Anomalous wave-packet transport on boundaries of Floquet topological systems

TL;DR

This paper explores the unusual transport behavior of wave packets at the edges of Floquet topological systems with zero winding number, revealing valley-dependent edge states and hybridization effects that influence boundary dynamics.

Contribution

It demonstrates the existence of valley-protected edge states and their hybridization in Floquet systems with zero winding number, highlighting controllable unidirectional edge transport.

Findings

Edge states at different valleys can hybridize to form bowtie-shaped bands.

Wave packet dynamics can be controlled via initial parameters and spin polarization.

Majority of particles exhibit confined bouncing while some follow chiral boundary transport.

Abstract

A two-dimensional periodically driven (Floquet) system with zero winding number in the absence of time-reversal symmetry is usually considered topologically trivial. Here, we study the dynamics of a Gaussian wave packet placed at the boundary of a two-dimensional driven system with zero winding numbers but multiple valley-protected edge states that can be realized in a square Raman lattice, and investigate the unidirectionally propagating topological edge currents. By carefully tuning the initial parameters of the wave packet including its spin polarization as well as the initial time of the periodic driving, we control the population of different edge states, where the speed of the resulting propagation establishes a direct correspondence with the target dispersions across different gaps and valleys. Interestingly, we find that the edge states at different valleys in the $\pi$ gap can hybridize and form bowtie-shaped edge bands fully detached from the bulk. This phase, not only presents a favorable regime with narrower bulk bands, but also exhibits distinct edge dynamics where the majority of particles bounce back-and-forth confined to a boundary while a small portion can follow a chiral transport around the sample.