Acoustic polaron in a ladder-type electron-lattice system

The properties of an acoustic polaron are numerically studied for an electr on-lattice system with a ladder-type structure where an electron transfer b etween two chains is allowed. Schrieffer-Heeger's (SSH) model extended to t he ladder system is used on the assumption that the periodic boundary condi tion in the chain direction is satisfied. When the interchain electronic tr ansfer integral t(x0) is comparable with the intrachain one t(y0), the latt ice 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 light er than the single chain case. On the other hand, when t(x0) is much smalle r 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 transit ion from the homogeneous to an inhomogeneous state is found to occur sudden ly 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 ene rgetics indicates that this is a discontinuous transition similar to the fi rst-order type phase transition. Dynamical simulation where the polaron is accelerated by an electric field shows that the homogeneous-inhomogeneous t ransition can occur when the velocity of the polaron increases and the pola ron width in the chain direction decreases.