Temperature-dependent optical spectroscopy studies of Nd1−xTiO3

The temperature dependent optical spectra are investigated in a well-characterized titanate system, Nd1−xTiO3, between 50 and 40 000cm−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 1.2eV 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 1eV 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.