Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (kx, ky, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å−1 diameter in a binding-energy range of several eV, resolving about 2.5 × 105 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe2 measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å−1, and a system response function of 150 fs.
VS 2 is a challenging material to prepare stoichiometrically in the bulk, and the single layer has not been successfully isolated before now. Here we report the first realization of single-layer VS 2 , which we have prepared epitaxially with high quality on Au(111) in the octahedral (1T) structure. We find that we can deplete the VS 2 lattice of S by annealing in vacuum so as to create an entirely new twodimensional compound that has no bulk analogue. The transition is reversible upon annealing in an H 2 S gas atmosphere. We report the structural properties of both the stoichiometric and S-depleted compounds on the basis of low-energy electron diffraction, X-ray photoelectron spectroscopy and diffraction, and scanning tunneling microscopy experiments.
Time-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe 2. We present a many-body approximation for the Green's function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions.
The possibility of triggering correlated phenomena by placing a singularity of the density of states near the Fermi energy remains an intriguing avenue toward engineering the properties of quantum materials. Twisted bilayer graphene is a key material in this regard because the superlattice produced by the rotated graphene layers introduces a van Hove singularity and flat bands near the Fermi energy that cause the emergence of numerous correlated phases, including superconductivity. Direct demonstration of electrostatic control of the superlattice bands over a wide energy range has, so far, been critically missing. This work examines the effect of electrical doping on the electronic band structure of twisted bilayer graphene using a back‐gated device architecture for angle‐resolved photoemission measurements with a nano‐focused light spot. A twist angle of 12.2° is selected such that the superlattice Brillouin zone is sufficiently large to enable identification of van Hove singularities and flat band segments in momentum space. The doping dependence of these features is extracted over an energy range of 0.4 eV, expanding the combinations of twist angle and doping where they can be placed at the Fermi energy and thereby induce new correlated electronic phases in twisted bilayer graphene.
We study the interplay between competitive substrate-C interaction processes occurring during chemical vapour deposition (CVD) of ethylene on Re(0001). At T < 500 K dissociative ethylene adsorption leads to the formation of a dimer species, producing an ordered (4 × 2) structure. In the range 500 − 700 K, the formation of a high-quality single-layer of graphene (GR) is strongly opposed by the formation of a surface carbide characterised by C trimer units, and, at higher temperatures, by carbon dissolution into the bulk. Our experimental and theoretical results demonstrate that, under UHV conditions, the formation of a long-range ordered GR layer on * Corresponding author. Tel.(+39)040 375-8719. E-mail:alessandro.baraldi@elettra.eu Preprint submitted to CarbonDecember 12, 2013Re(0001) without carbon bulk saturation is confined to a narrow window of growth parameters: substrate temperature, hydrocarbon gas pressure and exposure time. Our combined experimental and theoretical approach allowed us to validate a concept which had already been anticipated in some earlier works on Rh,Fe and Ni, namely that the epitaxial growth of GR is not necessarily restricted to surfaces where carburisation is precluded, but could take place, under given appropriate conditions, also on other metallic substrates exhibiting a strong C-substrate interaction.
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