Phonon-driven Floquet-Bloch states probed by quantum beat spectroscopy

Abstract: Controlling material excitations offers access to novel fundamental and technological properties. The paradigm of Floquet engineering, the manipulation of the electronic structure using a coherent and time-periodic driving source, has attracted significant attention. While most realizations rely on strong optical fields, coherent phonons provide an alternative route to realizing Floquet-Bloch states, and are expected to enable substantially longer-lived Floquet-Bloch states. We show that laser-excited coherent phonons drive Floquet-Bloch states. Using time-resolved multiphoton photoemission combined with quantum beat spectroscopy on graphene-covered Ir(111), we track the coherent electronic dynamics of the image-potential states dressed by coherent phonons. The beat signal indicates the presence of sideband structure with the coherent-phonon frequency as its fundamental period, consistent with Floquet theory. Furthermore, an independent oscillation in intensity at the same frequency was observed, confirming excitation of the coherent phonon mode. Compared with conventional light-driven Floquet-Bloch states, the observed phonon-driven Floquet-Bloch states persist for one to two orders of magnitude longer. These results establish a time-domain route to identifying phonon-driven Floquet-Bloch states and reveal their formation on ultrafast timescales.