Breakdown of helical edge state topologically protected conductance in time-reversal-breaking excitonic insulators

Gapless helical edge modes are a hallmark of the quantum spin Hall effect. Protected by time-reversal symmetry, each edge contributes a quantized zero-temperature conductance quantum G0 equivalent to e2/h. However, the experimentally observed conductance in WTe2 decreases below G0 per edge already at edge lengths around 100 nm, even in the absence of explicit time-reversal breaking due to an external field or magnetic impurities. In this work, we show how a time-reversal breaking excitonic condensate with a spin-spiral order that can form in WTe2 leads to the breakdown of conductance quantization. We perform Hartree-Fock calculations to compare time-reversal breaking and preserving excitonic insulators. Using these mean-field models we demonstrate via quantum transport simulations that weak nonmagnetic disorder reproduces the edge length scaling of resistance observed in the experiments. We complement this by analysis in the Luttinger liquid picture, shedding additional light on the mechanism behind the quantization breakdown.

Automated summary

In this work, we demonstrate how a time-reversal breaking excitonic condensate in WTe2 leads to the breakdown of conductance quantization. Through Hartree-Fock calculations and quantum transport simulations, we show that weak nonmagnetic disorder can reproduce the observed edge length scaling of resistance in experiments.