Within the scope of this thesis different transition-metal oxides with open d shells are investigated by means of spectroscopic ellipsometry in the energy range from 0.75eV to 5.5eV for temperatures ranging from 15K to 490K. The focus is on spin and orbital degrees of freedom and their impact on the optical spectra. The multipeak structures observed in the optical conductivity show a pronounced dependence on both temperature and polarization. We analyze the spectra in terms of multiplets which form the upper Hubbard band.
Our optical analysis of the multi-orbital Mott-Hubbard insulators YVO3, GdVO3, and CeVO3 yields a consistent description of the observed absorption bands in terms of 3d3 excited states which constitute the upper Hubbard band and thereby solves the discrepancies of the optical spectra of YVO3 reported in the literature. The temperature and polarization dependence of the optical spectra reflects the complex spin and orbital ordering phase diagram of RVO3 (R=Y, rare earth ion). A comparison of our data with theoretical predictions based on either rigid orbital order or strong orbital fluctuations leads us to the conclusion that orbital fluctuations cannot be strong in RVO3. The line shape and temperature dependence of a feature observed in the optical conductivity at around 2eV gives evidence for an excitonic Mott-Hubbard resonance, i.e. not a truly bound state below the gap but a resonance within the absorption band, and demonstrates the important role played by the kinetic energy for exciton formation in orbitally ordered Mott-Hubbard insulators.
Due to the layered structure of the correlated insulator LaSrFeO4 the optical spectra strongly depend on polarization. This anisotropy in combination with their different spectral weights offers an efficient tool to disentangle Mott-Hubbard excitations, corresponding to an electron transfer between neighboring Fe(3+) sites, and charge-transfer excitations, corresponding to an electron transfer from the oxygen 2p band to the Fe 3d band. We arrive at a consistent peak assignment and find that the lowest dipole-allowed excitation, which contributes to the in-plane optical conductivity only, is of Mott-Hubbard type. This result is rather unexpected at first sight as the 3d5 electron configuration of Fe(3+) is particularly stable due to the intra-atomic Hund exchange. We argue that the Fe 3d - O 2p hybridization and particularly the large splitting of the eg level originating from the tetragonal structure justify our result. The temperature dependence of the Mott-Hubbard excitations is only weak. This finding is in accordance with the fact that variations of nearest-neighbor spin-spin and orbital-orbital correlations are not strong below room temperature in LaSrFeO4 with a Néel temperature of 366K.
A further concern of this work lies on the correlated insulators LaCoO3 and EuCoO3 with Co(3+) 3d6 electron configuration, which have attracted a lot of interest because of the spin-state degree of freedom. The low-spin state, the intermediate-spin state, and the high-spin state, lie energetically close in these pseudocubic perovskites. It is well established that a thermal population of the high-spin state takes place from the low-spin ground state at T>25K in LaCoO3. This so-called spin-state transition is shifted to much higher temperatures in the sister compound EuCoO3.
In addition, we present optical data of the single-layered perovskites La(2-x)SrxCoO4 (x=0, 0.33, 0.45, 0.5, 0.9) and La1.5Ca0.5CoO4, which contain both Co(3+) 3d6 and Co(2+) 3d7 ions. In agreement with claims of the literature for a doping-induced spin-state transition from a Co(3+) low-spin state being realized in La(2-x)Sr(x)CoO4 for doping concentrations x<0.8 to a state of mixed Co(3+) low-spin and high-spin ions in LaSrCoO4, our optical spectra of La1.1Sr0.9CoO4 differ considerably from the spectra of the compounds with smaller doping concentrations. We assign the observed absorption bands to charge-transfer excitations from the oxygen 2p bands to the upper Hubbard bands. Surprisingly, we observe only small changes in the optical spectra of LaCoO3 across the spin-state transition temperature, our spectra of LaCoO3 resemble the spectra of La1.1Sr0.9CoO4 already at low temperatures
