Plasmon dispersion in layered transition-metal dichalcogenides (TMDC)

We propose to use inelastic x-ray scattering (IXS) spectroscopy to study the electronic structure of two prototypical layered transition-metal dichalcogenides (TMDC): 2H-NbSe2 and 2H-NbS2. These two materials have very similar band structures. However, while at low temperature 2H-NbSe2 undergoes a transition to a charge-density-wave (CDW) ordered phase, 2H-NbS2 does not. A very recent electron energy-loss spectroscopy (EELS) experiment [1] found an in-plane negative plasmon dispersion in 2HNbSe2, contrary to the positive slope reported for 2H-NbS2 by an older EELS measurement [2]. These results were discussed in terms of the incipient CDW order affecting the excitation spectra. However, ab initio simulations have now obtained for both materials a negative dispersion [3], which becomes
positive when the materials are doped with electrons (or holes) (which also leads to the suppression of the CDW in 2H-NbSe2). These discrepancies now pose a puzzle that we aim to answer with a new experimental technique. From an unambigious comparison between bulk-sensitive IXS results for 2H-NbSe2 and 2H-NbS2 we expect to be able to assess the connection between the CDW order, the band structure properties, and the in-plane negative plasmon dispersion. Finally, we aim to confirm the theoretical prediction [3] about the reappearance of the plasmon in the 􀀀A direction perpendicular to the planes. This is a resubmission of HC-730. The previous review found the proposal excellent and encouraged resubmission.