Infrared spectroscopy of the charge ordering transition in Na$_{0.5}$CoO$_2$

TL;DR

This study investigates the charge ordering transition in Na$_{0.5}$CoO$_2$ using infrared spectroscopy, revealing a metal-insulator transition, charge density wave formation, and strong electron-phonon interactions at low temperatures.

Contribution

It provides detailed infrared spectral evidence of charge ordering and electron-phonon coupling in Na$_{0.5}$CoO$_2$, highlighting the transition's spectral features and gap formation.

Findings

Charge ordering transition occurs near 50 K.

Optical gap of approximately 3.5 times k_B T_co.

Development of a hump near 800 cm$^{-1}$ and new phonon modes.

Abstract

We report infrared spectra of a Na$_{0.5}$CoO$_2$ single crystal which exhibits a sharp metal-insulator transition near 50 K due to the formation of charge ordering. In comparison with x=0.7 and 0.85 compounds, we found that the spectral weight associated with the conducting carriers at high temperature increases systematically with decreasing Na contents. The charge ordering transition only affects the optical spectra below 1000 cm$^{-1}$. A hump near 800 cm$^{-1}$ develops below 100 K, which is accompanied by the appearance of new lattice modes as well as the strong anti-resonance feature of phonon spectra. At lower temperature $T_{co}$, an optical gap develops at the magnitude of 2$\Delta\approx3.5k_BT_{co}$, evidencing an insulating charge density wave ground state. Our experimental results and analysis unequivocally point towards the importance of charge ordering instability and strong electron-phonon interaction in Na$_x$CoO$_2$ system.