The Role of Hydration and Magnetic Fluctuations in The Superconducting Cobaltate

Abstract

AbstractWe report electronic structure calculations within density functional theory for the hydrated superconductor Na1/3CoO21.33H2O and compare the results with the parent compound Na0.3CoO2. We find that intercalation of water into the parent compound has little effect on the Fermi surface outside of the predictable effects expansion, in particular increased two-dimensionality. This implies an intimate connection between the electronic properties of the hydrated and unhydrated phases. Additional density functional calculations are used to investigate the doping dependence of the electronic structure and magnetic properties in hexagonal NaxCoO2. 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 the 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.KeywordsHydrated superconductormagnetic quantum fluctuationstriplet superconductivity