Theory of high-energy optical conductivity and the role of oxygens in manganites

Recent experimental study revealed the optical conductivity of La _1-xCa_xMnO₃ over a wide range of energy and the occurrence of spectral weight transfer as the system transforms from a paramagnetic insulating to a ferromagnetic metallic phase. We propose a model and calculation within the dynamical mean-field theory to explain this phenomenon. We find the role of oxygens in mediating the hopping of electrons between manganeses as the key that determines the structures of the optical conductivity. In addition, by parametrizing the hopping integrals through magnetization, our result suggests a possible scenario that explains the occurrence of spectral weight transfer, in which the ferromagnetic ordering increases the rate of electron transfer from O_2p orbitals to upper Mn_eg orbitals while simultaneously decreasing the rate of electron transfer from O_2p orbitals to lower Mn_eg orbitals, as temperature is varied across the ferromagnetic transition. With this scenario, our optical conductivity calculation shows very good quantitative agreement with the experimental data.