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Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic FeRh. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process. Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established 350 ± 30 fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer. In FeRh, it is possible to optically drive a phase transition between ferromagnetic (FM) and anti-ferromagnetic (AFM) ordering. Here, using a combination of photoelectron spectroscopy and ab-initio calculations, the authors demonstrate the existence of a transient intermediate phase, explaining the delayed appearance of the FM phase.

Time-resolved momentum microscopy provides insight into the ultrafast interplay between structural and electronic dynamics. Here we extend orbital tomography into the time domain in combination with time-resolved momentum microscopy at a free-electron laser (FEL) to follow transient photoelectron momentum maps of excited states of a bilayer pentacene film on Ag(110). We use optical pump and FEL probe pulses by keeping FEL source conditions to minimize space charge effects and radiation damage. From the momentum microscopy signal, we obtain time-dependent momentum maps of the excited-state dynamics of both pentacene layers separately. In a combined experimental and theoretical study, we interpret the observed signal for the bottom layer as resulting from the charge redistribution between the molecule and the substrate induced by excitation. We identify that the dynamics of the top pentacene layer resembles excited-state molecular dynamics. Ultrafast pulses are useful to investigate the electron dynamics in excited atoms, molecules and other complex systems. Here, the authors measure transient photoelectron momentum maps following the free-electron laser pulse-induced ionization of a bilayer pentacene thin film on Ag (110) by using time-resolved orbital tomography.

Charge-transfer excitations are of paramount importance for understanding the electronic structure of copper-oxide based high-temperature superconductors. In this study, we investigate the response of a Bi 2 Sr 2 CaCu 2 O 8 + δ crystal to the charge redistribution induced by an infrared ultrashort pulse. Element-selective time-resolved core-level photoelectron spectroscopy with a high energy resolution allows disentangling the dynamics of oxygen ions with different coordination and bonds thanks to their different chemical shifts. Our experiment shows that the O 1 s component arising from the Cu–O planes is significantly perturbed by the infrared light pulse. Conversely, the apical oxygen, also coordinated with Sr ions in the Sr-O planes, remains unaffected. This result highlights the peculiar behavior of the electronic structure of the Cu–O planes. It also unlocks the way to study the out-of-equilibrium electronic structure of copper-oxide-based high-temperature superconductors by identifying the O 1 s core-level emission originating from the oxygen ions in the Cu–O planes. This ability could be critical to gain information about the strongly-correlated electron ultrafast dynamical mechanisms in the Cu–O plane in the normal and superconducting phases.

Correction to: Nature Communications https://doi.org/10.1038/s41467-021-25347-3, published online 24 August 2021In the authors final submission, the Supplementary Information was uploaded as a ZIP file, containing a non-compliable TeX file, rather than in PDF or word document. The Supplementary movies referred to within the text were not included in the final version as part of the Supplementary Information ZIP file.The attached files are the correct versions of the Supplementary Information and Supplementary movies referred to in the text, contained in the attached ZIP file and contain no changes.

Characterization of the electronic band structure of solid state materials is routinely performed using photoemission spectroscopy. Recent advancements in short-wavelength light sources and electron detectors give rise to multidimensional photoemission spectroscopy, allowing parallel measurements of the electron spectral function simultaneously in energy, two momentum components and additional physical parameters with single-event detection capability. Efficient processing of the photoelectron event streams at a rate of up to tens of megabytes per second will enable rapid band mapping for materials characterization. We describe an open-source workflow that allows user interaction with billion-count single-electron events in photoemission band mapping experiments, compatible with beamlines at 3rd and 4rd generation light sources and table-top laser-based setups. The workflow offers an end-to-end recipe from distributed operations on single-event data to structured formats for downstream scientific tasks and storage to materials science database integration. Both the workflow and processed data can be archived for reuse, providing the infrastructure for documenting the provenance and lineage of photoemission data for future high-throughput experiments.

Die winkelaufgelöste Photoemissionsspektroskopie (ARPES) ist eine effektive Methode zur Untersuchung der elektronischen Bandstruktur von kondensierter Materie. Durch die Pump-Probe-Technik kann ARPES um eine zeitliche Komponente erweitert werden, wodurch die Elektronendynamik auf einer Femtosekunden-Zeitskala beobachtet werden kann. Laserquellen für hohe Harmonische (HHG) und Freie-Elektronen-Laser (FEL) erzeugen extrem ultraviolette (EUV) oder Röntgenpulse mit einer zeitlichen Breite von 100 fs oder weniger. Bei der Erzeugung hoher Harmonischer wird sichtbare oder infrarote Laserstrahlung in EUV umgewandelt, während beim FEL relativistische Elektronen in einem alternierenden magnetischen Feld kohärente Synchrotronstrahlung emittieren. FLASH (Free electron LASer Hamburg) ist ein FEL mit hoher Pulswiederholrate, welcher Photonenenergien bis in den weichen Röntgenbereich liefert. Für zeitaufgelöste (tr-) ARPES-Messungen sind Elektronenspektrometer mit hoher Detektionseffizienz unerlässlich, da die Wiederholrate der Pump- und Probequelle oft einschränkend wirkt. Hierfür sind daher Flugzeit-Photoelektronenspektrometer mit einem 3DDetektionsschema am besten geeignet. Das Weitwinkel-Photoelektronenspektrometer WESPE und das Impulsmikroskop HEXTOF basieren auf der Flugzeittechnik und wurden speziell für die PG2-Beamline bei FLASH entwickelt. Diese Arbeit präsentiert zeitaufgelöste Untersuchungen von drei unterschiedlichen Quantenmaterialien.Das HEXTOF Impulsmikroskop wird häufig in Verbindung mit einer HHG-Quelle eingesetzt. In der ersten wissenschaftlichen Anwendung dieser Arbeit wird eine Monolage Graphen auf Iridium(111) untersucht, um das Potenzial dieses Versuchsaufbaus im HHG-Labor abzuschätzen. Dabei konnte eine Zeitauflösung von unter 100 fs erzielt werden und das Detektorbild deckt im reziproken Raum mehr als die erste BrillouinZone ab. Die multispektrale Natur des HHG-Spektrums ermöglicht die Messung von kz-abhängigen ARPES-Signalen, welche zur Rekonstruktion eines Tomogramms der Fermi-Oberfläche von Iridium verwendet wurden.Der Dirac-Kegel ist nicht ausschließlich ein Merkmal von Graphen, sondern tritt auch als charakteristisches Merkmal des Oberflächenzustands von topologischen Isolatoren auf, welche in einem weiteren Kapitel untersucht werden. Obwohl topologische Isolatoren in der Tiefe des Materials nicht elektrisch leitfähig sind, verfügen sie an der Oberfläche über einen faszinierenden leitenden topologischen Zustand. Mit Hilfe des HEXTOF-Aufbaus haben wir den topologischen Isolator Bismutselenid (Bi2Se3) untersucht und tr-ARPES-Daten sowohl für den topologischen Oberflächenzustand als auch für die Rumpfniveaus Bi 4d und Se 3d aufgezeichnet. Ziel unserer Studie war es, die Elektronendynamik bei hohem Pumpfluss innerhalb des topologischen Oberflächenzustands zu erforschen und die Robustheit des Zustands selbst zu untersuchen. Unsere Experimente haben gezeigt, dass der topologische Oberflächenzustand selbst bei hohen Pumpflüssen, die zu einer Oberflächenablation führen, intakt bleibt und seine Dynamik ihre Eigenschaften beibehält.Die letzte Experimentreihe in dieser Arbeit beschäftigt sich mit einem schweren fermionischen System, bei dem 4 f- und 5 f-Elektronen mit Leitungsbandelektronen wechselwirken und Quasiteilchen in der Bandstruktur bilden. Thuliumselenied (TmSe) ist ein schweres Fermionensystem mit „mixed-valence“ Charakter der Tm-Ionen. Die Tm-4 fElektronen hybridisieren mit den Elektronen im Leitungsband, was zur Bildung eines Multipletts im PES-Spektrum nahe des Ferminiveaus führt. Wenn TmSe mit Tellur dotiert wird, dehnt sich das Gitter aus, was einen Übergang vom „mixed-valence“ Zustand zu einem überwiegend divalenten Zustand bewirkt. Die Dynamik des 3H6-Multiplett Peaks zeigt eine Abhängigkeit von der Te-Konzentration. Im „mixed-valence“ Zustand wird eine verzögerte, langanhaltende Dynamik beobachtet.Diese Arbeit demonstriert nicht nur, wie vielseitig und leistungsfähig tr-ARPES für die Erforschung von Quantenmaterie im Allgemeinen ist, sondern zeigt auch explizit, dass die jüngsten Entwicklungen in der Impulsmikroskopie-Instrumentierung eine breite Palette von Anforderungen erfüllen können und den Weg zu einem „one fits all“- Design ebnen, das neben der Zeit- und Winkel- auch die Spin-Auflösung umfasst.

A 790-nm-driven high-harmonic generation source with a repetition rate of 6 kHz is combined with a toroidal-grating monochromator and a high-detection-efficiency photoelectron time-of-flight momentum microscope to enable time- and momentum-resolved photoemission spectroscopy over a spectral range of \\(23.6\\)-\\(45.5\\) eV with sub-100-fs time resolution. Three-dimensional (3D) Fermi surface mapping is demonstrated on graphene-covered Ir(111) with energy and momentum resolutions of $\\lesssim$$100\\( meV and \\)\\lesssim$$0.1\\( \\)Å^{-1}\\(, respectively. The table-top experiment sets the stage for measuring the \\)k_z$-dependent ultrafast dynamics of 3D electronic structure, including band structure, Fermi surface, and carrier dynamics in 3D materials as well as 3D orbital dynamics in molecular layers.

X-ray photoelectron diffraction is a powerful tool for determining the structure of clean and adsorbate-covered surfaces. Extending the technique into the ultrafast time domain will open the door to studies as diverse as the direct determination of the electron-phonon coupling strength in solids and the mapping of atomic motion in surface chemical reactions. Here we demonstrate time-resolved photoelectron diffraction using ultrashort soft X-ray pulses from the free electron laser FLASH. We collect Se 3d photoelectron diffraction patterns over a wide angular range from optically excited Bi\\(_2\\)Se\\(_3\\) with a time resolution of 140 fs. Combining these with multiple scattering simulations allows us to track the motion of near-surface atoms within the first 3 ps after triggering a coherent vibration of the A\\(_{1g}\\) optical phonons. Using a fluence of 4.2 mJ/cm\\(^2\\) from a 1.55 eV pump laser, we find the resulting coherent vibrational amplitude in the first two interlayer spacings to be on the order of 1 pm.

Function is dynamic and originates at atomic interfaces. Combining the degrees of freedom of molecules with the peculiar properties of 2D quantum materials can create novel functionality. Here, we report the manipulation and ultrafast imaging of a unidirectional gearing motion in molecules on a 2D quantum material. To visualize and disentangle the intertwined structural and electronic dynamics of such a hybrid interface, we record a 'full molecular movie' by imaging the atomic positions, the evolution of the molecular orbital wavefunctions and the modification of electronic states of the substrate. In a multimodal investigation in a single setup, we disentangle dynamics in valence and core electrons of both the molecule and the surface with femtosecond and sub-ångstr\"om precision. The ultrafast rotational motion is fueled by the transfer of hot holes into the molecules that results in 'supercharging' of the film. As hot carriers move through the interface, we track a transient modification of the frontier molecular orbitals and observe a chiral symmetry breaking associated with local structural rearrangements. Our calculations show that the 'supercharging' changes the interfacial potential energy landscape and triggers the gearing motion. The experiment offers all-in-one imaging of the electronic, molecular orbital, chemical and structural dynamics during the flow of charge and energy across the hybrid interface. Our approach provides detailed dynamical information on the mechanism underlying surface-adsorbed molecular gears and enables tailoring novel functionalities in hybrid active matter.

Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic FeRh. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process. Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established 350+- 30 fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer.