Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. We study optical excitations across the Mott gap in the multiorbital Mott-Hubbard insulators RVO 3 . The multipeak structure observed in the optical conductivity can be described consistently in terms of the different 3d 3 multiplets or upper Hubbard bands. The spectral weight is very sensitive to nearest-neighbor spin-spin and orbital-orbital correlations and thus shows a pronounced dependence on both temperature and polarization. Comparison with theoretical predictions based on either rigid orbital order or strong orbital fluctuations clearly rules out that orbital fluctuations are strong in RVO 3 . Both the line shape and the temperature dependence give clear evidence for the importance of excitonic effects.

Revealing the nature of charge excitations in strongly correlated electron systems is crucial to understanding their exotic properties. Here we use broadband ultrafast pump-probe spectroscopy in the visible range to study low-energy transitions across the Mott-Hubbard gap in the orbitally ordered insulator YVO 3 . Separating thermal and nonthermal contributions to the optical transients, we show that the total spectral weight of the two lowest peaks is conserved, demonstrating that both excitations correspond to the same multiplet. The pump-induced transfer of spectral weight between the two peaks reveals that the low-energy one is a Hubbard exciton, i.e., a resonance or a nearly bound state between a doublon and a holon. Finally, we speculate that the pump-driven spin disorder can be used to quantify the kinetic energy gain of the excitons in a ferromagnetic environment.

Compounds with intermediate-size transition metals such as Fe or Mn are close to the transition between charge-transfer systems and Mott-Hubbard systems. We study the optical conductivity σ(ω) of insulating layered LaSrFeO4 in the energy range 0.5 − 5.5 eV from 15 K to 250 K by the use of spectroscopic ellipsometry in combination with transmittance measurements. A multipeak structure is observed in both σ a (ω) and σ c (ω). The layered structure gives rise to a pronounced anisotropy, thereby offering a means to disentangle Mott-Hubbard and charge-transfer absorption bands. We find strong evidence that the lowest dipole-allowed excitation in LaSrFeO4 is of Mott-Hubbard type. This rather unexpected result can be attributed to Fe 3d -O 2p hybridization and in particular to the layered structure with the associated splitting of the eg level. In general, Mott-Hubbard absorption bands may show a strong dependence on temperature. This is not the case in LaSrFeO4, in agreement with the fact that spin-spin and orbital-orbital correlations between nearest neighbors do not vary strongly below room temperature in this compound with a high-spin 3d 5 configuration and a Néel temperature of TN = 366 K.