Broadband dielectric response of CaCu3Ti4O12: From dc to the electronic transition regime

TL;DR

This study investigates the phonon and electronic properties of CaCu3Ti4O12, revealing its colossal dielectric constant and electronic transition behaviors from infrared and broadband spectroscopy, with implications for understanding its dielectric response.

Contribution

It provides a detailed analysis of phonon dynamics and electronic transitions in CaCu3Ti4O12, highlighting the relationship between its dielectric properties and electronic structure.

Findings

Unusual temperature dependence of phonon eigenfrequencies and damping.

Electronic excitations identified as transitions between hybridized bands.

Small band gap (~200 meV) consistent with dc-conductivity measurements.

Abstract

We report on phonon properties and electronic transitions in CaCu3Ti4O12, a material which reveals a colossal dielectric constant at room temperature without any ferroelectric transition. The results of far- and mid-infrared measurements are compared to those obtained by broadband dielectric and millimeter-wave spectroscopy on the same single crystal. The unusual temperature dependence of phonon eigenfrequencies, dampings and ionic plasma frequencies of low lying phonon modes are analyzed and discussed in detail. Electronic excitations below 4 eV are identified as transitions between full and empty hybridized oxygen-copper bands and between oxygen-copper and unoccupied Ti 3d bands. The unusually small band gap determined from the dc-conductivity (~200 meV) compares well with the optical results.