Negative plasmon dispersion in 2H-NbS2 beyond charge-density-wave interpretation

New Journal Of Physics 18, 103050 (2016)

Negative plasmon dispersion in 2H-NbS2 beyond charge-density-wave interpretation

Pierluigi Cudazzo, Eric Múller, Carsten Habenicht, Matteo Gatti, Helmuth Berger, Martin Knupfer,Angel Rubio, Simo Huotari

We examine the experimental and theoretical electron-energy loss spectra in 2H-Cu0.2NbS2 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 °A−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.

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S.H. was supported by the Academy of Finland (projects 1254065, 1283136 and 1259526), M.G. by a Marie Curie FP7 Integration Grant within the 7th European Union Framework Programme, P.C. by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 660695. A.R. acknowledges financial support from the European Research Council (ERC-2010-AdG-267374), Spanish grant (FIS2013-46159-C3-1-P), Grupos Consolidados (IT578-13), and AFOSR Grant No. FA2386-15-1-0006 AOARD 144088, H2020-NMP-2014 project MOSTOPHOS, GA no.H2020-NMP-2014 project MOSTOPHOS (GA no. 646259) and COST Action MP1306 (EUSpec).Computational time was granted by GENCI (Project No. 544).

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