Quantized Transport in Floquet Topological Insulators

TL;DR

This paper investigates quantized quantum transport in Floquet topological insulators, revealing a sum rule for conductance quantization involving Floquet sidebands and providing insights accessible to experimental verification.

Contribution

It introduces a Floquet sum rule for conductance quantization in driven topological systems, supported by exact numerics and analytical understanding.

Findings

Longitudinal conductance quantized as |W_ε| e^2/h

Hall conductance quantized as W_ε e^2/h

Sum over Floquet sidebands yields exact quantization

Abstract

We study quantum transport in a periodically driven (Floquet) topological system coupled to static fermionic reservoirs. Using the Floquet nonequilibrium Green's-function (NEGF) formalism we show, from exact numerics for a strip geometry, that the two-terminal (longitudinal) conductance is quantized as $|W_{\varepsilon}|\,e^2/h$, while the Hall (transverse) conductance is quantized as $W_{\varepsilon}\,e^2/h$, where $W_{\varepsilon}$ is the Floquet winding invariant associated with the quasienergy gap at $\varepsilon = 0$ or $\varepsilon = \Omega/2$. Quantization is achieved only after summing over the contribution of all Floquet sidebands. We provide an analytic understanding of this Floquet conductance sum rule, by considering the Hall conductance in the weak coupling limit. In that limit, we show that the Floquet Hall conductance gets contributions from the Floquet sidebands, which includes the signs of the velocities of the edge modes. Their sum yields exact quantization, as predicted by the Floquet sum rule. We find that in a wide range of parameter regime, the convergence is fast, making observation of the sum rule and Floquet winding numbers accessible to experiments.