Optical investigation of the metal-insulator transition in the manganite films with the thickness dependence

We investigated the optical response of La_0.67Sr_0.33MnO₃ (LSMO) films which show the metal-insulator transition (MIT) with the film thickness dependence. By using spectroscopic ellipsometric technique we found that the optical spectra below the charge transfer gap exhibited the coherent-incoherent crossover behavior through MIT. The formation of the incoherent mode near 1.5 eV was reminiscent of the polaron absorption which had been widely observed in various manganites. We suggest that the electron-phonon coupling could be enhanced due to low dimensionality in the ultrathin LSMO film in consideration of orbital polarization.