Time-reversal symmetry breaking fractional quantum spin Hall insulator in moir\'e MoTe2

TL;DR

This paper reports the discovery of spontaneous time-reversal symmetry breaking and ferromagnetism in twisted bilayer MoTe2, providing new insights into the nature of fractional quantum spin Hall insulators in moiré materials.

Contribution

It demonstrates spontaneous time-reversal symmetry breaking in twisted bilayer MoTe2 and explores its implications for fractional quantum spin Hall insulators, a novel phenomenon in this material system.

Findings

Spontaneous time-reversal symmetry breaking observed at various fillings.

Finite anomalous Hall response and magnetic hysteresis at low magnetic fields.

Tendency towards ferromagnetism in doped Chern bands.

Abstract

Twisted bilayer transition metal dichalcogenide semiconductors, which support flat Chern bands with enhanced interaction effects, realize a platform for fractional Chern insulators and fractional quantum spin Hall (FQSH) insulators. A recent experiment has reported the emergence of a FQSH insulator protected by spin-Sz conservation at a moir\'e lattice filling factor {\nu}=3 in 2.1-degree twisted bilayer MoTe2. Theoretical studies have proposed both time-reversal symmetric and asymmetric ground states as possible candidates for the observed FQSH insulator, but the nature of the state remains unexplored. Here we report the observation of spontaneous time-reversal symmetry breaking at generic fillings in 2.1-degree twisted bilayer MoTe2 from {\nu}<1 all the way to {\nu}>6 except at {\nu}=2, 4, and 6. Although zero Hall response is observed at {\nu}=3 for magnetic fields higher than 20 mT, a finite anomalous Hall response accompanied by a magnetic hysteresis is observed at lower magnetic fields, demonstrating spontaneous time-reversal symmetry breaking. Our work shows the tendency towards ferromagnetism by doping the first three pairs of conjugate Chern bands in the material; it also sheds light on the nature of the FQSH insulator at {\nu}=3.