A possibility of high spin hole states in doped CoO$_2$ layered systems

TL;DR

This paper models doped CoO2 layers to explore high spin states, revealing doping-induced high-spin Co4+ ions forming stripe-like magnetic structures, with implications for understanding magnetic properties in cobalt oxides.

Contribution

Introduces a five-band model for doped cobalt oxides, revealing doping-induced high-spin states and complex magnetic stripe formations in CoO2 layers.

Findings

Doping generates high S=5/2 spins at Co4+ ions.

High-spin states form singlet pairs and stripe-like magnetic structures.

Ground states remain insulating across doping levels.

Abstract

We introduce and investigate an effective five-band model for $t_{2g}$ and $e_g$ electrons to describe doped cobalt oxides with Co$^{3+}$ and Co$^{4+}$ ions in two-dimensional CoO$_2$ triangular lattice layers, as in Na$_{1-x}$CoO$_2$. The effective Hamiltonian includes anisotropic kinetic energy (due to both direct Co-Co and indirect Co-O-Co hoppings), on-site Coulomb interactions parameterized by intraorbital Hubbard repulsion $U$ and full Hund's exchange tensor, crystal-field terms and Jahn-Teller static distortions. We study it using correlated wave functions on $6\times 6$ clusters with periodic boundary conditions. The computations indicate low S=0 spin to high S=2 spin abrupt transition in the undoped systems when increasing strength of the crystal field, while intermediate S=1 spins are not found. Surprisingly, for the investigated realistic Hamiltonian parameters describing low spin states in CoO$_2$ planes, doping generates high $S=\frac{5}{2}$ spins at Co$^{4+}$ ions that are pairwise bound into singlets, seen here as pairs of up and down spins. It is found that such singlet pairs self-organize at higher doping into lines of spins with coexisting antiferromagnetic and ferromagnetic bonds, forming stripe-like structures. The ground states are insulating within the investigated range of doping because computed HOMO-LUMO gaps are never small enough.