Ferromagnetism and topology of the higher flat band in a fractional Chern insulator

H Park and JQ Cai and E Anderson and XW Zhang and XY Liu and W Holtzmann and WJ Li and C Wang and CW Hu and YZ Zhao and T Taniguchi and K Watanabe and JH Yang and D Cobden and JH Chu and N Regnault and BA Bernevig and L Fu and T Cao and D Xiao and XD Xu, NATURE PHYSICS, 21 (2025).

DOI: 10.1038/s41567-025-02804-0

The recent observation of the fractional quantum anomalous Hall effect in moir & eacute; fractional Chern insulators provides an opportunity to investigate zero magnetic field anyons. One approach for potentially realizing non-abelian anyons is to engineer higher flat Chern bands that mimic higher Landau levels. We investigate the interaction, topology and ferromagnetism of the second moir & eacute; miniband in twisted MoTe2 bilayers. At half-filling of the second miniband, we observed spontaneous ferromagnetism and an incipient Chern insulator state. The Chern numbers of the top two moir & eacute; flat bands exhibited opposite signs for twist angles above 3.1 degrees but had the same sign near 2.6 degrees, consistent with theoretical predictions. In the 2.6 degrees device, increasing the magnetic field induced a topological phase transition due to band-crossing between opposite valleys, resulting in an emergent state with Chern number C = -2. Additionally, an insulating state at half-filling of the second valley-polarized band indicates that a charge-ordered state is favoured over the fractional Chern insulator state. These findings lay a foundation for understanding the higher flat Chern bands, which are crucial for the discovery of non- abelian fractional Chern insulators.

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