Observation of Floquet states in graphene

by Marco Merboldt
Doctoral thesis
Date of Examination:2025-01-30
Date of issue:2025-09-17
Advisor:Prof. Dr. Stefan Mathias
Referee:Prof. Dr. Stefan Mathias
Referee:PD Dr. Salvatore R. Manmana
Persistent Address: http://dx.doi.org/10.53846/goediss-11509

 

 

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Abstract

English

Recent advances in the field of condensed-matter physics have unlocked the potential to realize and control emergent material phases that are not accessible in thermal equilibrium. One of the most promising concepts in this regard is Floquet engineering, the coherent dressing of matter via time-periodic perturbations. However, the broad applicability of Floquet engineering to quantum materials is still in question. Crucially, the theoretically predicted Floquet-induced effects in monolayer graphene, the most sought-after case, could not be verified or only be explored in indirect experimental approaches. More specifically, a report of the light-induced anomalous Hall effect in graphene is critically debated. Furthermore, a time-resolved photoemission experiment even indicates that Floquet effects might not be realizable in graphene and other (semi-)metals that exhibit relatively short decoherence times. Here, we resolve this long-standing debate by using electronic structure measurements to provide direct experimental evidence of Floquet engineering in graphene. We report light-matter-dressed Dirac bands by measuring the contribution of Floquet sidebands, Volkov sidebands, and their quantum path interference to graphene’s photoemission spectral function. Fully supported by experiment and theory, we demonstrate that Floquet engineering in graphene is possible, even though ultrafast decoherence processes occur on few tens of femtosecond timescales. Our results pave the way for the experimental realization of Floquet engineering strategies in (semi-)metallic systems, promising the coherent stabilization of light-induced states with potentially non-trivial topological properties.
Keywords: Ultrafast momentum microscopy; trARPES; Graphene; light-matter interaction; Floquet engineering; Floquet-Bloch; Dirac materials; Non-equilibrium quantum phases; Time-periodic driving; Ultrafast dynamics; Condensed-matter physics