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We studied the pump-induced anisotropy of the intraband excitation in bilayer graphene in degenerate terahertz pump-probe experiments. The differential transmission signal increases approximately linearly with the excitation field, in qualitative agreement with our microscopic model.

The raw data and lab book pages are given in the .zip folders. The results of the theoretical calculations in comparison to the experiment are given in Roozbeh_plot.opju. In the other two Origin files, the differential transmission signals and other relevant values are calculated from the raw data.The 2 THz measurement was performed in 2019, the 3.4 THz measurement in 2020.

© 2018 IOP Publishing Ltd. Van der Waals epitaxy enables the integration of 2D transition metal dichalcogenides with other layered materials to form heterostructures with atomically sharp interfaces. However, the ability to fully utilize and understand these materials using surface science techniques such as angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) requires low defect, large area, epitaxial coverage with ultra-clean interfaces. We have developed a chemical vapor deposition van der Waals epitaxy growth process where the metal and chalcogen sources are separated such that growth times can be extended significantly to yield high coverage while minimizing surface contamination. We demonstrate the growth of high quality 2D WS2over large areas on graphene. The as-grown vertical heterostructures are exceptionally clean as demonstrated by ARPES, STM and spatially resolved photoluminescence mapping. With these correlated techniques we are able to relate defect density to electronic band structure and, ultimately, optical properties. We find that our synthetic approach provides ultra-clean, low defect density (~1012 cm-2), ~10 μm large WS2monolayer crystals, with an electronic band structure and valence band effective masses that perfectly match the theoretical prediction for pristine WS2.

Here we study the alkali metal induced effects on an ordered and aligned sexiphenyl monolayer on Cu(110) with angle-resolved UV spectroscopy (ARUPS). The caesium (Cs) induced gap states could clearly be identified by orbital tomography, a method based on ARUPS, which allows both the orbital character of these states and the molecular orientation to be determined. We show that with increasing alkali metal dose, doping proceeds in three distinct steps. Initially, Cs decouples the molecular monolayer from the substrate, with emptying of the lowest unoccupied molecular orbital (LUMO) that had been filled on hybridization with the substrate. Further Cs exposure refills the LUMO. Finally a filling of the LUMO + 1 by charge transfer from the alkali metal occurs. Remarkably, although long range order is not preserved and the molecular planes tilt away from the surface, the molecules remain aligned parallel to the [1¯10] azimuth during the whole doping process.

Growth of nanocrystalline graphene films on (6√3 × 6√3)R30°-reconstructed SiC surfaces was achieved by molecular beam epitaxy, enabling the investigation of quasi-homoepitaxial growth. The structural quality of the graphene films, which is investigated by Raman spectroscopy, increases with growth time. X-ray photoelectron spectroscopy proves that the SiC surface reconstruction persists throughout the growth process and that the synthesized films consist of sp2-bonded carbon. Interestingly, grazing incidence x-ray diffraction measurements show that the graphene domains possess one single in-plane orientation, are aligned to the substrate, and offer a noticeably contracted lattice parameter of 2.450 Å. We correlate this contraction with theoretically calculated reference values (all-electron density functional calculations based on the van der Waals corrected Perdew–Burke–Ernzerhof functional) for the lattice parameter contraction induced in ideal, free-standing graphene sheets by: substrate-induced buckling, the edges of limited-size flakes and typical point defects (monovacancies, divacancies, Stone–Wales defects).

We report on highly resolved core-level and valence-band photoemission spectroscopies of hydrogenated, unreconstructed 6H-SiC(0001) and (000 (1) over bar) using synchrotron radiation. In the C 1s core level spectra of 6H-SiC(000 (1) over bar) a chemically shifted surface component due to C-H bonds is observed at a binding energy (0.47+/-0.02) eV higher than that of the bulk line. The Si 2p core-level spectra of SiC(0001) suggest the presence of a surface component as well but a clear identification is hindered by a large Gaussian width, which is present in all spectra and which is consistent with values found in the literature. The effect of thermal hydrogen desorption was studied. On 6H-SiC(0001) the desorption of hydrogen at 700-750 degreesC is accompanied by a simultaneous transformation to the Si-rich (root3xroot3)R30degrees reconstruction. On 6H-SiC(000 (1) over bar) first signs of hydrogen desorption, i.e., the formation of a dangling bond state in the fundamental band gap of SiC, are seen at temperatures around 670 degreesC while the (1x1) periodicity is conserved. At 950 degreesC a (3x3) reconstruction is formed. The formation of these reconstructions on thermally hydrogenated 6H-SiC(0001) and (000 (1) over bar) is discussed in the light of earlier studies of 6H-SiC{0001} surfaces. It will be shown that by using the hydrogenated surfaces as a starting point it is possible to gain insight into how the (root3xroot3)R30degrees and (3x3) reconstructions are formed on 6H-SiC(0001) and 6H-SiC(000 (1) over bar), respectively. This is due to the fact that only hydrogen-terminated 6H-SiC{0001} surfaces possess a surface carbon to silicon ratio of 1:1.