Jan Philipp Bange

We report atomic scale structure and phonon thermal transport in (LaMnO ₃ ) _m /(SrMnO ₃ ) _n /SrTiO ₃ (100) superlattices (LMO/SMO SLs) with the thickness of individual layers m , n = 1 − 10 u.c. and the thickness ratio m / n = 1, 2. Optical transient thermal reflectivity measurements reveal pronounced differences in the thermal conductivity between SLs with m / n = 1 and SLs with m / n = 2. State‐of‐the art electron microscopy techniques and ab initio density functional calculations enable us to assign the origin of this difference to the differences in the oxygen octahedral rotation (OOR) angle φ _OOR within the LMO layers in these SLs. Namely, LMO and SMO layers within the ( m / n = 1) SLs possess the same φ _OOR = 180°, which is characteristic of a cubic structure. In contrast, the m / n = 2 SLs reveal a significantly smaller value φ _OOR = 165° in LMO and a large φ _OOR = 180° in SMO. The analysis of experimental data shows that the thermal conductance of the LMO/SMO interfaces in m / n = 2 SLs with OOR misfit is low 0.3 GWm ⁻² K ⁻¹ , whereas a surprisingly large value of interfacial conductance 1.8 GWm ⁻² K ⁻¹ was determined for m / n = 1 SLs without OOR misfit. Moreover, a minimum of thermal conductivity of κ _min = 0.3–0.4 Wm ⁻¹ K ⁻¹ has been observed at the interface density 1/(Λ = m+n)∼0.11–0.25 u.c. ⁻¹ , denoting the crossover between the interface‐ to volume‐dependent thermal conductivity. Our results point out the importance of MnO ₆ octahedral rotations/tilts as an effective structural control parameter to tune thermal transport and magnetism in LMO/SMO SLs by changing the LMO/SMO thickness ratio.

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.

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).

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 − MoS 2 , 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.

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.

Two-dimensional transition metal dichalcogenides and organic semiconductors have emerged as promising material platforms for optoelectronic devices. Combining the two is predicted to yield emergent properties while retaining the advantages of each. In organic semiconductors, the optoelectronic response is typically dominated by localized Frenkel-type excitons, whereas transition metal dichalcogenides host delocalized Wannier-type excitons. However, much less is known about the characteristics of excitons at hybrid interfaces between these materials, which determine the possible energy- and charge-transfer pathways. Here we identify a hybrid exciton at one such interface using ultrafast momentum microscopy and many-body perturbation theory. We show that this hybrid exciton, formed predominantly via resonant Förster energy transfer, has both Frenkel- and Wannier-type contributions: intralayer electron–hole transitions within the organic semiconductor layer and interlayer transitions across the interface give rise to an exciton wavefunction with mixed character. This work advances our understanding of charge and energy transfer processes across 2D–organic heterostructures.

Floquet engineering—the coherent dressing of matter via time-periodic perturbations—is a mechanism to realize and control emergent phases in materials out of equilibrium. However, its applicability to metallic quantum materials and semimetals such as graphene is an open question. The report of light-induced anomalous Hall effect in graphene remains debated, and a time-resolved photoemission experiment has suggested that Floquet effects might not be realizable in graphene and other semimetals with relatively short decoherence times. Here we provide direct spectroscopic evidence of Floquet effects in graphene through electronic structure measurements. We observe light–matter-dressed Dirac bands by measuring the contribution of Floquet sidebands, Volkov sidebands and their quantum path interference to graphene’s photoemission spectrum. Our results demonstrate that Floquet engineering in graphene is possible, even though ultrafast decoherence processes occur on the timescale of a few tens of femtoseconds. Our approach offers a way to experimentally realize Floquet engineering strategies in metallic and semimetallic systems and for the coherent stabilization of light-induced states with potentially non-trivial topological properties.

Excitons are realizations of a correlated many-particle wave function, specifically consisting of electrons and holes in an entangled state. Excitons occur widely in semiconductors and are dominant excitations in semiconducting organic and low-dimensional quantum materials. To efficiently harness the strong optical response and high tuneability of excitons in optoelectronics and in energy-transformation processes, access to the full wavefunction of the entangled state is critical, but has so far not been feasible. Here, we show how time-resolved photoemission momentum microscopy can be used to gain access to the entangled wavefunction and to unravel the exciton’s multiorbital electron and hole contributions. For the prototypical organic semiconductor buckminsterfullerene (C₆₀), we exemplify the capabilities of exciton tomography and achieve unprecedented access to key properties of the entangled exciton state including localization, charge-transfer character, and ultrafast exciton formation and relaxation dynamics.