Composite fermions in the $ν=3$ fractional quantum spin Hall effect

Abstract

Well-understood fractional quantum Hall states in GaAs and graphene can be described in terms of weakly interacting composite fermions. It is natural to expect that the same unifying principle applies to the putative fractional quantum spin Hall effect in MoTe$_2$. Since the quantum spin Hall effect involves two spin components, two types of composite fermions must be present. We classify all two-component composite-fermion states at the filling factor $ν=3$. The classification includes the three classes of states, which were introduced from different physical perspectives in Refs. Sodemann Villadiego, Phys. Rev. B 110, 045114 (2024), Jian et al., Phys. Rev. X 15, 021063 (2025), and May-Mann et al., Phys. Rev. B 111, L201111, (2025), as well as two new classes of states. A majority of the composite-fermion states break the time-reversal symmetry. We review quasiparticle charges, statistics, and edge theories for each possible state. We also address a way of identifying the experimentally relevant state or states. This can be accomplished by combining three probes. First, the shot noise technique provides information about fractional charges. Second, thermal conductance helps count edge modes. The third probe is based on a new idea and involves transport between two quantum point contacts along a single edge. We find that the current from one contact to the other depends on the shape of the edge channel, which can be controlled with a side gate. The probe reveals the emergent symmetry group of the low-energy edge theory.