Infrared probe of the electronic structure and charge dynamics in Na$_{0.7}$CoO$_2$

TL;DR

This study investigates the electronic structure and charge dynamics of Na$_{0.7}$CoO$_2$ using optical spectroscopy, revealing complex interband transitions and unusual temperature-dependent charge response.

Contribution

It provides new insights into the interband transitions and low-frequency charge dynamics of Na$_{0.7}$CoO$_2$ through optical spectra analysis and comparison with band structure and ARPES data.

Findings

Two broad interband transition peaks at 1.6 eV and 3.2 eV

A weak midinfrared peak at 0.4 eV

Low-frequency peak shifts to higher frequencies with temperature

Abstract

We present measurements of the optical spectra on Na$_{0.7}$CoO$_2$ single crystals. The optical conductivity shows two broad interband transition peaks at 1.6 eV and 3.2 eV, and a weak midinfrared peak at 0.4 eV. The intraband response of conducting carriers is different from that of a simple Drude metal. A peak at low but finite frequency is observed, which shifts to higher frequencies with increasing temperature, even though the dc resistivity is metallic. We discussed the origin of the interband transitions and the low-frequency charge dynamics based on the results of band structure calculations and quasiparticle behavior observed in ARPES experiment.