Effects of local oxygen distortions on electronic structures of Na$_{x}$CoO$_{2}$

TL;DR

This study investigates how local oxygen distortions influence the electronic structures of Na$_{x}$CoO$_{2}$, revealing phase transitions and the suppression or enhancement of ferromagnetism depending on Na content and structural strain.

Contribution

It provides new insights into the effects of local oxygen distortions on electronic properties and magnetic phases of Na$_{x}$CoO$_{2}$ using first-principles calculations.

Findings

Decreased cobalt-oxygen hybridization with increasing Na content.

Predicted phase transition from ferromagnetic to paramagnetic metals.

Strain enhances hybridization and ferromagnetism in hydrated layers.

Abstract

By using pseudopotential method with local spin density functional approximation, the electronic band structures of Na$_{x}$CoO$_{2}$ are calculated for $x=0.25$, 0.5, 0.75, and $x=1$ in the presence of the structure relaxations. As increasing Na content, the hybridization between cobalt and oxygen orbitals is decreased, and a phase transition is predicted from a wide-band ferromagnetic to a narrow band paramagnetic metals. The itinerant ferromagnetism is strongly suppressed by the local distortions of the oxygens around the cobalts. Moreover, straining the CoO$_{2}$ layers corresponding to the hydrated superconductor Na$_{0.35}$CoO$_{2}\cdot $1.3H$%_{2}$O strongly enhances both the hybridization and ferromagnetism.