Magnetic Signatures of a Putative Fractional Topological Insulator in Twisted MoTe2

The interplay among electronic correlation, topology, and time-reversal-symmetry (TRS) often leads to exotic quantum states of matter. Primary examples include the recently realized fractional Chern insulators (FCIs) in twisted MoTe2 bilayers (tMoTe2) 1–4 and multilayer graphene aligned with hBN 5, where TRS is broken in partially filled flat moiré Chern bands. Among the FCIs in tMoTe2, the most robust is at a hole filling of v = -2/3 per moiré unit cell 1–4,6,7. Interestingly, transient optical sensing 8 and more recent transport measurements 9,10 revealed a correlated state at v = -4/3, twice the filling factor for the v = -2/3 FCI. Here, employing pump-probe circular dichroism (CD) measurements on tMoTe2 with twist angles θ = 3.9^o and 3.7^o, we find that the v = -4/3 state exhibits vanishing magnetization (m = 0) in finite windows of out-of-plane magnetic field |µ0H| ≤ ~2-4 mT, and a first order phase transition to ±m states at higher fields. This out-of-plane antiferromagnetic (AFM) like behavior is notably absent for all other correlated states and disappears for the v = -4/3 state at higher or lower twist angles of θ = 4.0^o and 3.3^o. The observed magnetic signature at v = -4/3 is consistent with a predicted fractional topological insulator (FTI) with TRS, consisting of two copies of -2/3 FCIs with opposite chiralities 11. We support these findings with calculations in the interacting continuum model of tMoTe2. Our work presents a candidate for fractional topological insulators with TRS 12–14.