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 LaTiO3 crystal exhibiting the G-type antiferromagnetic order at the Néel temperature TN = 146 K, we present a comprehensive spectroscopic ellipsometry study in the spectral range 0.5–5.6 eV at temperatures 10 ⩽ T ⩽ 300 K. We found that the complex dielectric function spectra of LaTiO3 crystal are almost featureless, nearly isotropic, and weakly temperature dependent in the range of the intersite d–d optical transitions. Nonetheless, analyzing the difference spectra below the TN, we have identified the LS-state d1d1⇌d2d0 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 JH ∼ 0.5 eV, which define the energy of the HS-state d1d1⇌d2d0 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 TN. The discovered properties indicate that the 1.3 eV band in LaTiO3 can be associated with a Mott–Hubbard exciton.