POLARON THEORY OF MID-INFRARED CONDUCTIVITY - AN EXACT CLUSTER SOLUTION

Mid infrared spectra are obtained by exact calculations of the optical conductivity sigma(omega) of a finite size Holstein model. sigma(omeg a) shows a number of peaks corresponding to the bound states of polaro ns with different number of phonons. It is found that for intermediate coupling the peak in sigma(omega) is strongly asymmetric. The optical conductivity of the 2-site model in presence of two electrons is stud ied. Numerical results show a shift of the peak in sigma(omega) to the low energy region with an increasing Hubbard U for strong electron-ph onon interaction (E(p) > U) whereas the peak moves to high energy regi on for U > E(p). A new peak in high energy region starts to develop in large U limit in the presence of phonons. Significance of these calcu lations for the experimental observations of the mid infrared spectra of high-T-c cuprates is discussed.