Ground-state phase diagram and excitation spectrum of a Bose-Einstein condensate with spin-orbital-angular-momentum coupling

TL;DR

This paper explores the phase diagram and excitation spectrum of a spinor Bose-Einstein condensate with spin-orbital-angular-momentum coupling, revealing new angular-stripe phases and their distinct excitation features.

Contribution

It introduces the existence of asymmetric angular-stripe phases at large detuning and large OAM difference, expanding understanding of phase behavior in SOAM-coupled BECs.

Findings

Angular-stripe phases can exist at large detuning with large OAM difference.

Distinct excitation spectra characterize different phases, including gapless bands and roton-like structures.

Predictions are applicable to recent experiments with SOAM-coupled BECs.

Abstract

We investigate the ground-state phase diagram and excitation spectrum of an interacting spinor Bose-Einstein condensate with spin-orbital-angular-momentum (SOAM) coupling realized in recent experiments by introducing atomic Raman transition with a pair of copropagating Laguerre-Gaussian laser beams that carry different orbital angular momenta (OAM) [Chen et al., Phys. Rev. Lett. 121, 113204 (2018) and Zhang et al., Phys. Rev. Lett. 122, 110402 (2019)]. Because of the ground-state degeneracy of the single-particle Hamiltonian at vanishing detuning, several angular-stripe phases, which are superposition of states with different angular quantum numbers, appear in the phase diagram. However, these phases normally exist at small detuning, which makes them hard to be probed in experiments. We show that for a large OAM difference of the laser beams, an asymmetric kind of angular-stripe phase can exist even at large detuning. The excitation spectra in different phases exhibit distinct features: In the angular-stripe phase there exist two gapless bands corresponding to the broken U(1) and rotational symmetries, while in the half-skyrmion phase the gapless band exhibits a roton-like structure. Our predictions of the angular-stripe phases and the low-energy excitations can be examined in recently realized BECs with SOAM coupling.