Evidence of frustrated magnetic interactions in a Wigner-Mott insulator

Electrons in two-dimensional semiconductor moire materials are more delocalized around the lattice sites than those in conventional solids(1,2). The non-local contributions to the magnetic interactions can therefore be as important as the Anderson superexchange(3), which makes the materials a unique platform to study the effects of competing magnetic interactions(3,4). Here we report evidence of strongly frustrated magnetic interactions in a Wigner-Mott insulator at a two-thirds (2/3) filling of the moire lattice in angle-aligned WSe2/WS2 bilayers. Magneto-optical measurements show that the net exchange interaction is antiferromagnetic for filling factors below 1 with a strong suppression at a 2/3 filling. The suppression is lifted on screening of the long-range Coulomb interactions and melting of the Wigner-Mott insulators by a nearby metallic gate. The results can be qualitatively captured by a honeycomb-lattice spin model with an antiferromagnetic nearest-neighbour coupling and a ferromagnetic second-neighbour coupling. Our study establishes semiconductor moire materials as a model system for lattice-spin physics and frustrated magnetism(5).