Terahertz conductivity of localized photoinduced carriers in Mott insulator YTiO${}_3$ at low excitation density, contrasted with metallic nature in band semiconductor Si

Abstract

We performed optical-pump terahertz-probe measurements of a Mott insulator YTiO${}_3$ and a band semiconductor Si using a laser diode (1.47 eV) and a femtosecond pulse laser (1.55 eV). Both samples possess long energy-relaxation times (1.5 ms for YTiO${}_3$ and 15 $\mu$s for Si); therefore, it is possible to extract terahertz complex conductivities of photoinduced carriers under equilibrium. We observed highly contrasting behavior - Drude conductivity in Si and localized conductivity possibly obeying the Jonscher law in YTiO${}_3$. The carrier number at the highest carrier-concentration layer in YTiO${}_3$ is estimated to be 0.015 per Ti site. Anisotropic conductivity of YTiO${}_3$ is determined. Our study indicates that localized carriers might play an important role in the incipient formation of photoinduced metallic phases in Mott insulators. In addition, this study shows that the transfer-matrix method is effective for extracting an optical constant of a sample with a spatially inhomogeneous carrier distribution.

16 pages,7 figures, to appear in J. Phys.: Condens. Matter