Optical evidence for the proximity to a spin-density-wave metallic state in Na$_{0.7}$CoO$_2$

TL;DR

This study investigates the optical properties of Na$_{0.7}$CoO$_2$ across a broad spectral range, revealing signs of proximity to a spin-density-wave metallic state through analysis of charge carrier dynamics.

Contribution

The paper provides detailed optical measurements and analysis indicating Na$_{0.7}$CoO$_2$ is near a spin-density-wave transition, using the generalized Drude model to analyze charge carrier scattering.

Findings

Scattering rate $\Gamma(\omega)$ scales linearly with frequency at low temperatures.

Na$_{0.7}$CoO$_2$ exhibits characteristics suggesting proximity to a spin-density-wave phase.

Optical conductivity analysis supports the material's closeness to a magnetic instability.

Abstract

We present the optical properties of \na single crystals, measured over a broad spectral range as a function of temperature ($T$). The capability to cover the energy range from the far-infrared up to the ultraviolet allows us to perform reliable Kramers-Kronig transformation, in order to obtain the absorption spectrum (i.e., the complex optical conductivity). To the complex optical conductivity we apply the generalized Drude model, extracting the frequency dependence of the scattering rate ($\Gamma$) and effective mass ($m^*$) of the itinerant charge carriers. We find that $\Gamma(\omega)\sim \omega$ at low temperatures and for $\omega > T$. This suggests that \na is at the verge of a spin-density-wave metallic phase.