Fingerprints of Mott-Hubbard physics in optical spectra of antiferromagnetic LaTiO3

Abstract

Magnetic properties of Mott-Hubbard insulators are determined by superexchange interactions mediated by the high-spin (HS) and low-spin (LS) intersite d-d charge excitations, which can be associated with the HS- and LS-Hubbard subbands in optical experiments. To explore the Mott-Hubbard physics in orthorhombic LaTiO_3 crystal exhibiting the G-type antiferromagnetic order at the Neel temperature T_N = 146 K, we present a comprehensive spectroscopic ellipsometry study in the spectral range 0.5-5.6 eV at temperatures 10 K < T < 300 K. We found that the complex dielectric function spectra of LaTiO_3 crystal are almost featureless, nearly isotropic, and weakly temperature dependent in the range of d-d optical transitions. Nonetheless, analyzing the difference spectra below the T_N, we have identified the LS-state d^1d^1 - d^2d^0 excitations at ~3.7 and ~5.15 eV and estimated values of the on-site Coulomb repulsion U ~4.2 eV and Hund's exchange constant J_H ~0.5 eV, which define the energy of the HS-state d^1d^1 - d^2d^0 excitation at ~2.7 eV. In addition, we discovered that the pronounced lowest-energy 1.3 eV optical band displays the critical intensity behavior and anomalous broadening with decreasing temperature below the T_N. The discovered properties indicate that the 1.3 eV band in LaTiO_3 can be associated with a Mott-Hubbard exciton.