Ferromagnetism and Topology of the Higher Flat Band in a Fractional Chern Insulator

Abstract

The recent observation of the fractional quantum anomalous Hall effect in moir\'e fractional Chern insulators provides an opportunity to investigate zero magnetic field anyons. To potentially realize non-Abelian anyons, one approach is to engineer higher flat Chern bands that mimic higher Landau levels. Here, we investigate the interaction, topology, and ferromagnetism of the second moir\'e miniband in twisted MoTe2 bilayers. At half filling of the second miniband, we observe spontaneous ferromagnetism and an incipient Chern insulator state. The Chern numbers of the top two moir\'e flat bands exhibit opposite signs for twist angles above 3.1{\deg}, but share the same sign near 2.6{\deg}, consistent with theoretical predictions. In the 2.6{\deg} device, increasing magnetic field induces a topological phase transition via 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 suggests a charge-ordered state is favored over the fractional Chern insulator state. These findings lay a foundation for understanding the higher flat Chern bands, crucial for the discovery of non-Abelian fractional Chern insulators.