Quantum spin Hall crystals at fractional filling of twisted MoTe${}_2$

Abstract

We predict and classify interaction-driven quantum spin Hall crystals (QSHCs), a class of states emerging at fractional filling through an interplay of topology and spontaneous translation-symmetry breaking. QSHCs form nearly degenerate manifolds whose members can realize distinct topological phases protected by time-reversal or valley $U(1{)}_v$ symmetry, with time-reversal acting nontrivially within the manifold. As a representative of this broad class of states we provide evidence for 9-fold quasi-degenerate $\sqrt{3}\times \sqrt{3}$ charge ordered QSHCs at $\nu =-8/3$ of twisted bilayer MoTe${}_2$ near a $5^{\circ }$ twist. Here a ${\mathbb{Z}}_3$ index organizes states related by lattice translation into three time-reversal invariant states with nontrivial ${\mathbb{Z}}_2$ topology and three time-reversal related doublets whose individual members spontaneously break time-reversal and carry a $U(1{)}_v$ protected spin-Chern number. Finally, we determine the conditions that favor QSHCs over closely competing intervalley-coherent crystals.

15 pages, 6 figures. Comments are welcome!