Abstract
We examine the experimental and theoretical electron-energy loss spectra in 2H-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 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.
Highlights
The charge density wave (CDW) is a broken symmetry state of metals induced by electron-phonon or by electron-electron interactions
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
As a matter of fact, being related to the inverse macroscopic dielectric function, it can be directly measured in electron energy loss spectroscopy (EELS) and in inelastic X-ray scattering (IXS) experiments
Summary
The charge density wave (CDW) is a broken symmetry state of metals induced by electron-phonon or by electron-electron interactions. As a matter of fact, being related to the inverse macroscopic dielectric function, it can be directly measured in electron energy loss spectroscopy (EELS) and in inelastic X-ray scattering (IXS) experiments In this context, J. van Wezel and coworkers[7] have established a direct link between CDW instability and collective excitations (plasmons) that represent the poles of the charge-charge response function. Good agreement between TDDFT calculations and IXS spectra at high energy and large momentum transfer was recently found for NbSe2 and Cu0.2NbS224 Motivated by these observations, we combined EELS experiments and first-principle TDDFT calculations, and investigated the collective excitations and their dispersion as a function of the momentum transfer in 2HCu0.2NbS2 with the aim to clarify the origin of the negative plasmon dispersion in TMDs. From our measurements we did not find a positive quadratic dispersion in this system, in contradiction to what has been reported before. Based on our new experimental findings, that match well the prediction of TDDFT, we argue that the negative plasmon dispersion is a general property of 2H-TMDs related to the particular band structure of these materials and that there is no need to invoke a coupling with the CDW
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