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Tunable correlated Chern insulator and ferromagnetism in a moiré superlattice

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...

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Published in:Nature (London) 2020-03, Vol.579 (7797), p.56-61
Main Authors: Chen, Guorui, Sharpe, Aaron L., Fox, Eli J., Zhang, Ya-Hui, Wang, Shaoxin, Jiang, Lili, Lyu, Bosai, Li, Hongyuan, Watanabe, Kenji, Taniguchi, Takashi, Shi, Zhiwen, Senthil, T., Goldhaber-Gordon, David, Zhang, Yuanbo, Wang, Feng
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Language:English
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Summary: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.
ISSN:0028-0836
1476-4687
DOI:10.1038/s41586-020-2049-7