants at resonance now become universal functions of λ/k F , and we want to understand the behaviors of these functions. And for iii), the question is whether T c /T F will be enhanced or suppressed by SOC. (Here the units k F , E F and T F are the Fermi momentum, the Fermi energy and the Fermi temperature for non-interacting system without SOC. a s is the s-wave scattering length.)In this letter we address these issues using both meanfield (MF) theory and exact solution of two-body (TB) problem, and the main results are summarized as follows: (A1) T...

Rashbon Bound States Associated with a Spherical Spin–Orbit Coupling in an Ultracold Fermi Gas with an s-Wave Interaction

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Harmonically trapped two-atom systems: Interplay of short-ranges-wave interaction and spin-orbit coupling

“…A variety of intriguing phenomena such as non-equilibrium dynamics [9,10], the spin-orbit coupling assisted formation of molecules [7], and the engineering of band structures [11] have been investigated. At the mean-field level, spin-orbit coupled gases exhibit rich phase diagrams [4,[12][13][14][15][16][17][18]. Effects beyond mean-field theory [19][20][21][22], associated with the renormalization of interactions, are enhanced by the spin-orbit coupling, especially in the pure Rashba case, and can qualitatively change the mean-field results.…”

Itinerant magnetic phases and quantum Lifshitz transitions in a three-dimensional repulsively interacting Fermi gas with spin-orbit coupling

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