The realization of spin-orbit-coupling in ultracold atoms has opened a door to many exotic quantum phases that can be designed and demonstrated in physics laboratories. More recently, by coupling the atomic internal “spin” degree of freedom to its orbital angular momentum, a new type of spin-orbit coupling, named spin-orbital-angular-momentum coupling (SOAMC), was proposed and then realized in spinor Bose-Einstein condensate. Very different from the coupling between spin and linear momentum, the SOAMC preserves an axial rotation symmetry, resulting in a unique discrete single-particle spectrum. This can significantly affect the many-body behaviors of a Fermi gas, especially on the pairing and superfluid physics, which however has not been addressed so far. A natural and intriguing question here is:what superfluid state will strong SOAMC lead to?

This question has now been answered by a joint research team from Westlake University and Capital Normal University in a paper titled “Exotic Vortex States with Discrete Rotational Symmetry in Atomic Fermi Gases with Spin-Orbital–Angular-Momentum Coupling”, published inPhysical Review Letterson May 12th. In this paper, the team reports on the discovery of an unprecedented vortex state that spontaneously breaks the continuous rotational symmetry in a Fermi superfluid with SOAMC. Dr. Liang-Liang Wang, presently a research scientist working in Dr. Jian Li’s research group in Westlake University, is the first author of this paper, with Dr. Jian Li from Westlake University and Prof. Qing Sun from Capital Normal University being co-corresponding authors.

Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.193401

In this work, Wanget al.first show that a family of single-vortex ground state can exist in a broad regime of the phase diagram for finite two-photon detuning by solving the Bogoliubov-de Gennes equations self-consistently. This is in sharp contrast to the conventional cases where an external rotation or magnetic field is required to create vortices. What is more striking as an outcome of their further investigation, however, is the formation and stabilization of an unprecedented vortex state that spontaneously breaks the continuous rotational symmetry down to a discrete one, thus exhibiting a periodical spatial modulation in the angular dimension. The underlying mechanism for the occurrence of these vortex states is the unconventional pairing between fermions with quantized orbital angular momenta, which in turn originates from the nontrivial interplay of SOAMC, two-photon detuning and atom-atom interaction. These exotic vortex states showcase a latest surprise that quantum states of matter bear to offer.

This research work was supported by the National Natural Science Foundation of China, Westlake Education Foundation, Zhejiang Natural Science Foundation, and Academy for Multidisciplinary Studies in Capital Normal University.