Helical Quantum Edge Gears in 2D Topological Insulators

A remarkable and as-yet-unexploited aspect of topological insulator (TI) physics is the topology of the edge states, i.e. the fact that the edge liquid of a 2D TI forms a closed, unbreakable loop in the absence of electrical contacts or magnetic fields. We propose a novel experimental setup in which edge loops rotate as interlocking “gears” through Coulomb drag, in TIs with Rashba spin-orbit coupling. We show that two-terminal transport can measure the Luttinger liquid parameter $K$, a quantity that is otherwise notoriously difficult to measure. In the low-temperature ($T \rightarrow 0$) perfect drag regime, the conductance is $(e^2/h)(2 K + 1)/(K + 1)$. At higher $T$ we predict a conductivity $\sim T^{-4K+3}$. Our results should trigger new experiments and may open a new venue for edge gear-based electronic devices.\\ Ref: Phys. Rev. Lett. 115, 186404 (2015)