Electron Correlations in Molecular Chains

Abstract

It is shown that in a one-dimensional molecular chain two extra electrons with opposite spins interacting with acoustical phonons in the harmonic approximation form a bisoliton, which is a bound singlet state of the electrons trapped by a self-created deformation of the chain. The harmonic approximation in description of lattice vibrations can be violated for molecular chains which are relatively rigid, for systems with relatively strong electron–lattice interaction (resulting in large deformation of the chain) or under the external pressure applied to the system. In this case, the deformation can be described taking into account anharmonic terms. It is shown that the account of these terms leads to stabilization of bisolitons up to the sound velocity in the chain, and to formation of bisolectrons, which can propagate with the supersound velocity. In the case of finite number of electrons in a molecular chain their ground state is a self-consistent cnoidal wave in the deformation potential. This cnoidal wave at sufficiently small doping level corresponds to the condensate of autolocalized bisolitons (bisolectrons). The dependence of the condensate gap width on concentration of charge carriers is shown to have a nonmonotonic character. The pressure dependence of the condensate critical temperature is analyzed and shown to be of the type characteristic for HTSCs under pressure.