Crystal stability and optical properties of organic chain compounds

– 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. Ever since the discovery of the mixed stack organic chain compounds (OCCs), some twenty five years ago, two important questions remained open [1]. The first is the stability of their crystal structure, with a fractional charge transfer between the molecules of na ≈ 0.6 electrons per molecule. Furthermore, in some lattices like TTF-TCNQ the equally charged (TTF or TCNQ) chains stack side by side, to form charged sheets, instead of alternating in both directions, as suggested by the simple Madelung argument. Such a simple alternation does occur in other lattices, like HMTTF-TCNQ, which, in spite of the radically different chain stacking, have similar na. Neither the fractional charge transfer nor the formation of charged sheets are expected within the standard ionic cohesion scheme which assumes the dominance of affinity-ionicity and electrostatic (Madelung) contributions. Besides, calculations of cohesion energy within this scheme suggest that almost 2 eV per donor-acceptor molecular pair is lacking. This ”Madelung defect” [2] clearly signifies that the cohesion at a scale of few eVs has to be stabilized by some mechanism(s) beyond the standard ionic scheme. Although various suggestions [2]-[7] have been put forward, they have failed to explain all the aspects of the cohesion, mostly because they could not account for the Madelung defect. Typeset using EURO-TEX 2 EUROPHYSICS LETTERS On the other hand, Friedel [8] pointed out that the van der Waals interaction between the molecules favors the formation of charged sheets observed in TTF-TCNQ. In this model the intramolecular (interband) and the intermolecular (intraband) charge fluctuations are separated. Such separation however fails in the low frequency range in which TTF-TCNQ shows strong anomalies, observed in numerous microwave/infrared optical measurements [9][13] (fig.1). The origin of these anomalies is the second puzzle of OCCs. E. g. in TTF-TCNQ the frequency of the anomaly at 10 meV [10, 11] is comparable to the value of the (pseudo)gap ∆L produced by the 2kF lattice instability at low temperatures (T < 50K). However the anomalies in fig.1 can not be related to this instability, since they persist at high temperatures, where the 2kF lattice fluctuations are practically absent. In this Letter we present a theory which for the first time treats interband (van der Waals) and intraband correlations on equal footing, and explains simultaneously the crystal stability and the optical anomalies of OCCs. We investigate a simple model with two orbitals per donor and acceptor molecule, i. e. two bands per molecular chain, the lower bands being partially filled. Applying the recently proposed tight-binding formalism for the dielectric matrix in the random phase approximation (RPA) [14], we determine the hybridized collective modes associated with the electron polarization processes, and the corresponding zero-point energy contributions to the cohesive energy. We show that the above unusual cohesive and optical properties are related through the presence of a soft interband collective mode and its hybridization with the intraband charge fluctuations. In the tight-binding formalism [14] the matrix elements of the screened Coulomb interaction V̄ (r, r) are represented in terms of interband and intraband polarization processes, and the corresponding system of RPA Dyson equations reads