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Negative plasmon dispersion in 2H-NbS2 beyond the charge-density-wave interpretation
We examine the experimental and theoretical electron-energy loss spectra in 2H-${\mathrm{Cu}}_{0.2}$NbS2 and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range $q\lt 0.35$ Å−1. We argue that for a stoichiometric pure 2H-NbS2 the dispersion relation is expected to have a negative slope as is the case for other transition-metal dichalcogenides. The presence of Cu impurities, required to stabilize the crystal growth, tends to shift the negative plasmon dispersion into a positive one, but the doping level in the current system is small enough to result in a nearly-non-dispersing plasmon. We conclude that a negative-slope plasmon dispersion is not connected with the existence of a charge-density-wave order in transition metal dichalcogenides.
Loss spectroscopy of molecular solids: Combining experiment and theory
The nature of the lowest-energy electronic excitations in prototypical molecular solids is studied here in detail by combining electron energy loss spectroscopy (EELS) experiments and state-of-the-art many-body calculations based on the Bethe–Salpeter equation. From a detailed comparison of the spectra in picene, coronene and tetracene we generally find a good agreement between theory and experiment, with an upshift of the main features of the calculated spectrum of 0.1–0.2 eV, which can be considered the error bar of the calculation. We focus on the anisotropy of the spectra, which illustrates the complexity of this class of materials, showing a high sensitivity with respect to the three-dimensional packing of the molecular units in the crystal. The differences between the measured and the calculated spectra are explained in terms of the small differences between the crystal structures of the measured samples and the structural model used in the calculations. Finally, we discuss the role played by the different electron–hole interactions in the spectra. We thus demonstrate that the combination of highly accurate experimental EELS and theoretical analysis is a powerful tool to elucidate and understand the electronic properties of molecular solids.
Electronic properties of molecular solids: The peculiar case of solid picene
Recently, a new organic superconductor, K-intercalated picene, with high transition temperatures Tc (up to 18 K) has been discovered. We have investigated the electronic properties of an undoped relative of this superconductor, solid picene, using a combination of experimental and theoretical methods. Our results provide deep insights into the occupied and unoccupied electronic states.