Tunable exciton valley-pseudospin orders in moiré superlattices.

Excitons in two-dimensional (2D) semiconductors have offered an attractive platform for optoelectronic and valleytronic devices. Further realizations of correlated phases of excitons promise device concepts not possible in the single particle picture. Here we report tunable exciton "spin" orders in WSe₂/WS₂ moiré superlattices. We find evidence of an in-plane (xy) order of exciton "spin"—here, valley pseudospin—around exciton filling v_ex = 1, which strongly suppresses the out-of-plane "spin" polarization. Upon increasing v_ex or applying a small magnetic field of ~10 mT, it transitions into an out-of-plane ferromagnetic (FM-z) spin order that spontaneously enhances the "spin" polarization, i.e., the circular helicity of emission light is higher than the excitation. The phase diagram is qualitatively captured by a spin-1/2 Bose–Hubbard model and is distinct from the fermion case. Our study paves the way for engineering exotic phases of matter from correlated spinor bosons, opening the door to a host of unconventional quantum devices. Control of correlated excitonic states is a key goal of modern optoelectronic physics. Here, the authors demonstrate filling- and field-tunable exciton valley-pseudospin orders in a moiré heterostructure. [ABSTRACT FROM AUTHOR]