Probing topological Floquet states in graphene with ultrafast terahertz scanning tunneling microscopy

TL;DR

This paper introduces ultrafast terahertz scanning tunneling microscopy (THz-STM) as a novel real-space, energy-resolved method to directly observe Floquet topological states and gaps in graphene, advancing ultrafast quantum materials research.

Contribution

It develops a nonequilibrium Green's-function formalism for time-dependent tunneling and applies it to demonstrate THz-STM's capability to detect Floquet gaps and edge states in graphene systems.

Findings

THz-STM can directly detect Floquet-induced gaps in graphene.

Time- and space-resolved imaging of Floquet edge states is possible.

Chiral impurities produce characteristic THz-STM signatures.

Abstract

Floquet control of band topology is a central theme in ultrafast quantum materials science. Established experimental probes of light-induced topological states include ultrafast transport and time- and angle-resolved photoemission spectroscopy, each with important strengths but also well-known limitations. Here we propose ultrafast terahertz scanning tunneling microscopy (THz-STM) as a real space energy-resolved probe of Floquet physics. We show that THz-STM enables direct local detection of bulk Floquet gaps and distinct Floquet edge state signatures. We derive a nonequilibrium Green's-function formalism for time-dependent tunneling that directly extends standard STM theory and provides an intuitive interpretation of rectified ultrafast tunneling currents. We apply the approach to bulk graphene and graphene nanoribbons of variable width. For the bulk, we show that THz-STM provides direct spectroscopic access to Floquet-induced gap openings, and we contrast pulsed pump-probe protocols with the continuous-wave Floquet steady-state limit. For finite ribbons, we demonstrate time- and space-resolved imaging of Floquet-induced topological edge states and identify the ribbon-width scale below which edge state protection breaks down. We further show how band structures of graphene nanoribbons and Floquet chiral edge modes can be reconstructed via Floquet quasiparticle interference. Finally we demonstrate that chiral impurities that break time-reversal symmetry induce characteristic spatial THz-STM signatures that can be used as a direct probe of Floquet edge state chirality.