The origin of strong correlations and superconductivity in Na$_x$CoO$_2$

TL;DR

This paper introduces a minimal model explaining the strong correlations and superconductivity in sodium-rich Na$_x$CoO$_2$, highlighting the role of spin-state quasidegeneracy and correlated hopping in its unusual electronic properties.

Contribution

The paper presents a novel minimal model that accounts for the observed correlations and predicts singlet superconductivity in Na$_x$CoO$_2$, emphasizing the importance of spin-state quasidegeneracy.

Findings

Correlated hopping leads to spin-polaron physics at high doping.

The model predicts extended s-wave pairing at larger doping.

It explains the enigmatic correlations observed in sodium-rich Na$_x$CoO$_2$.

Abstract

We propose a minimal model resolving a puzzle of enigmatic correlations observed in sodium-rich Na$_x$CoO$_2$ where one expects a simple, free motion of the dilute $S=1/2$ holes doped into a band insulator NaCoO$_2$. The model also predicts singlet superconductivity at experimentally observed compositions. The model is based on a key property of cobalt oxides -- the spin-state quasidegeneracy of CoO$_6$ octahedral complex -- leading to an unusual physics of, {\it e.g.}, LaCoO$_3$. We show that correlated hopping between $t_{2g}$ and $e_g$ states leads to the spin-polaron physics at $x\sim 1$, and to an extended s-wave pairing at larger doping when coherent fermionic bands are formed.