Mott insulator to correlated metal: Optical study of ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{TiO}}_3$

The room-temperature reflectance of a well-characterized series of samples (x-ray, neutron activation and thermigravimetric analyses, resistivity, magnetization) in the ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{TiO}}_3$ system has been measured between 50 and 40 000 ${\mathrm{cm}}^{\mathrm{-}1}$ on samples ranging from the antiferromagnetic insulating (${\mathrm{LaTiO}}_3$) to the metallic (${\mathrm{La}}_{0.88}$${\mathrm{TiO}}_3$) part of the phase diagram. The electronic portion of the low-frequency optical conductivity increases with frequency at the lowest frequencies, similar to several barely metallic systems. This non-Drude behavior can be modeled as the sum of the two low-frequency oscillators, a Drude contribution that increases systematically with doping and a broad midinfrared continuum. The midinfrared band, which may be associated with transitions across the Hubbard gap, persists in highly doped samples in agreement with theoretical predictions. If one assumes a single low-frequency component with frequency-dependent scattering rate, one finds a negative mass enhancement below 150 ${\mathrm{cm}}^{\mathrm{-}1}$ in metallic samples close to the metal-insulator phase boundary.