Doping effects on the electronic and structural properties of CoO2: An LSDA+U study

TL;DR

This study uses LSDA+U calculations to explore how doping affects the electronic, structural, and phononic properties of single-layer CoO2, revealing complex interactions and potential electron-phonon coupling phenomena.

Contribution

It provides a detailed LSDA+U analysis of doping effects on CoO2, highlighting the sensitivity of electronic structure to doping levels and phonon behavior.

Findings

Undoped CoO2 is a charge transfer insulator.

Doped CoO2 systems are metallic unless charge orderings occur.

Softening of phonon modes suggests strong electron-phonon coupling.

Abstract

A systematic LSDA+U study of doping effects on the electronic and structural properties of single layer CoO2 is presented. Undoped CoO2 is a charge transfer insulator within LSDA+U and a metal with a high density of states (DOS) at the Fermi level within LSDA. (CoO2)$^{1.0-}$, on the other hand, is a band insulator with a gap of 2.2 eV. Systems with fractional doping are metals if no charge orderings are present. Due to the strong interaction between the doped electron and other correlated Co d electrons, the calculated electronic structure of (CoO2)$^{x-}$ depends sensitively on the doping level x. Zone center optical phonon energies are calculated under the frozen phonon approximation and are in good agreement with measured values. Softening of the $E_g$ phonon at doping x ~0.25 seems to indicate a strong electron-phonon coupling in this system. Possible intemediate spin states of Co ions, Na ordering, as well as magnetic and charge orderings in this system are also discussed.