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arXiv:2006.08898 · Physical Review Letters

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

Ke-Ji Chen, Fan Wu, Shi-Guo Peng, Wei Yi, Lianyi He

Published Dec 31, 2020• 21 citations
arXivHTMLPDF

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)PRLTAO0031-900710.1103/PhysRevLett.121.113204 and Zhang et al., Phys. Rev. Lett. 122, 110402 (2019)PRLTAO0031-900710.1103/PhysRevLett.122.110402], where the orbital angular momentum imprinted upon bosons leads to quantized vortices. For fermions, such an exotic synthetic gauge field can provide fertile ground for fascinating pairing schemes and rich superfluid phases, which are yet to be explored. Here we demonstrate how SOAM coupling stabilizes vortices in Fermi superfluids through a unique mechanism that can be viewed as the angular analog to that of the spin-orbit-coupling-induced Fulde-Ferrell state under a Fermi surface deformation. Remarkably, the vortex size is comparable with the beam waist of Raman lasers generating the SOAM coupling, which is typically much larger than previously observed vortices in Fermi superfluids. With tunable size and core structure, these giant vortex states provide unprecedented experimental access to topological defects in Fermi superfluids.

DOI
10.1103/physrevlett.125.260407

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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)PRLTAO0031-900710.1103/PhysRevLett.121.113204 and Zhang et al., Phys. Rev. Lett. 122, 110402 (2019)PRLTAO0031-900710.1103/PhysRevLett.122.110402], where the orbital angular momentum imprinted upon bosons leads to quantized vortices. For fermions, such an exotic synthetic gauge field can provide fertile ground for fascinating pairing schemes and rich superfluid phases, which are yet to be explored. Here we demonstrate how SOAM coupling stabilizes vortices in Fermi superfluids through a unique mechanism that can be viewed as the angular analog to that of the spin-orbit-coupling-induced Fulde-Ferrell state under a Fermi surface deformation. Remarkably, the vortex size is comparable with the beam waist of Raman lasers generating the SOAM coupling, which is typically much larger than previously observed vortices in Fermi superfluids. With tunable size and core structure, these giant vortex states provide unprecedented experimental access to topological defects in Fermi superfluids.
Abstract We briefly review the recent progress of theories and experiments on spin-orbital-angular-momentum (SOAM)-coupled quantum gases. The coupling between the intrinsic degree of freedom of particles and their external orbital motions widely exists in the universe and leads to a broad variety of fundamental phenomena in both classical physics and quantum mechanics. The recent realization of synthetic SOAM coupling in cold atoms has attracted a great deal of attention and stimulated a large amount of considerations on exotic quantum phases in both Bose and Fermi gases. In this review, we present a basic idea of engineering SOAM coupling in neutral atoms, starting from a semiclassical description of atom-light interaction. Unique features of single-particle physics in the presence of SOAM coupling are discussed. The intriguing ground-state quantum phases of weakly interacting Bose gases are introduced, with emphasis on a so-called angular stripe phase, which has not yet been observed at present. It is demonstrated how to generate a stable giant vortex in a SOAM-coupled Fermi superfluid. We also discuss the topological characters of a Fermi superfluid in the presence of SOAM coupling. We then introduce the experimental achievement of SOAM coupling in 87^{87}87 87 Rb Bose gases and its first observation of phase transitions. The most recent development of SOAM-coupled Bose gases in experiments is also summarized. Regarding the controllability of ultracold quantum gases, it opens a new era, from the quantum simulation point of view, to study the fundamental physics resulting from SOAM coupling as well as newly emergent quantum phases.
The experimental realization of emergent spin-orbit coupling through laser-induced Raman transitions in ultracold atoms paves the way for exploring novel superfluid physics and simulating exotic many-body phenomena. A recent proposal with the use of Laguerre-Gaussian lasers enables another fundamental type of coupling between spin and orbital angular momentum (SOAM) in ultracold atoms. We hereby study quantum phases of a realistic Bose-Einstein condensate (BEC) with this synthetic SOAM coupling in a disk-shaped geometry, respecting radial inhomogeneity of the Raman coupling. We find that the experimental system naturally resides in a strongly interacting regime in which the phase diagram significantly deviates from the single-particle picture. The interplay between SOAM coupling and interaction leads to rich structures in spin-resolved position and momentum distributions, including a stripe phase and various types of immiscible states. Our results would provide a guide for an experimental investigation of SOAM-coupled BECs.
We study the two-body bound states in a spin-orbital-angular-momentum (SOAM) coupled quantum gas of fermions within a tightly confined ring geometry in two dimensions. Two different configurations are considered: an attractive sss-wave interaction exists between two spin species that are SOAM coupled and an atom with SOAM coupled internal spins interacts state selectively with another atom. For both cases, we identify the condition for the emergence of molecular states with finite orbital angular momenta. These molecular states with quantized orbital angular momenta correspond to the SOAM-coupling-induced vortices in the corresponding Fermi superfluid. We propose to detect the molecules through Raman spectroscopy with Laguerre-Gaussian lasers. As the molecular states can form above the superfluid temperature, they offer an experimentally more accessible route toward the study of the underlying pairing mechanism under SOAM coupling.
We show that pairing in an ultracold Fermi gas under spin-orbital-angular-momentum coupling (SOAMC) can acquire topological characters encoded in the quantized angular degrees of freedom. The resulting topological superfluid is the angular analog of its counterpart in a one-dimensional Fermi gas with spin-orbit coupling, but characterized by a Zak phase defined in the angular-momentum space. Upon tuning the SOAMC parameters, a topological phase transition occurs, which is accompanied by the closing of the quasiparticle excitation gap. Remarkably, a topological vortex state can also be stabilized by deforming the Fermi surface, which is topologically nontrivial in both the coordinate and angular-momentum space, offering interesting potentials for applications in quantum information and quantum control. We discuss how the topological phase transition and the exotic vortex state can be detected experimentally.
Spin-orbit coupling (SOC) plays a crucial role in many branches of physics. In this context, the recent experimental realization of the coupling between spin and linear momentum of ultracold atoms opens a completely new avenue for exploring new spin-related superfluid physics. Here we propose that another important and fundamental SOC, the coupling between spin and orbital angular momentum (SOAM), can be implemented for ultracold atoms using higher-order Laguerre-Gaussian laser beams to induce Raman coupling between two hyperfine spin states of atoms. We study the ground-state phase diagrams of SOAM-coupled Bose-Einstein condensates on a ring trap and explore their applications in gravitational force detection. Our results may provide the basis for further investigation of intriguing superfluid physics induced by SOAM coupling, such as collective excitations.
Abstract Ultracold atoms endowed with tunable spin-orbital-angular-momentum coupling (SOAMC) represent a promising avenue for delving into exotic quantum phenomena. Building on recent experimental advancements, we propose the generation of synthetic gauge fields, and by including exotic vortex phases within spinor Bose–Einstein condensates, employing a combination of a running wave and Laguerre–Gaussian laser fields. We investigate the ground-state characteristics of the SOAMC condensate, revealing the emergence of exotic vortex states with controllable orbital angular momenta. It is shown that the interplay of the SOAMC and conventional spin-linear-momentum coupling induced by the running wave beam leads to the formation of a vortex state exhibiting a phase stripe hosting single multiply quantized singularity. The phase of the ground state will undergo the phase transition corresponding to the breaking of rotational symmetry while preserving the mirror symmetry. Importantly, the observed density distribution of the ground-state wavefunction, exhibiting broken rotational symmetry, can be well characterized by the synthetic magnetic field generated through light interaction with the dressed spin state. Our findings pave the way for further exploration into the rotational properties of stable exotic vortices with higher orbital angular momenta against splitting in the presence of synthetic gauge fields in ultracold quantum gases.