Temperature-dependent optical spectroscopy studies ofNd1−xTiO3

Abstract

The temperature dependent optical spectra are investigated in a well-characterized titanate system, ${\mathrm{Nd}}_{1\ensuremath{-}x}\mathrm{Ti}{\mathrm{O}}_{3}$, between 50 and $40 000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ at three different doping levels, $x=0.019$, 0.046, and 0.095, corresponding to a Mott-Hubbard insulator, a semiconductor and a correlated metal, respectively. Mid-gap states develop inside the Hubbard gap with hole doping. Based on the room-temperature spectra of the optical conductivity, the evolution rate of the excitations below $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with doping is dependent on the electron correlation strength $(U∕W)$ of the parent insulator, which has been observed in other titanates as well. In the metallic sample $(x=0.095)$, an anomalous enhancement of spectral weight below $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ develops with decreasing temperature. The partial spectral weight shows a quadratic dependence on temperatures up to the plasma frequency. Meanwhile, the metallic sample displays a Fermi-liquid behavior at low frequencies.