14.05.2025 New publication in Nature Physics: "Observation of Floquet states in graphene"

Floquet engineering works. As the Kapitza pendulum finds new equilibria due to time-dependent shaking (check-out Youtube videos!), so can quantum materials.

Over the last years, there have been fascinating theoretical predictions on how time-periodic light fields can be used to coherently control the electronic band structure and even the topological phase of quantum materials. It all started with a pioneering paper by Oka and Aoki 16 years ago [Phys. Rev. B 79, 081406 (2009)]: They proposed that in graphene, time-reversal symmetry can be coherently broken via irradiation with circular polarized light. This would realize the Haldane model—one of the most paradigmatic models of topology in condensed-matter physics. However, no experiment could provide unambiguous proof for the existence of such Floquet effects in graphene.

In our new publication, we use femtosecond momentum microscopy to experimentally probe Floquet states in graphene. Specifically, we measure the quantum path interference of Floquet states and Volkov states, and thus demonstrate the feasibility of Floquet engineering in graphene.

We thank the key players of this research: Marco Merboldt and David Schmitt. Moreover, a big thanks to the teams around Michael Schüler (PSI, Schweiz), Klaus Pierz (PTB Braunschweig), Michael Sentef (Uni Bremen), and Stefan Mathias (Uni Göttingen). 

Publication:

Merboldt, M., Schüler, M., Schmitt, D. et al. Observation of Floquet states in graphene. Nat. Phys. (2025). DOI: 10.1038/s41567-025-02889-7