Acoustic Polaron in a Ladder-Type Electron-Lattice System

Abstract

The properties of an acoustic polaron are numerically studied for an electron-lattice system with a ladder-type structure where an electron transfer between two chains is allowed. Su-Schrieffer-Heeger's (SSH) model extended to the ladder system is used on the assumption that the periodic boundary condition in the chain direction is satisfied. When the interchain electronic transfer integral t_x0 is comparable with the intrachain one t_y0, the lattice deformation related to the polaron is small compared to the case of one-dimensional systems because the electronic density profile is distributed equally in the two chains, and as a result the effective mass becomes lighter than the single chain case. On the other hand, when t_x0 is much smaller than t_y0, the polaron is localized in one of the two chains instead of being distributed with a homogeneous weight to the two chains. This transition from the homogeneous to an inhomogeneous state is found to occur suddenly at a certain value of t_x0 as we decrease it, where the critical value of t_x0 depends on the electron-lattice coupling constant. Analysis of energetics indicates that this is a discontinuous transition similar to the first-order type phase transition. Dynamical simulation where the polaron is accelerated by an electric field shows that the homogeneous-inhomogeneous transition can occur when the velocity of the polaron increases and the polaron width in the chain direction decreases.