Quantum Critical Behavior and Possible Triplet Superconductivity in Electron Doped CoO2 Sheets

TL;DR

This study uses density functional calculations to explore the electronic and magnetic properties of electron-doped CoO2 sheets, suggesting potential for triplet superconductivity similar to Sr2RuO4 due to ferromagnetic tendencies.

Contribution

It provides a detailed theoretical analysis of doping effects on electronic structure and magnetic behavior in Na_xCoO2, proposing possible triplet superconductivity.

Findings

Electronic structure is highly two-dimensional.

Weak itinerant ferromagnetism predicted across doping levels.

Potential for triplet superconductivity based on Fermi surface and magnetic fluctuations.

Abstract

Density functional calculations are used to investigate the doping dependence of the electronic structure and magnetic properties in hexagonal Na$_x$CoO$_2$. The electronic structure is found to be highly two dimensional, even without accounting for the structural changes associated with hydration. At the local spin density approximation level, a weak itinerant ferromagnetic state is predicted for all doping levels in the range $x=0.3$ to $x=0.7$, with competing but weaker itinerant antiferromagnetic solutions. The Fermi surface, as expected, consists of simple rounded hexagonal cylinders, with additional small pockets depending on the $c$ lattice parameter. Comparison with experiment implies substantial magnetic quantum fluctuations. Based on the Fermi surface size and the ferromagnetic tendency of this material,it is speculated that a triplet superconducting state analogous to that in Sr$_2$RuO$_4$ may exist here.