Effects of magnetic ordering on the anisotropy and temperature dependence of the optical conductivity in ${\mathrm{LaMnO}}_3:$ A tight-binding approach

A tight-binding parametrization of the band structure, along with a mean-field treatment of the Hund, electron-electron, and electron-lattice couplings, is used to obtain the full optical conductivity tensor of ${\mathrm{LaMnO}}_3$ as a function of temperature. We predict striking changes with temperature in the functional form and magnitude of the optical absorption. Comparison of our results with existing data makes it possible to determine the electron-lattice and electron-electron couplings. The effective “Hubbard U” is found to be $\approx 1.6$ eV, rather less than the full bandwidth $\approx 3.6$ eV, putting the material in the weak-intermediate coupling regime.