Abstract
Pumping graphene with circularly polarized light is the archetype of light-tailoring topological bands. Realizing the induced Floquet-Chern-insulator state and demonstrating clear experimental evidence for its topological nature has been a challenge, and it has become clear that scattering effects play a crucial role. We tackle this gap between theory and experiment by employing microscopic quantum kinetic calculations including realistic electron-electron and electron-phonon scattering. Our theory provides a direct link to the build up of the Floquet-Chern-insulator state in light-driven graphene and its detection in time- and angle-resolved photoemission spectroscopy (ARPES). This approach allows us to study the robustness of the Floquet features against dephasing and thermalization effects. We also discuss the ultrafast Hall response in the laser-heated state. Furthermore, the induced pseudospin texture and the associated Berry curvature give rise to momentum-dependent orbital magnetization, which is reflected in circular dichroism in ARPES (CD-ARPES). Combining our nonequilibrium calculations with an accurate one-step theory of photoemission allows us to establish a direct link between the build up of the topological state and the dichroic pump-probe photoemission signal. The characteristic features in CD-ARPES are shown to be stable against heating and dephasing effects. Thus, tracing circular dichroism in time-resolved photoemission provides new insights into transient topological properties.
1 More- Received 25 March 2020
- Revised 23 June 2020
- Accepted 25 August 2020
DOI:https://doi.org/10.1103/PhysRevX.10.041013
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Many attempts have been made to turn graphene into a topological insulator, an exotic material that conducts on its surface yet insulates in its interior. The marriage of topological insulators with the unique electronic structure of graphene could offer a rich experimental playground for exploring many novel quantum phenomena as well as applications such as fault-tolerant quantum computing. But realizing and demonstrating topological states in graphene remains a significant experimental challenge. As a step toward overcoming this hurdle, we propose a new probe of transient topological states in graphene that relies on spectroscopic analysis of circularly polarized light.
Our analysis focuses on a state-of-the-art tool for measuring the energy and momenta of electrons known as time- and angle-resolved photoemission spectroscopy (TR-ARPES). In this technique, one laser excites electrons in a sample and a subsequent laser probes the evolving electronic behavior. Our proposal to probe topological states is based on circular dichroism in TR-ARPES. Through simulations, we show that a circularly polarized excitation laser can induce a topological insulating state in graphene. This leaves a signature in the graphene that reveals itself via a circularly polarized probe laser: One polarization direction will be absorbed more than the other.
Analysis of circular dichroism in TR-ARPES-based investigations can therefore provide a powerful tool for realizing and investigating transient topological properties in graphene.