Terahertz conductivity of localized photoinduced carriers in Mott insulator YTiO$_{3}$ at low excitation density, contrasted with metallic nature in band semiconductor Si

TL;DR

This study compares terahertz conductivity of photoinduced carriers in a Mott insulator YTiO₃ and a band semiconductor Si, revealing localized behavior in YTiO₃ and metallic behavior in Si, with implications for understanding photoinduced phase transitions.

Contribution

It demonstrates the use of terahertz spectroscopy to distinguish localized versus metallic photoinduced carriers in Mott insulators and semiconductors, highlighting the role of localized carriers in phase transitions.

Findings

YTiO₃ exhibits localized conductivity possibly following Jonscher law.

Si shows Drude metallic conductivity under photoexcitation.

Localized carriers in YTiO₃ may influence the formation of metallic phases.

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.