Crystal stability and optical properties of organic chain compounds

Abstract

The solution to the long standing problem of the cohesion of organic chain compounds is proposed. We consider the tight-binding dielectric matrix with two electronic bands per chain, determine the corresponding hybridized collective modes, and show that three among them are considerably softened due to strong dipole-dipole and monopole-dipole interactions. By this we explain the unusual low frequency optical activity of TTF-TCNQ, including the observed 10meV anomaly. The softening of the modes also explains the cohesion of the mixed-stack lattice, the fractional charge transfer almost independent of the material, and the formation of the charged sheets in some compounds.Typeset using EURO-T E X

Collective mode dispersions of organic chain compounds

“…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]. However, closer inspection of the RPA application to the two band insulator (see section 4.5 in reference [9]) shows that so called interband plasmon is coincident with the longitudinal dipolar mode in our approach [10,11]. Regarding the role of electron-phonon interaction, it is of secondary importance at these energies.…”

“…[11], we have determined the energy dispersions of the hybridized collective modes associated with the electron polarization processes. According to the results from Ref.…”

“…found by Županović et al within the random phase approximation (RPA) [11]. The indices 0 ( ) = (0 ( ) 0 ( ) ) and 1 ( ) = (0 ( ) 1 ( ) ) represent intraband and interband one-electron transitions on the acceptor (donor) chains, respectively.…”

“…The lower orbitals participate in the intraband and interband transitions due to partial filling of corresponding bands. Extensive calculations of the stability of TTF-TCNQ show that it is not Madelung energy but dispersive forces, accounted by such orbitals, that stabilize this crystal [11][12][13][14]. Thus, motivated by the above questions, in this paper we investigate collective excitations within the model of a quasione-dimensional conductor, proposed in Ref.…”

“…Thus, motivated by the above questions, in this paper we investigate collective excitations within the model of a quasione-dimensional conductor, proposed in Ref. [11], with two dipolar active orbitals per donor and acceptor molecules, i.e. two one-dimensional electron bands per each type of molecular chain, with the lower bands being partially filled and the upper empty, and the three-dimensional long-range Coulomb electron-electron interaction.…”