POLARON THEORY OF MIDINFRARED CONDUCTIVITY - A NUMERICAL CLUSTER SOLUTION

Mid-infrared spectra are obtained with numerical calculations of the o ptical conductivity sigma(omega) of a finite-size Holstein model. The results show that the analytic formula of Reik for the optical conduct ivity is valid only in a strong electron-phonon coupling regime. Sigma (omega) shows a number of peaks corresponding to the bound states of p olarons with a different number of phonons. Calculation of sigma(omega ) has also been done in the adiabatic limit in the one-dimensional cas e. It is found that for intermediate coupling the peak in sigma(omega) is strongly asymmetric. The optical conductivity of the two-site mode l in the presence of two electrons is studied. Numerical results show a shift of the peak in sigma(omega) to the low-energy region with an i ncreasing Hubbard U for the strong electron-phonon interaction (E(p)>U ) whereas the peak moves to the high-energy region for U>E(p). A new p eak in the high-energy region starts to develop in the large U limit i n the presence of phonons. The significance of these calculations for the experimental observations of the mid-infrared spectra of high-T(c) cuprates is discussed.