Low-energy collective electronic excitations in Pd metal

Abstract

A theoretical study of collective electronic excitations in Pd at low-energy ͑from 0 to ϳ3 eV͒ domain is reported. The calculations were performed with full inclusion of the electron band structure obtained within self-consistent pseudopotential approach. We show that the presence in Pd of two kinds of carriers ͑in s-p and d bands͒ at the Fermi level produces dramatic modification of the excitation spectra in this energy range in comparison with free-electron-like model prediction. In particular, at small momenta a peculiar plasmon mode with characteristic sound-like dispersion-an acoustic plasmon-is predicted to exist in this material. This mode has strong directional dependence on the momentum transfer: whereas for momenta along the ͗100͘ and ͗111͘ symmetry directions it arises as a single mode up to ϳ1 eV, along the ͗110͘ direction two acoustic modes ͑one of which disperses up to ϳ2.5 eV͒ with different slope exist. As in many metallic systems, e.g., in transition metals, there are energy bands with large difference in the Fermi velocities, we expect that the existence of such plasmon mode must be a rather general phenomenon. Additionally, present calculations reveal other well-defined features in the energy-loss spectra in this low-energy range due to numerous interband transitions.

Low-energy plasmonic structure in CaC6

“…It can be observed in the energy interval between 0 and ∼2.1 eV (∼1.2 eV) for momentum transfers along the a * (b * ) axis. This dispersion corresponds to an AP mode recently predicted 37,38 to exist in Pd and MgB 2 . However, it is for the first time that such kind of low-energy collective excitation is observed in a GIC system.…”

“…At the same time, recent ab initio calculations 37,38 of the excitation spectra in bulk MgB 2 and Pd demonstrated that an AP can indeed exist in real materials. The crucial role of band structure in formation of a such mode was demonstrated.…”

“…[45][46][47][48][49][50][51][52] The approach we adopt here consists 46,49 in the evaluation, at the first step, of the spectral function S 0 GG (q,ω) directly related to the imaginary part of χ 0 GG (q,ω). In this case, instead of η, a relevant numerical parameter is a broadening parameter 37,46 γ which was set here to 10 meV. The sum over the 1BZ in Eq.…”

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

“…The latter can act to screen the repulsion between the former, resulting in the appearance of a plasma mode with a soundlike dispersion. This is, for instance, what has been proved in MgB 2 [33], which has two bands with π (fast carriers) and σ (slow carriers) character that are metallic (i.e., cross the Fermi energy) or in other materials such as CaC 6 [34], Pb [35], and Pd [36]. With respect to this, 2H-TMD are similar to MgB 2 since the d z 2 and p z bands can play the same role of the σ and π bands, respectively.…”

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

“…57 We relay the appearance of this peak to the existence of a low-energy acoustic plasmon, also predicted to exist on base of the first-principles calculations in other systems such as MgB 2 and Pd. 31,58 Previously such kind of plasmon was intensively investigated in various layered systems [23][24][25][26][27][28][29][30][31][32][33][34] and other materials with two kinds of carriers at the Fermi level. [59][60][61][62][63][64] The appearance of this plasmon is related to the presence of two kinds of charge carriers characterized by different maximal Fermi velocity components in a given direction in the energy bands crossing the Fermi surface.…”