MOTT INSULATOR TO CORRELATED METAL - OPTICAL STUDY OF LA1-XTIO3

The room-temperature reflectance of a well-characterized series of sam ples (x-ray, neutron activation and thermigravimetric analyses, resist ivity, magnetization) in the La1-xTiO3 system has been measured betwee n 50 and 40 000 cm-1 on samples ranging from the antiferromagnetic ins ulating (LaTiO3) to the metallic (La0.88TiO3) part of the phase diagra m. The electronic portion of the low-frequency optical conductivity in creases with frequency at the lowest frequencies, similar to several b arely metallic systems. This non-Drude behavior can be modeled as the sum of the two low-frequency oscillators, a Drude contribution that in crease systematically with doping and a broad midinfrared continuum. T he midinfrared band, which may be associated with transitions across t he Hubbard pp, persists in highly doped samples in agreement with theo retical predictions. If one assumes a single low-frequency component w ith frequency-dependent scattering rate, one finds a negative mass enh ancement below 150 cm-1 in metallic samples close to the metal-insulat or phase boundary.