Negative plasmon dispersion in 2H-NbS₂beyond the charge-density-wave interpretation

Abstract

We examine the experimental and theoretical electron-energy loss spectra in 2H-Cu 0.2 NbS 2 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 0.35 Å −1 . We argue that for a stoichiometric pure 2H-NbS 2 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.

Doping dependent plasmon dispersion in2H-transition metal dichalcogenides

“…Then, it is also clear that doping electrons into these bands and shifting the Fermi level accordingly will have an impact on the plasmon behavior. Indeed, shifting the Fermi level in such calculations to simulate a doping effect results in a positive plasmon dispersion for high doping levels in very good agreement to the experimental data [17,35]. Therefore, our data more support the picture that the particular band structure in the 2H -TMD systems is responsible for the peculiar behavior of the plasmon dispersion.…”

Plasmons in MoS2 studied via experimental and theoretical correlation of energy loss spectra

“…Other metallic TMDs such as PtTe2 show lower energy Dirac intraband plasmons (~0.5 eV) [20,21]. Also, TaS2, TaSe2, and NbSe2 have excitations known as charge-carrier plasmons (~1 eV) [22][23][24].…”

Electron-Driven In Situ Transmission Electron Microscopy of 2D Transition Metal Dichalcogenides and Their 2D Heterostructures

“…Monolayered MoS 2 and WS 2 are the most studied 2D TMD materials; however, there are many other 2D-layered materials with specific properties that are only recently attracting interest. Among the most exciting TMD family members to date are the semiconducting − MoS 2 and WS 2 members, the WTe 2 and TiSe 2 semimetal members, , and the true metal members, − NbS 2 and VSe 2 . There are also superconducting members, − namely, NbSe 2 and TaS 2 .…”