New Lattice Softening Mechanism in the Quasi-One-Dimensional TCNQ Conductors

Abstract

A 1-d model system embodying the features of the TCNQ conductors is worked out in order to study on the softening of the TA phonon mode caused by the interaction with the intramolecular electric polarizations of the TCNQ⁻ ions. We obtain Green’s function for the renormalized phonon mode in two approximate methods and find that this mode is actually softened around the so-called 2K_F wavenumber. The polarization wave mode softens at this wavenumber, pressing down the phonon mode, as the electron distribution develops the Wigner-crystal-like short range order with decreasing temperature. This situation is simulated numerically for a simple case and found very similar to that observed experimentally. Appearance of the spontaneous polarization at 0 K proves a sufficient condition for completely softening the phonon mode. The softening in TTF-TCNQ is attributed to the 2-d tight arrangement of the TCNQ molecules in the bc-plane. This reasoning is reinforced by a numerical simulation of a 2-d model of classical electrons.