Surface-phase transitions in the charge-density-wave systems 1T-TaSe2 and 1T-TiSe2

Das Ziel der Arbeit ist es, die elektronische Struktur der Oberfläche der Übergangsmetalldichalkogenide 1T-TaSe2 und 1T-TiSe2 zu untersuchen. 1T-TaSe2 zeigt einen Metall-Isolator-Übergang der mit Mott-Physik in Verbindung gebracht wird und sich in der Volumenleitfähigkeit nicht nachweisen lässt. Zeit- und winkelaufgelöste Photoelektronenspektroskopie wurde genutzt, um die Dynamik der elektronischen Struktur an der Oberfläche von 1T-TaSe2 impulsaufgelöst nach Anregung mit einem Infrarot-Laserpuls zu untersuchen. Es konnten zwei zeitlich getrennte Abläufe beobachtet werden: Zum einen der quasi sofortige Verlust der elektronischen Ordnung im Bereich der Zeitauflösung von t < 40 fs und zum anderen die kohärente Unterdrückung der periodischen Gitterverzerrung auf der Zeitskala der Oszillationsfrequenz eines kohärenten Phonons. Die schnellen Reaktionszeiten zeigen, dass tatsächlich elektronische Wechselwirkung die Ursache dieses Metall-Isolator-Übergangs ist. Des Weiteren wurde die Tiefenabhängigkeit der Ladungsdichtewelle in 1T-TaSe2 bei tiefen Temperaturen anhand der Aufspaltung der Ta-4f-Rumpfniveaus von 1T-TaSe2 studiert. Die Analyse zeigt eine Vergrößerung der Aufspaltung von 40 meV an der Oberfläche und damit eine Stärkung der Ladungsdichtewelle. Die Größe der Aufspaltung korreliert mit der des thermischen Metall-Isolator-Übergangs, welcher folglich lediglich an der Oberfläche des Materials auftritt. Im zweiten Teil der Arbeit wird der gezielte Einfluss von Elektronendotierung auf die elektronische Bandstruktur an der Oberfläche des Ladungsdichtewelle-Materials 1T-TiSe2 mit winkelauflösender Photoelektronenspektroskopie analysiert. Im Detail wird der Einfluss der zusätzlichen Ladungsträger auf die Ladungsdichtewelle untersucht. Bei hohen Elektronendotierung konnte eine vollständige energetische Entkopplung der obersten Einheitszelle nachgewiesen werden.The goal of the present work is to investigate the electronic structure of the surfaces of the transition-metal dichalcogenides 1T-TaSe2 and 1T-TiSe2. In particular the modification of the electronic structure by enhanced interactions at the surface is studied. 1T-TaSe2 shows a pronounced metal-to-insulator transition, which may be related to Mott physics and is not observable in bulk sensitive transport measurements. The first part of this work investigates the origin and the spatial extend of this transition. Time- and angle-resolved extreme ultraviolet photoelectron spectroscopy is used to determine the momentum-dependent electronic structure dynamics in the layered material 1T-TaSe2. Extracted spectroscopic order parameters display a global two-time-scale dynamics indicating a quasi-instantaneous loss of the electronic orders and a subsequent coherent suppression of the lattice distortion on a time scale related to the frequency of the charge-density-wave amplitude mode. The fast reaction of the system reveals electronic interplay as the driving force to this phase transition. Moreover, the depth dependency of the charge-density wave in 1T-TaSe2 at low temperatures is studied by hard X-ray photoelectron spectroscopy. The additional charge- density-wave-induced splitting of the Ta-4f-core levels, which serves as an excellent order parameter for the strength of the charge-density wave, is investigated. The findings corroborate the idea of a surface confined metal-to-insulator transition triggered by an enhancement of the charge-density wave at the surface. The second part of the present work focuses on the modification of the electronic structure at the surface of 1T-TiSe2 enforced by electron doping. Angle-resolved photo- electron spectroscopy is used to investigate the influence on the charge-density wave in this material in detail. At high doping levels the charge-density wave vanishes and the complete electronically decoupling of the uppermost layer is observed

The Heisenberg-RIXS instrument at the European XFEL

Resonant Inelastic X-ray Scattering (RIXS) is an ideal X-ray spectroscopy method to push the combination of energy and time resolutions to the Fourier transform ultimate limit, because it is unaffected by the core-hole lifetime energy broadening. And in pump-probe experiments the interaction time is made very short by the same core-hole lifetime. RIXS is very photon hungry so it takes great advantage from high repetition rate pulsed X-ray sources like the European XFEL. The hRIXS instrument is designed for RIXS experiments in the soft X-ray range with energy resolution approaching the Fourier and the Heisenberg limits. It is based on a spherical grating with variable line spacing (VLS) and a position-sensitive 2D detector. Initially, two gratings are installed to adequately cover the whole photon energy range. With optimized spot size on the sample and small pixel detector the energy resolution can be better than 40 meV at any photon energy below 1000 eV. At the SCS instrument of the European XFEL the spectrometer can be easily positioned thanks to air-pads on a high-quality floor, allowing the scattering angle to be continuously adjusted over the 65-145 deg range. It can be coupled to two different sample interaction chamber, one for liquid jets and one for solids, each equipped at the state-of-the-art and compatible for optical laser pumping in collinear geometry. The measured performances, in terms of energy resolution and count rate on the detector, closely match design expectations. hRIXS is open to public users since the summer of 2022.Comment: 43 pages, 12 figures, Supplemental Materia

Suppression of the Charge Density Wave State in Two-Dimensional 1T−TiSe2\mathrm{1T-TiSe_{2}} by Atmospheric Oxidation

Two‐dimensional (2D) metallic transition‐metal dichalcogenides (TMDCs), such as 1T‐TiSe$_2$, have recently emerged as unique platforms for exploring their exciting properties of superconductivity and the charge density wave (CDW). 2D 1T‐TiSe$_2$ undergoes rapid oxidation under ambient conditions, significantly affecting its CDW phase‐transition behavior. We comprehensively investigate the oxidation process of 2D TiSe$_2$ by tracking the evolution of the chemical composition and atomic structure with various microscopic and spectroscopic techniques and reveal its unique selenium‐assisting oxidation mechanism. Our findings facilitate a better understanding of the chemistry of ultrathin TMDCs crystals, introduce an effective method to passivate their surfaces with capping layers, and thus open a way to further explore the functionality of these materials toward devices