Dynamical characterization of Weyl nodes in Floquet Weyl semimetal phases

TL;DR

This paper introduces a dynamical invariant to distinguish Weyl points at different quasienergies in Floquet Weyl semimetals, aiding their topological characterization and experimental detection.

Contribution

It proposes a new dynamical topological invariant for Floquet Weyl semimetals, enabling differentiation of Weyl points at distinct quasienergies and advancing understanding of their topological properties.

Findings

Dynamical invariant effectively characterizes Weyl points at different quasienergies.

Model exhibits unlimited Weyl points with increasing system parameters.

Transport signatures linked to Weyl points are analyzed.

Abstract

Due to studies in nonequilibrium (periodically-driven) topological matter, it is now understood that some topological invariants used to classify equilibrium states of matter do not suffice to describe their nonequilibrium counterparts. Indeed, in Floquet systems the additional gap arising from the periodicity of the quasienergy Brillouin zone often leads to unique topological phenomena without equilibrium analogues. In the context of Floquet Weyl semimetal, Weyl points may be induced at both quasienergy zero and $\pi/T$ ($T$ being the driving period) and these two types of Weyl points can be very close to each other in the momentum space. Because of their momentum-space proximity, the chirality of each individual Weyl point may become hard to characterize in both theory and experiments, thus making it challenging to determine the system's overall topology. In this work, inspired by the construction of dynamical winding numbers in Floquet Chern insulators, we propose a dynamical invariant capable of characterizing and distinguishing between Weyl points at different quasienergy values, thus advancing one step further in the topological characterization of Floquet Weyl semimetals. To demonstrate the usefulness of such a dynamical topological invariant, we consider a variant of the periodically kicked Harper model (the very first model in studies of Floquet topological phases) that exhibits many Weyl points, with the number of Weyl points rising unlimitedly with the strength of some system parameters. Furthermore, we investigate the two-terminal transport signature associated with the Weyl points. Theoretical findings of this work pave the way for experimentally probing the rich topological band structures of some seemingly simple Floquet semimetal systems.