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 2H -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 ∼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.

Interplay between structure and electronic properties of layered transition-metal dichalcogenides: Comparing the loss function of1Tand2Hpolymorphs

“…2 we compare the low-energy (0-4 eV) EELS spectra of 1T-VSe 2 and 1T-TiTe 2 with that of 2H-NbSe 2 calculated in RPA for momentum transfers q along M, i.e., belonging to the plane of the layer. While in 2H-NbSe 2 (and similarly for the other metallic compounds of the 2H family [19][20][21]) the loss function at small q is dominated by a single sharp peak at ∼1 eV, in the 1T materials it is strongly broadened with two main features (at ∼0.6 and 1.7 eV) and several other minor structures involving independent electron-hole pairs. Moreover, while in the 2H polytypes the sharp peak has a negative dispersion as q increases [19][20][21], in 1T-VSe 2 and 1T-TiTe 2 the lowerand higher-energy excitations have positive and negative dispersion, respectively.…”

“…Since in our preliminary tests we found that in the small-energy range considered here the effect of f xc in the adiabatic local-density approximation is negligible (see also Refs. [20] and [21]), in the following we will consider results obtained in the random-phase approximation (RPA) in which f xc = 0 and that has already been proven to be sufficient to obtain a good agreement with available experimental data [18,19,22]. This approximation has been used to evaluate the loss function of single-sheet TMD as well since previous works on graphene and single-wall carbon nanotubes, see e.g.…”

“…We calculate the dynamical charge-density response function of these materials by means of first-principles time-dependent density functional theory (TDDFT). We compare the loss function of 1T polymorphs with that of members of the 2H family [15][16][17][18][19][20][21][22], both in the bulk and in the 2D form [23]. We characterize the different plasmon dispersions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone.…”

Plasmons in metallic monolayer and bilayer transition metal dichalcogenides

“…Metallic TMDCs have much higher charge carrier densities leading to plasmon energies of around 1 eV in bulk TMDCs. [12][13][14][15] Together with the possibility of tuning the plasmon energies and lifetimes through quantum confinement or plasmon hybridization, 16,17 this makes few-layer TMDCs interesting candidates for nanoplasmonic applications in the optical frequency regime. We mention that the efficient coupling of 2D plasmons and light generally requires some kind of scattering of the light to overcome the momentum mismatch.…”

High-energy collective electronic excitations in layered transition-metal dichalcogenides

“…In fact, for both metals and insulators, it is possible to identify two plasmons in the EELS spectra measured for in-plane momentum transfer: a low-energy plasmon at about 9 eV and a high-energy plasmon involving all valence electrons at about 23 eV. Beside these two features, metallic TMD present also an intraband plasmon at about 1 eV involving only the charge-carrier electrons [17][18][19][20]. However, neutral excitations in these materials have never been studied at large momentum transfer where the short-range part of the Coulomb interaction plays a key role.…”