Evidence of frustrated magnetic interactions in a Wigner-Mott insulator.

Electrons in two-dimensional semiconductor moiré 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 moiré lattice in angle-aligned WSe 2 /WS 2 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 moiré materials as a model system for lattice-spin physics and frustrated magnetism 5 .