An ab-initio evaluation of the local effective interactions in the superconducting compound $\rm Na\_{0.35} Co O\_2-1.3H\_2O$

TL;DR

This study employs ab-initio quantum chemical methods to accurately determine local effective interactions, including ligand field splitting, Hund's exchange, Coulomb repulsion, and exchange integrals, in the superconducting compound $ m Narac{0.35}{1.3} Co O_2$.

Contribution

The paper provides detailed ab-initio calculations of effective interactions in $ m Na_{0.35} Co O_2$, highlighting the relevance of a three-band model and quantifying key parameters.

Findings

Ligand field splitting $ o$ $ m \, ext{delta} \, ext{sim} \, 300 \, meV$

Hund's exchange $J_H \, ext{sim} \, 280 \, meV$

Nearest-neighbor exchange $J \, ext{sim} \, -66 \, meV$

Abstract

We used ab-initio quantum chemical methods, treating explicitly the strong correlation effects within the cobalt 3d shell, as well as the screening effects on the effective integrals, for accurately determining on-site and nearest-neighbor (NN) interactions in the $\rm Na\_{0.35} Co O\_2-1.3H\_2O$ superconducting compound. The effective ligand field splitting within the $t\_{2g}$ orbitals was found to be $\delta \sim 300 \rm meV$, the $a\_{1g}$ orbital being destabilized compared to the $e\_g^\prime$ ones. The effective Hund's exchange and Coulomb repulsion were evaluated to $J\_H\sim 280 \rm meV$ and $U\sim 4.1$--$4.8 \rm eV$. The NN hopping parameters were determined within the three $t\_{2g}$ orbitals and found to be of the same order of magnitude as the $t\_{2g}$ ligand field splitting, supporting the hypothesis of a three band model for this system. Finally we evaluated the NN effective exchange integral to be antiferromagnetic and $J=-66 \rm meV$.