Unusual electron correlations in NaxCoO2 due to the spin-state quasidegeneracy of cobalt ions

TL;DR

This paper investigates the unusual electron correlations in NaxCoO2, attributing them to the spin-state quasidegeneracy of cobalt ions, and proposes a minimal orbital-degenerate model predicting spin-polaron behavior and potential superconductivity.

Contribution

It introduces a minimal model incorporating orbital degeneracy and spin-state quasidegeneracy to explain the strong correlations and superconductivity in NaxCoO2, highlighting the role of cobalt spin states.

Findings

Spin-polaron physics at x~1 due to spin-state fluctuations.

Prediction of extended s-wave singlet superconductivity at higher doping.

Suppression of charge carrier coherence by spin-state scattering.

Abstract

Recent studies exposed many remarkable properties of layered cobaltates NaxCoO2. Surprisingly, many-body effects have been found to increase at sodium-rich compositions of NaxCoO2 where one expects a simple, nearly free motion of the dilute S=1/2 holes doped into a band insulator NaCoO2. Here we discuss the origin of enigmatic correlations that turn a doped NaCoO2 into a strongly correlated electronic system. A minimal model including orbital degeneracy is proposed and its predictions are discussed. The model is based on a key property of cobalt oxides - the spin-state quasidegeneracy of CoO6 octahedral complex - which has been known, e.g., in the context of an unusual physics of LaCoO3 compound. Another important ingredient of the model is the 90-degree Co-O-Co bonding in NaxCoO2 which allows nearest-neighbor $t_{2g}-e_g$ hopping. This hopping introduces a dynamical mixture of electronic configurations $t_{2g}^6, S=0$ and $t_{2g}^5e_g^1, S=1$ of neighboring cobalt ions. We show that scattering of charge carriers on spin-state fluctuations suppresses their coherent motion and leads to the spin-polaron physics at $x\sim 1$. At larger doping when coherent fermionic bands are formed, the model predicts singlet superconductivity of extended s-wave symmetry. The presence of low-lying spin states of Co$^{3+}$ is essential for the pairing mechanism. Implications of the model for magnetic orderings are also discussed.