Spin-orbital-angular-momentum coupling in ultracold atoms

Coupling between particle’s spin and orbital motion is ubiquitous in atoms, photons, solid materials and many other systems. It contributes to the topological properties like quantum-Hall effect in solid materials and electronic fine structure in atoms. Ultracold atoms with a high tunability provides an ideal platform to study spin-orbit (SO) coupling. Spin-linear-momentum (SLM) coupling has been observed in quantum gases and subsequently a variety of exotic quantum states have been explored. While the experimental study on the other kind of SO coupling, namely the spin-orbital-angular-momentum (SOAM) coupling, is still lacking.In this talk, I will report the experimental observation of the ground-state phase diagram of the SOAM coupled Bose-Einstein condensate. By inducing a Raman transition using a pair of Gaussian and Laguerre-Gaussian (LG) laser beams, we realize SOAM coupling of ultracold atoms. We observe phase transitions when the two-photon Raman coupling strength or detuning approaches the critical value. The phase transitions are classified as the first order, which feature a discontinuous jump of the angular momentum (OAM) and the spin polarization. We demonstrate the hysteresis loop across the first-order phase transition. The role of interatomic interaction on the phase transition is also elucidated.