Evidence of the fractional quantum spin Hall effect in moiré MoTe 2

Quantum spin Hall (QSH) insulators are two-dimensional electronic materials that have a bulk band gap similar to an ordinary insulator but have topologically protected pairs of edge modes of opposite chiralities 1 – 6 . So far, experimental studies have found only integer QSH insulators with counter-propagating up-spins and down-spins at each edge leading to a quantized conductance G 0 = e 2 / h (with e and h denoting the electron charge and Planck’s constant, respectively) 7 – 14 . Here we report transport evidence of a fractional QSH insulator in 2.1° twisted bilayer MoTe 2 , which supports spin- S z conservation and flat spin-contrasting Chern bands 15 , 16 . At filling factor ν = 3 of the moiré valence bands, each edge contributes a conductance 3/2G_0 with zero anomalous Hall conductivity. The state is probably a time-reversal pair of the even-denominator 3/2-fractional Chern insulators. Furthermore, at ν = 2, 4 and 6, we observe a single, double and triple QSH insulator with each edge contributing a conductance G 0 , 2 G 0 and 3 G 0 , respectively. Our results open up the possibility of realizing time-reversal symmetric non-abelian anyons and other unexpected topological phases in highly tunable moiré materials 17 – 19 .