Tunable transient valley-pseudospin orders in a moiré Bose-Hubbard model

We demonstrate exciton-filling and magnetic field tunable exciton valley-pseudospin orders in a doped correlated insulator of excitons. We find evidence of an in-plane order of exciton ‘spin’ – here, valley pseudospin – around exciton filling vex = 1, which strongly suppresses the out-of-plane ‘spin’ polarization. Upon increasing vex 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. The phase diagram is qualitatively captured by a spin-1/2 Bose–Hubbard model.