Generating Giant Vortex in a Fermi Superfluid via Spin-Orbital-Angular-Momentum Coupling

Abstract

Spin-orbital-angular-momentum (SOAM) coupling has been realized in recent experiments of Bose-Einstein condensates [Chen et al., Phys. Rev. Lett. 121, 113204 (2018) and Zhang et al., Phys. Rev. Lett. 122, 110402 (2019)], where the orbital angular momentum imprinted upon bosons directly leads to quantized vortices. However, an s-wave Fermi pairing superfluid under the same SOAM coupling is typically vortex-less, as the two fermion species acquire opposite angular momenta in the center-of-mass motion. Here we show that, by introducing a moderate two-photon detuning in the Raman process generating the SOAM coupling, a quantized vortex, with size comparable to the beam waist of Raman lasers, can be stabilized in a Fermi superfluid. Such a giant vortex state can be viewed as the angular analogue of the Fulde-Ferrell states under a spin-orbit-coupling-induced Fermi-surface deformation, now with Cooper pairs carrying quantized angular momenta. Due to the spin-polarized nature of vortex bound states, these vortices feature a large spin polarization at the vortex core, thus providing an ideal signal for their experimental detection.

Molecular state in a spin-orbital-angular-momentum coupled Fermi gas

“…The underlying pairing mechanism is similar to that of the SOAMC-induced vortex state in Ref. [21,22], and is the angular analogue of the spin-orbit-coupling-induced Fulde-Ferrell state in Ref. [27].…”

“…Therein, different ground hyperfine states of an atom are coupled by a pair of co-propagating Laguerre-Gaussian beams with distinct orbital angular momentum, giving rise to the experimental observation of spin-dependent vortices in spinor Bose-Einstein condensates under SOAMC [10,11]. Further, as a direct consequence of the deformed single-particle dispersion in the discretized angular-momentum space, a unique vortexforming mechanism exists in the SOAM coupled Fermi superfluids [21,22]. And it has been proposed very recently that an angular topological superfluid can be induced by SOAMC [23], whose topological defect, in the form of giant vortices, has interesting implications for topological quantum computation.…”

Angular Topological Superfluid and Topological Vortex in an Ultracold Fermi Gas

“…We also impose a hardwall box potential V ext (r) with a radius R, to provide a natural boundary. Note that a gauge transformation U = e −ilθσz [42] is imposed to derive Hamiltonian (1). Consistent with previous experiments on SOAMC [31,32], the Raman coupling and the ac Stark potential are written as Ω(r) = Ω 0 I(r) and χ(r) = χ 0 I(r), respectively.…”

“…As the Raman lasers are Laguerre-Gaussian beams with different orbital angular momenta, this angular-momentum difference of light is imprinted onto the hyperfine spins of each single atom, with profound many-body implications [33][34][35][36][37][38][39][40][41]. For instance, while the SOAMC-driven vortex formation and phase transitions are recently observed in Bose-Einstein condensates [31,32], theoretical studies reveal that the interplay of SOAMC and pair-ing interactions underlies a unique vortex-forming mechanism in Fermi superfluids [42,43]. Here a series of intriguing questions arise: whether topological superfluid can also be stabilized under SOAMC?…”

Quantum phases of spin-orbital-angular-momentum coupled bosonic gases in optical lattices

“…The atomic system obtains orbital angular momentum from the copropagating LG beams via Raman transitions among the internal hyperfine states of atoms, whereas the transfer of photon momentum into atoms is suppressed [19,20]. Within SOAM coupling, several in-triguing quantum phases have been predicted theoretically [21][22][23][24][25][26][27][28][29][30][31][32] and observed experimentally [19,20,33,34]. In these studies, however, interactions between atoms play tiny role in the various quantum phases, and one mainly focus on the weakly interacting regime.…”