Hauptinhalt

2026

  • W. Bennecke, T. L. Dinh, J. P. Bange, D. Schmitt, M. Merboldt, L. Weinhagen, B. v. Wingerden, F. Frassetto, L. Poletto, M. Reutzel, D. Steil, D. R. Luke, S. Mathias, G. S. M. Jansen, Table-top three-dimensional photoemission orbital tomography with a femtosecond extreme ultraviolet light source, Nat. Commun. 17, 5457 (2026).

    Following electronic processes in molecules and materials at the level of the quantum mechanical electron wavefunction with ångström-level spatial resolution and with full access to its femtosecond temporal dynamics is at the heart of ultrafast condensed matter physics. A breakthrough invention allowing experimental access to electron wavefunctions was the reconstruction of molecular orbitals from angle-resolved photoelectron spectroscopy data in 2009, termed photoemission orbital tomography (POT). This invention opens a route towards ultrafast three-dimensional (3D) POT, with many new prospects for the study of ultrafast light-matter interaction, femtochemistry, and photo-induced phase transitions. Here, we develop a synergistic experimental-algorithmic approach to realize the first 3D-POT experiment using a short-pulse extreme ultraviolet light source. We combine a new variant of photoelectron spectroscopy, namely ultrafast momentum microscopy, with a table-top spectrally-tunable high-harmonic generation light source and a tailored algorithm for efficient 3D reconstruction from sparse, undersampled data. This combination dramatically speeds up the experimental data acquisition, while at the same time reducing the sampling requirements to achieve complete 3D information. We demonstrate the power of this approach by full 3D imaging of the frontier orbitals of a prototypical organic semiconductor adsorbed on pristine Ag(110).

    DOI: 10.1038/s41467-026-74308-1

  • K. Gadge, M. Merboldt, M. Schüler, J. P. Bange, W. Bennecke, M. A. Sentef, M. Reutzel, S. Mathias, S. R. Manmana, Comparative study of perturbative and nonequilibrium Green's function approaches for Floquet sidebands in periodically driven quantum systems, Phys. Rev. B 114, 125402 (2026)

    We compare two complementary theoretical approaches to compute and interpret Floquet sidebands in periodically driven quantum materials: a first-order perturbative approach (first-order perturbative Born approximation, PB1) and time-dependent nonequilibrium Green's functions (tdNEGF). Using graphene as a model Dirac system, we disentangle in pump-probe setups Floquet-dressed initial states, Volkov-dressed final states (also known as laser-assisted photoelectric effect), and their interference. We quantify how photoemission matrix elements, polarization, incidence angle, and near-surface screening shape the momentum-resolved sideband intensity observed in tr-ARPES. PB1 yields an analytical expression for the momentum-dependent sideband intensity and for graphene it captures the correct symmetry trends, such as the magnitude of the intensities when considering the interference between the Floquet and the Volkov states and photoemission matrix elements. tdNEGF reproduces the full energy-momentum-resolved spectra, including hybridization gaps and spectral-weight redistribution. We find qualitative agreement between PB1 and tdNEGF once matrix elements are included; quantitative differences arise near hybridization regions and at specific angles where higher-order processes and self-energies are essential. Thus, for systems with simple band structures and away from these regions, the two approaches can be used in a complementary way.

    DOI: 10.1103/h59r-hyc4

  • G. Meneghini, M. Löwe, R. Perea-Causin, J. P. Bange, W. Bennecke, M. Reutzel, S. Mathias, E. Malic, ARPES Signatures of Trions in van der Waals Materials, Nano Lett. 2026, 26, 17, 5866–5872 (2026).

    Angle-resolved photoemission spectroscopy (ARPES) has recently emerged as a direct probe of excitonic correlations in two-dimensional semiconductors, resolving their dispersion and dynamics in energy–momentum space, including dark exciton states inaccessible to optical techniques. However, the ARPES fingerprint of charged excitons (trions), which plays a key role in all doped and gated two-dimensional (2D) material systems, has remained unknown so far. We present a first theoretical analysis of trion signatures in monolayer transition-metal dichalcogenides, highlighting how the additional charge carrier modifies the spectral position and shape relative to neutral excitons in ARPES spectra. Interestingly, we further predict that mass-imbalanced trions yield a characteristic double-peak structure, clearly separated in energy and line shape from neutral excitons. The predicted temperature dependence of these features offers guidance for experimental investigations aimed at identifying trionic states, thereby establishing a framework for ARPES studies of many-body Coulomb complexes in doped 2D semiconductors.

    DOI: 10.1021/acs.nanolett.6c00729

  • M. A. Sentef & M. Reutzel, A Floquet gap finally opens in graphene, Nat. Mater. (2026).

    Time- and angle-resolved photoemission spectroscopy now resolves the long-sought avoided crossing in light-driven graphene, establishing a practical recipe for Floquet band engineering and sharpening prospects for light-induced topology in quantum materials.

    DOI: 10.1038/s41563-026-02548-z

  • P. Werner, W. Bennecke, J. P. Bange, G. Meneghini, D. Schmitt, M. Merboldt, A. M. Seiler, A. AlMutairi, K. Watanabe, T. Taniguchi, G. S. M. Jansen, J. Liu, D. Steil, S. Hofmann, R. T. Weitz, E. Malic, S. Mathias, M. Reutzel, Role of Nonequilibrium Populations in Dark-Exciton Formation, Phys. Rev. Lett. 136 (2026).

    The optical excitation of a bright exciton may be followed by the formation of lower-energy dark states. In these formation and relaxation processes, nonequilibrium exciton and phonon populations play a dominant role but remain so far largely unexplored, as most states are inaccessible by regular spectroscopies. Here, on the example of homobilayer 2⁢H−MoS2, we realize direct access to the full exciton relaxation cascade from experiment and theory. We find distinct changes in the time-, energy-, and in-plane momentum-resolved photoemission spectral function that can be explained only when considering the formation and subsequent thermalization of excitonic nonequilibrium occupation distributions. In agreement with microscopic many-particle calculations, we quantify the timescales for the formation of a nonequilibrium dark-excitonic occupation and its subsequent thermalization to 85 and 150 fs, respectively. Our results provide a previously inaccessible view of the complete exciton relaxation cascade, which is of importance for the future characterization of nonequilibrium excitonic phases and the efficient design of optoelectronic devices.

    DOI: 10.1103/w29j-z48v

  • Y. Yen, M. Reutzel, A. Li, Z. Wang, H. Petek, M. Schüler, Observation of Nonadiabatic Landau-Zener Tunneling among Floquet States, Phys. Rev. X 16 (2026).

    Electromagnetic fields not only induce electronic transitions but also fundamentally modify the quantum states of matter through strong light-matter interactions. As one established route, Floquet engineering provides a powerful framework to dress electronic states with time-periodic fields, giving rise to quasi-stationary Floquet states. With increasing field strength, non-perturbative responses of the dressed states emerge, yet their nonlinear dynamics remain challenging to interpret. In this work we explore the emergence of non-adiabatic Landau-Zener transitions among Floquet states in Cu(111) under intense optical fields. At increasing field strength, we observe a transition from perturbative dressing to a regime where Floquet states undergo non-adiabatic tunneling, revealing a breakdown of adiabatic Floquet evolution. These insights are obtained through interferometrically time-resolved multi-photon photoemission spectroscopy, which serves as a sensitive probe of transient Floquet state dynamics. Numerical simulations and the theory of instantaneous Floquet states allow us to directly examine real-time excitation pathways in this non-perturbative photoemission regime. Our results establish a direct connection the onset of light-dressing of matter, non-perturbative ultrafast lightwave electronics, and high-optical-harmonic generation in the solids.

    https://arxiv.org/abs/2502.18269, https://arxiv.org/abs/2503.04431