LAUSR

Studies of two-dimensional electron systems in a strong magnetic field revealed the quantum Hall effect 1 , a topological state of matter featuring a finite Chern number C and chiral edge states 2 , 3 . Haldane 4 later theorized that Chern insulators with integer quantum Hall effects could appear in lattice models with complex hopping parameters even at zero magnetic field. The ABC-trilayer graphene/hexagonal boron nitride (ABC-TLG/hBN) moiré superlattice provides an attractive platform with which to explore Chern insulators because it features nearly flat moiré minibands with a valley-dependent, electrically tunable Chern number 5 , 6 . Here we report the experimental observation of a correlated Chern insulator in an ABC-TLG/hBN moiré superlattice. We show that reversing the direction of the applied vertical electric field switches the moiré minibands of ABC-TLG/hBN between zero and finite Chern numbers, as revealed by large changes in magneto-transport behaviour. For topological hole minibands tuned to have a finite Chern number, we focus on quarter filling, corresponding to one hole per moiré unit cell. The Hall resistance is well quantized at h /2 e 2 (where h is Planck’s constant and e is the charge on the electron), which implies C  = 2, for a magnetic field exceeding 0.4 tesla. The correlated Chern insulator is ferromagnetic, exhibiting substantial magnetic hysteresis and a large anomalous Hall signal at zero magnetic field. Our discovery of a C  = 2 Chern insulator at zero magnetic field should open up opportunities for discovering correlated topological states, possibly with topological excitations 7 , in nearly flat and topologically nontrivial moiré minibands. A topological Chern insulating state is reported to emerge from strong correlations in flat moiré bands in a graphene superlattice and by applying a vertical electric field the Chern number is switched.