The Doping and Temperature Dependence of Optical Properties of Nd1-xTI03

Abstract

<p> A well characterized titanate system, Nd1_xTi03, has been studied by temperature dependent reflectance spectroscopy between 50 and 40 000 cm-1 at three different doping levels, x = 0.019, 0.046, and 0.095, which yield a Matt-Hubbard insulator, a semiconductor and a correlated metal, respectively. Two main issues are discussed regarding the optical properties of the current system. The first is the variation of the low-lying electronic structure with hole concentration. The doping-dependent optical conductivity of Nd1_xTi03 shows several obvious differences when compared to the superconducting cuprates. We observed mid-infrared absorption bands in the doped samples, suggesting that mid-gap states develop inside the Hubbard gap with hole doping in the context of a two-component model. A quantitative analysis of the spectral weight below 1.2 e V as a function of doping indicates that the evolution rate of the optical excitations below 1.2 eV is related to the electron correlation strength of the parent insulator, which has been observed in other titanates as well. The second issue addressed in this thesis is the temperature-dependent optical features of the correlated metallic sample with x = 0.095, a composition close to the metal-insulator transition at x '"'-~ 0.08. The optical conductivity shows an anomalous enhancement of spectral weight below leV, in both the Drude and midinfrared part, that develops with decreasing temperature. The dynamical mean field theory (DMFT) may explain this feature. Meanwhile, the metallic sample displays a Fermi-liquid like behavior in the low-frequency limit, which can be established from the spectra of the scattering rate as a function of both frequency and temperature. We found a good agreement between the experimental results extracted from the scattering rate and the Fermi-liquid theory. </p>