Quantum critical behavior and possible triplet superconductivity in electron-doped CoO₂ sheets

Density-functional calculations are used to investigate the doping dependence of the electronic structure and magnetic properties in hexagonal Na_xCoO₂. The electronic structure is 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 range x=0.3 to x=0.7, with competing but weaker itinerant antiferromagnetic solutions. Comparison with experiment implies substantial magnetic quantum fluctuations. Based on the simple Fermi surface and the ferromagnetic tendency of this material, it is speculated that a triplet superconducting state analogous to that in Sr₂RuO₄ may exist here.