Dynamics of an acoustic bipolaron in one-dimensional electron-lattice systems

The dynamical properties of an acoustic bipolaron in non-degenerate conjugated polymers such as polydiacetylene (PDA), which can be regarded as one-dimensional electron-lattice systems, are studied through numerical simulations. In these simulations, use has been made of Su-Schrieffer-Heeger's model extended to involve on-site as well as nearest-neighbor electron-electron Coulomb repulsion terms. The electron-electron interactions are treated within the unrestricted Hartree-Fock approximation. It is confirmed that the velocity of the acoustic bipolaron accelerated by a constant electric field saturates at the sound velocity of the system; and that the width of the bipolaron shrinks to zero as its velocity approaches the saturation velocity (= sound velocity). The effective mass of a bipolaron is estimated to be several hundred times as heavy as the free electron mass, in the case of PDA. These properties are compared with those of a polaron.