Frank Stefan Tautz

Strong light fields have unlocked entirely new possibilities to tailor coherent electron trajectories, engineer band structures and shape emergent phases of matter all-optically ^1–9 . Unravelling the underlying quantum mechanisms requires a visualization of the lightwave-driven electron motion ^10,11 directly in the band structure. Although photoelectron momentum microscopy has imaged photoexcited electrons averaged over many cycles of light ^12–19 , subcycle band-structure videography has been limited to small electron momenta ^8,20 . Yet, lightwave-driven elementary processes in quantum materials often occur throughout momentum space. Here we introduce attosecond-precision, subcycle band-structure videography covering the entire first Brillouin zone and visualize one of the most fundamental but elusive strong-field processes: non-adiabatic Landau–Zener–Majorana tunnelling. The interplay of strong-field-driven acceleration within the Dirac-like band structure of graphene and periodic Landau–Zener–Majorana interband tunnelling manifests in a coherent subcycle displacement and distortion of the carrier distribution at the Brillouin zone edge. The extremely non-thermal electron distributions also allow us to disentangle competing scattering processes and assess their impact on coherent electronic control. Our panoramic view of field-driven electron motion enables a new quality of microscopic understanding of strong-field control of quantum materials.

Van der Waals (vdW) heterostructures allow the engineering of magnetic and electronic properties via interfacial orbital hybridization and charge transfer. The resulting electric fields can give rise to Rashba spin-orbit coupling and Dzyaloshinskii-Moriya interactions, which may stabilize novel two-dimensional magnetic phases. While such emergent magnetic ‘interphases’ may offer a promising platform for spin-based electronics, direct spectroscopic evidence for them is still lacking. Here, we report electric-field tunable Zeeman effects with Landé g -factors up to ≈ 230 at the interface of graphene and the vdW ferromagnet Fe ₃ GeTe ₂ . They arise from an interplay of local-moment canting and the itinerant orbital moments of Fe ₃ GeTe ₂ , producing a giant asymmetric level splitting. Exploiting the inelastic phonon gap of graphene, scanning tunneling spectroscopy allows us to access the buried interface properties, and quantify Faraday-like screening. Our findings provide new insights and opportunities for electrically controlled interface magnetism, e.g. for spintronics and orbitronics devices.

Angle-resolved photoemission spectroscopy (ARPES) provides the most direct experimental access to electronic band structures and many-body interactions in solids. Beyond energy and momentum, the measured intensity also carries detailed information about the initial-state wavefunction through the complex photoemission matrix element governing the transition to—and scattering character of—the photoelectron final state. Orbital tomography seeks to exploit this sensitivity to reconstruct electronic wavefunctions. However, its quantitative implementation has been hindered by the lack of direct information on the phase of the matrix element and by the nontrivial role of solid-state scattering. Here, we overcome these limitations by achieving an experimental reconstruction of the complex photoemission matrix element in quasi-freestanding monolayer graphene (QFMLG), including both amplitude and phase. By combining circular-dichroism ARPES with linearly polarized measurements within a controlled scattered-wave framework, we determine the full matrix element over a broad kinetic-energy range. The reconstructed amplitudes exhibit pronounced channel-selective resonances and Cooper minima, directly revealing substantial solid-state modifications beyond commonly used simplified models. Leveraging this quantitative reconstruction, we establish QFMLG as an easily available intensity reference for ARPES. Its known complex matrix element enables direct normalization for the product of photon flux and detector response, allowing the determination of photoemission cross sections in other materials under identical experimental geometries. This advances orbital tomography by ARPES to a quantitatively normalized approach that separates the initial-state wavefunction from final-state scattering contributions, and more broadly may provide access to the associated quantum geometry.

A pointlike defect in a uniform current-carrying conductor induces a dipole in the electrochemical potential, which counteracts the original transport field. If the mean free path of the carriers is much smaller than the size of the defect, then the dipole results from the purely diffusive motion of the carriers around the defect. In the opposite limit, ballistic carriers scatter from the defect—for this situation, Landauer postulated the emergence of residual resistivity dipoles that are independent of the defect size and thus impose a fundamental limit on the resistance of the parent conductor. Here, we study resistivity dipoles around holes of different sizes in two-dimensional Bi films on Si(111). Using scanning tunneling potentiometry to image the dipoles, we find a crossover from linear to constant scaling behavior of their amplitudes with defect size, manifesting the transition from diffusive to Landauer dipoles. The extracted parameters of the transition allow us to estimate the Fermi wave vector and the carrier mean free path in our Bi films.

High-quality crystalline growth of a thin film on sapphire requires sufficient substrate preparation, often achieved via aggressive chemical cleaning. Direct thermal reconstruction of the sapphire substrate via a CO 2 laser beam may provide an alternative method to prepare the substrate for epitaxy without the use of any chemical processing. In this work, we demonstrate that the thermal annealing of sapphire into its ( 31 × 31 ) R ± 9 ∘ reconstruction is a valid alternative preparation technique for sapphire substrates. TiN films grown via plasma-assisted molecular beam epitaxy upon these substrates exhibit greater crystallinity than those grown on chemically-cleaned sapphire substrates. Superconducting resonators fabricated from these films exhibit similar performance, with many possessing internal quality factors at single photon levels greater than 10 6 for both substrate preparation methods.

Excitons, the correlated electron-hole pairs governing optical and transport properties in organic semiconductors, have long resisted direct experimental access to their full quantum-mechanical wave functions. Here, we use femtosecond time-resolved photoemission orbital tomography (trPOT) combining high-harmonic probe pulses with time- and momentum-resolved photoelectron spectroscopy to directly image the momentum-space distribution and ultrafast dynamics of excitons in α -sexithiophene thin films. We introduce a model that enables reconstruction of the exciton wave function in real space, including both its spatial extent and its internal phase structure. The reconstructed wave function reveals coherent delocalization across approximately three molecular units and exhibits a characteristic phase modulation, consistent with calculations within the framework of many-body perturbation theory. Time-resolved measurements further indicate an approximately 25% contraction of the exciton radius within 400 fs, suggesting self-trapping driven by exciton-phonon coupling. These results establish trPOT as a general and experimentally accessible approach for resolving exciton wave functions—with spatial, phase, and temporal sensitivity—in a broad class of molecular and low-dimensional materials.

Circular dichroism in the angular distribution (CDAD) is the effect that the angular intensity distribution of photoemitted electrons depends on the handedness of the incident circularly polarized light. The origin of CDAD can be manifold, including intrinsic properties of the system under study, such as chirality, spin-orbit interaction, or quantum-geometrical properties, but CDAD can also originate from final-state effects influenced by the experimental geometry. For example, CDAD has been reported for achiral organic molecules at the interface to metallic substrates. For this latter case, we investigate two prototypical π -conjugated molecules, namely, tetracene and pentacene, whose frontier orbitals have a similar shape but exhibit distinctly different symmetries. By comparing experimental CDAD momentum maps with simulations within time-dependent density functional theory, we show how the final state of the photoelectron must be regarded as the source of the CDAD in such otherwise achiral and quantum-geometrically trivial systems. We gain additional insight into the mechanism by employing a simple scattering model for the final state, which allows us to decompose the CDAD signal into partial wave contributions.

Due to their high kinetic inductance, highly disordered superconducting thin films are a potential hardware for the realization of compact, low-noise elements in cryoelectronic applications. However, high disorder typically results in structural defects that cause spatial inhomogeneity of the superconducting order parameter, thereby impairing the film’s properties. Here, we employ scanning tunneling microscopy to demonstrate that NbN thin films fabricated by plasma-enhanced atomic layer deposition exhibit unusual spatial homogeneity, even at thicknesses approaching the superconductor–insulator transition. Tunneling spectra acquired across the sample surface show only small variations of the order parameter with a standard deviation of 2%–3%, on length scales that significantly exceed the film’s grain size. At the same time, the films achieve a relatively high sheet resistance (up to 1400 Ohm) and, consequently, a high sheet kinetic inductance (up to approximately 200 pH), making them well-suited for applications.