Role of band structure and local-field effects in the low-energy collective electronic excitation spectra of 2H-NbSe2

Abstract

We present a study of the electron dynamics in the layered compound 2$H$-NbSe${}_{2}$. First-principles calculations are used to obtain the band structure employed in the evaluation of the loss function with inclusion of local-field (LF) effects. Two different symmetry directions [(100) and (010)] were explored in the hexagonal basal plane. In both cases, a low-energy charge-carrier plasmon (CCP) at $\ensuremath{\sim}$1 eV presenting a negative dispersion over a wide momentum transfer range is found, in agreement with recent experimental results [Wezel et al., Phys. Rev. Lett. 107, 176404 (2011)]. On the contrary, in the (001) perpendicular direction, the CCP has negative dispersion at small momenta only, presenting strong positive dispersion at larger momenta. Our calculations reveal that this behavior can be explained without invoking many-body effects, as long as band structure effects are properly included in the evaluation of the excitation spectra. In addition to this CCP mode, we find another one with an arclike oscillating dispersion along the perpendicular direction, as well as the appearance of a CCP replica at high momenta due to LF effects.