Title: Optical signatures of strain-induced ferromagnetism in LaCoO$_3$ thin film

Abstract: Using spectroscopic ellipsometry, we studied the optical conductivity of LaCoO$_3$ with various degrees of strain. The optical response of the compressively strained \lco\ film is qualitatively similar to the one of the unstrained LaCoO$_3$ polycrystalline sample and exhibits redistribution of the spectral weight between about 0.2 and 6 eV, which is most likely related to the thermal excitation of the high-spin states. The optical response of the ferromagnetic tensile strained film exhibits clear signatures due to the ferromagnetic state. Below the Curie temperature $T_c=82$ K, the spectral weight is transferred with the increasing temperature from low energies between 0.2 and 3.3 eV to energies between 3.3 and 5.6 eV. The temperature dependence of the low-energy spectral weight between 0.2 and 3.3 eV can be understood in the framework of the high-spin biexciton model of Sotnikov and Kune\v{s} as corresponding to the variation of the concentration of high-spin states that are stabilized below $T_c$. The magnitude of redistribution of spectral weight due to the formation of the ferromagnetic state is sizable. We estimate that it corresponds to a lowering of the kinetic energy of 13 meV per Co ion, which is about two times $k_BT_c$. The latter shows that the saving of the kinetic energy is important and may be the leading energy contribution in the formation of the ferromagnetic phase.