Crystal stability and optical properties of organic chain compounds

TL;DR

This paper explains the cohesion and optical properties of organic chain compounds by analyzing their dielectric matrix and collective modes, revealing mode softening responsible for observed optical anomalies and lattice stability.

Contribution

It introduces a model considering hybridized collective modes and dipole interactions to explain cohesion and optical phenomena in organic chain compounds.

Findings

Identification of three softened collective modes due to dipole interactions

Explanation of the 10meV optical anomaly in TTF-TCNQ

Insight into the formation of charged sheets and fractional charge transfer

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.