Light-induced ferromagnetism in moire superlattices

Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would allow the possibility to access and control complex electronic phase diagrams. Recently, two-dimensional moire superlattices have emerged as a promising platform for quantum engineering such phenomena(1-3). The power of the moire system lies in the high tunability of its physical parameters by adjusting the layer twist angle(1-3), electrical field(4-6), moire carrier filling(7-11) and interlayer coupling(12). Here we report that optical excitation can highly tune the spin-spin interactions between moire-trapped carriers, resulting in ferromagnetic order in WS2/WSe2 moire superlattices. Near the filling factor of -1/3 (that is, one hole per three moire unit cells), as the excitation power at the exciton resonance increases, a well-developed hysteresis loop emerges in the reflective magnetic circular dichroism signal as a function of magnetic field, a hallmark of ferromagnetism. The hysteresis loop persists down to charge neutrality, and its shape evolves as the moire superlattice is gradually filled, indicating changes of magnetic ground state properties. The observed phenomenon points to a mechanism in which itinerant photoexcited excitons mediate exchange coupling between moire-trapped holes. This exciton-mediated interaction can be of longer range than direct coupling between moire-trapped holes(9), and thus magnetic order arises even in the dilute hole regime. This discovery adds a dynamic tuning knob to the rich many-body Hamiltonian of moire quantum matter(13-19).

Automated summary

Many-body interactions between carriers play a crucial role in correlated physics. This study demonstrates the ability to highly tune spin-spin interactions between moire-trapped carriers using optical excitation, resulting in ferromagnetic order in WS2/WSe2 moire superlattices. The observed phenomenon adds a dynamic tuning knob to the rich many-body Hamiltonian of moire quantum matter.