Electronic structure of the $\rm Ca_3Co_4O_9$ compound from ab initio local interactions

TL;DR

This study employs ab initio calculations to analyze the electronic structure of Ca3Co4O9, revealing large electron correlations, orbital localization, and magnetic exchange fluctuations, with implications for thermoelectric properties and phase transitions.

Contribution

It provides the first detailed ab initio determination of Hubbard and t-J model parameters for Ca3Co4O9, highlighting correlation effects and magnetic fluctuations.

Findings

Large electron correlation parameter U/t~26.

Partial localization of a1g electrons.

Magnetic exchange fluctuates from AFM to FM.

Abstract

We used fully correlated ab initio calculations to determine the effective parameters of Hubbard and t - J models for the thermoelectric misfit compound $\rm Ca_3Co_4O_9$. As for the $\rm Na_xCoO_2$ family the Fermi level orbitals are the $a_{1g}$ orbitals of the cobalt atoms ; the $e'_g$ being always lower in energy by more than 240\,meV. The electron correlation is found very large $U/t\sim 26$ as well as the parameters fluctuations as a function of the structural modulation. The main consequences are a partial $a_{1g}$ electrons localization and a fluctuation of the in-plane magnetic exchange from AFM to FM. The behavior of the Seebeck coefficient as a function of temperature is discussed in view of the ab initio results, as well as the 496\,K phase transition.