Tunable valley ferromagnetism of excitons in a moiré Bose-Hubbard model

Pump-probe spectroscopy provides a rich toolbox for probing and controlling quantum materials. Here we utilize a novel pump-probe technique to demonstrate a correlated insulator phase of excitons in WSe2/WS2 moire superlattices. Further doping the correlated insulator with more excitons, we find evidence of an in-plane order of exciton "spin" – here, valley pseudospin – which strongly suppresses the out-of-plane "spin" polarization. Upon increasing exciton filling or applying a small magnetic field of ~10 mT, it transitions into an out-of-plane ferromagnetic spin order that spontaneously enhances the "spin" polarization, i.e., the circular helicity of emission light is higher than the excitation. Our study established long-lived excitons in WSe2WS2 as a platform for investigating correlated excitons and Bose-Hubbard model physics.