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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., Comment: 7 pages, 4 figures

Here we report on Landau level spectroscopy of an epitaxially grown thin film of the topological insulator Sb2Te3, complemented by ellipsometry and magneto-transport measurements. The observed response suggests that Sb2Te3 is a direct-gap semiconductor with the fundamental band gap located at the \Gamma point, or along the trigonal axis, and its width reaches Eg = 190 meV at low temperatures. Our data also indicate the presence of other low-energy extrema with a higher multiplicity in both the conduction and valence bands. The conclusions based on our experimental data are confronted with and to a great extent corroborated by the electronic band structure calculated using the GW method., Comment: 11 pages, 8 figures, to be published in Phys. Rev. B

We have grown La$_{0.3}$Sr$_{0.7}$CoO$_{3-{\delta}}$ thin films by pulsed laser deposition on (LaAlO$_3$)$_{0.3}$(Sr$_2$TaAlO$_3$)$_{0.7}$ substrates with and without a protective LaAlO$_3$ capping layer and investigated their structural and magnetic properties. We have observed that, in the uncapped films, the Curie temperature strongly decreased after annealing in helium atmosphere, and it significantly decreased even in samples stored for several weeks at room temperature. The decrease of the Currie temperature is caused by an increase of the concentration of oxygen vacancies, {\delta}. However, we show that already a 3 nm thin LaAlO$_3$ capping layer can essentially conserve {\delta} at room temperature, and it considerably slows down the formation of oxygen vacancies at elevated temperatures.

We have studied ultrafast dynamics of thin films of LaCoO$_3$ and La$_{0.5}$Sr$_{0.5}$CoO$_3$ with femtosecond pump-probe ellipsometry in the energy range of 1.6-3.4 eV. We have observed a large pump-induced transfer of spectral weight in LaCoO$_3$ that corresponds to an insulator-to-metal transition. The photo-induced metallic state initially relaxes via a fast process with a decay constant of about 200 fs. Both LaCoO$_3$ and La$_{0.5}$Sr$_{0.5}$CoO$_3$ exhibit a significant secondary transient structure in the 1-30 ps range. Results of measurements on films with different thicknesses demonstrate that it corresponds to a propagation of an acoustic strain pulse. On timescales longer than 100 ps, heat diffusion to the substrate takes place that can be modelled with a bi-exponential decay., Comment: 8 pages, 4 figures