Optical investigation of LaMnO_3 thin films: A study of the 2-eV band

摘要

We measure the transmittance of LaMnO_3 (LMO) thin films in the spectral range from 10 000 to 38 000 cm-1 as a function of temperature. The optical conductivity obtained from the transmittance data is decomposed into five Drude-Lorentz contributions: three of them show evident changes below the Néel temperature T N ～ 140 K. We discuss the band assignment in the framework of the tight-binding and of the orbitally degenerate Hubbard models. Our results ascribe the overall low-frequency spectrum of LMO to charge-transfer mechanisms involving the t _(2g) and e_ g electrons of the Mn ions, without invoking the presence of excitonic contributions. This finding definitely promotes a Mott-Hubbard scenario for LMO and provides an estimate of the Hamiltonian parameters.