Simulation of the time-dependent optical conductivity of LaMnO₃ in a laser-pulse pump-probe experiment

We simulated numerically the time-dependence of the optical conductivity of LaMnO₃ excited and probed by using an ultrashort laser pulse in a pump-probe experiment. To simpify the calculation, we built a small cluster model consisting of four Mn ions for the sample system. We utilized the Hubbard Hamiltonian and solved the Hamiltonian by using a numerical diagonalization method to obtain the eigenstates of the system. Using the eigenstates, we calculated the optical conductivity at the equilibrium state and found a qualitative similiarity to the experimental result. We also observed a change in the integrated optical conductivity as a function of the delay time between the pump pulse and the probe pulse. We found a phonon oscillation due to the Q₂ phonon mode and an optical anisotropy that depended on the polarzation configurations of the pump pulse and the probe pulse. We found the breaking of the geometrical orbital ordering by the pump pulse to be the cause of the anisotropy in the optical conductivity.