Mott-Hubbard exciton in the optical conductivity of YTiO3 and SmTiO3

In the Mott-Hubbard insulators YTiO3 and SmTiO3 we study optical excitations from the lower to the upper Hubbard band, vertical bar d(1)d(1)> -> vertical bar d(0)d(2)>. The multipeak structure observed in the optical conductivity reflects the multiplet structure of the upper Hubbard band in a multiorbital system. Absorption bands at 2.55 and 4.15 eV in the ferromagnet YTiO3 correspond to final states with a triplet d(2) configuration, whereas a peak at 3.7 eV in the antiferromagnet SmTiO3 is attributed to a singlet d(2) final state. A strongly temperature-dependent peak at 1.95 eV in YTiO3 and 1.8 eV in SmTiO3 is interpreted in terms of a Hubbard exciton, i.e., a charge-neutral (quasi-) bound state of a hole in the lower Hubbard band and a double occupancy in the upper one. The binding to such a Hubbard exciton may arise both due to Coulomb attraction between nearest-neighbor sites and due to a lowering of the kinetic energy in a system with magnetic and/or orbital correlations. Furthermore, we observe anomalies of the spectral weight in the vicinity of the magnetic ordering transitions, both in YTiO3 and SmTiO3. In the G-type antiferromagnet SmTiO3, the sign of the change of the spectral weight at T-N depends on the polarization. This demonstrates that the temperature dependence of the spectral weight is not dominated by the spin-spin correlations, but rather reflects small changes of the orbital occupation.