Temperature evolution of polaron dynamics and Jahn-Teller distortion modes in strongly correlated La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ manganite film

Abstract

Reflectivity as a function of temperature for the La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) film has been measured across the metal-insulator phase transition. The optical properties and their temperature dependence were determined in the infrared and visible range by fits to a Drude-Lorentz model, using exact formula for the thin film optics and the measured properties of the substrate. The phonon modes were identified and verified with lattice dynamical calculations for the ideal and distorted perovskite structure of the material. The optical conductivity shows agreement with the double exchange mechanism in conjunction with the Jahn-Teller distortion term in the Hamiltonian. Low temperature metallic phase is dominated by large polaron dynamics, a key component of electron-orbital coupling in a strongly corrrelated system. Free carrier dynamics in the metallic phase is described in terms of coherent heavy polaronic motion in the DC limit with incoherent and asymmetric polaronic background in the mid-IR range. The strength and line width of Jahn-Teller modes has been discussed across the phase transition and their temperature evolution is qualitatively discussed on account of existing electron-phonon coupling. The localized Holstein polaron formation in the high temperature insulative phase is identified as optical conductivity peaks in the visible range above the critical temperature.