Mott-Hubbard exciton in the optical conductivity ofYTiO3andSmTiO3

Abstract

In the Mott-Hubbard insulators ${\text{YTiO}}_{3}$ and ${\text{SmTiO}}_{3}$ we study optical excitations from the lower to the upper Hubbard band, $|{d}^{1}{d}^{1}⟩\ensuremath{\rightarrow}|{d}^{0}{d}^{2}⟩$. 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 ${\text{YTiO}}_{3}$ correspond to final states with a triplet ${d}^{2}$ configuration, whereas a peak at 3.7 eV in the antiferromagnet ${\text{SmTiO}}_{3}$ is attributed to a singlet ${d}^{2}$ final state. A strongly temperature-dependent peak at 1.95 eV in ${\text{YTiO}}_{3}$ and 1.8 eV in ${\text{SmTiO}}_{3}$ 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 ${\text{YTiO}}_{3}$ and ${\text{SmTiO}}_{3}$. In the $G$-type antiferromagnet ${\text{SmTiO}}_{3}$, 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.