Plasmon spectrum of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ)

Abstract

The plasmon excitations in a quasi-one-dimensional organic material are analyzed in terms of a simple tightbinding energy-band model, %'ithin the random-phase approximation, the calculated plasmon dispersion and anisotropy are in quite good agreement with the electron-loss data on tetrathiafulvalene-

Collective mode dispersions of organic chain compounds

“…The numerical calculations performed in this paper show that the collective mode with the lowest energy is placed between the acceptor and donor quasi-continuums in the longwavelength limit in accordance with the predictions based on the numerical calculations within the two-band model used in Ref. [5]. Additionally there are three collective modes above both electron-hole quasi-continuums, the first is a low energy branch that is acoustic in all directions except along the chains, and the other two are optic branches.…”

“…However, unlike in Ref. [5] and here, in Ref. [4], electrons have been confined to strands of finite radius and the extent of the localization has had a strong influence on plasmon dispersions.…”

“…The frequency of this mode is comparable to the value of the (pseudo) gap ∆ L produced by the 2 F lattice instability at low temperatures (T < 50 K), but it cannot be related to this instability since it exists only at high temperatures, where 2 F lattice fluctuations are absent. This is an indication that this mode could be of plasmon origin, but it certainly cannot be understood within a simple plasmon picture [3][4][5]. On the other hand, the existence of the mode at 0.75 eV even below 50 K, resembles the plasmon excitation in semiconductors at high energy (∼ 10−20 eV) [9].…”

“…Further analyses of Williams and Bloch, as well as of Kahn et al, within the extended models [4,5] with two one-dimensional electron bands, each on a different type of chain, have predicted a second excitation. Williams and Bloch have found that the second coupled plasmon mode has been acoustic, but still both obtained modes have been placed above the electron-hole quasi-continuum in contrast to the experimental findings which did not resolve the two branches [2], whereas Kahn et al have shown that coupling between sublattice plasmons leads to the appearance of the acoustic collective mode in the narrow region between the quasi-continuums of two sublattices where its weak strength makes it unobservable in agreement with experiment.…”

“…[3][4][5] do not apply directly to TTF-TCNQ which has bands for which the interband transitions are dipole active (two per each type of molecular chain corresponding to energetically rather close highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) on each TCNQ and TTF site). The lower orbitals participate in the intraband and interband transitions due to partial filling of corresponding bands.…”