Relativistic Mott transition in twisted WSe2 tetralayers.

The realization of graphene has provided a bench-top laboratory for quantum electrodynamics. The low-energy excitations of graphene are two-dimensional massless Dirac fermions with opposite chiralities at the ±K valleys of the graphene Brillouin zone. It has been speculated that the electron-electron interactions in graphene could spontaneously break the chiral symmetry to induce a finite mass for Dirac fermions, which is also known as the relativistic Mott transition. However, the phenomenon has not been observed in pristine graphene because the interaction strength is insufficient. Here, we report the realization of strongly correlated artificial graphene and the observation of the relativistic Mott transition in twisted WSe2 tetralayers. Using magnetotransport, we show that the first Γ-valley moiré valence band mimics the low-energy graphene band structure. At half band filling, the system exhibits hallmarks of massless Dirac fermions, including an anomalous Landau fan originated from a π Berry phase and a square-root density dependence of the cyclotron mass. We tune the interaction across the semimetal-insulator transition by reducing the twist angle below about 2.7°. The emergent insulator is compatible with an antiferromagnetic Mott insulator. Our results open the possibility of studying strongly correlated Dirac fermions in a condensed matter system.