Charge order and phonon renormalizations: Possible implications for cobaltates

TL;DR

This paper explores how charge ordering proximity in cobaltates affects phonon modes, predicting observable anomalies like softening and broadening of specific optical phonons as water content increases.

Contribution

It introduces a $t-V$ model coupled to phonons to qualitatively analyze phonon anomalies related to charge ordering in cobaltates.

Findings

Optical phonon modes at K and M points are sensitive to charge order proximity.

Increasing water content pushes the system closer to charge order, causing phonon softening.

Phonon broadening is also expected near charge ordering conditions.

Abstract

Several experimental and theoretical studies in cobaltates suggest the proximity of the system to charge ordering (CO). We show, qualitatively, in the frame of a $t-V$ model coupled to phonons that optical phonon modes at the $K$ and $M$ points of the Brillouin zone, which involves only $O$-ions displacement around a $Co$-ion, are good candidates to display anomalies due to the CO proximity. If by increasing of $H_2O$ content the system is pushed closer to CO, the mentioned phonon modes should show softening and broadening.