Novel Electronic States in Sodium Cobaltates NaxCoO2

Abstract

Transition metal oxides occupy an important arena in strongly correlated materials. We focus here on the triangular lattice sodium cobaltates which display a very rich phase diagram as a function of Na doping x with many new surprises. We review a spectrum of recent experiments and argue that strong correlation and Na dopant potential play essential roles in understanding the unconventional behaviors of the charge, spin, and orbital degrees of freedom of the Co 3d electrons. The strong Co intra-atomic Coulomb repulsion renormalizes the crystal field splitting and the bandwidths of the t 2g complex in Na x CoO 2 , resulting in a single band crossing the Fermi level at all doping levels x explored by ARPES experiments. On this basis, we study the electronic states using a minimal one-band Hubbard model with large U on the triangular lattice. The strong correlation renormalizes the Stoner criterion and stabilizes the paramagnetic state for x < x c ~ 0.67. The important role played by the off-plane Na dopants is taken into account by including the ionic electrostatic potential. In the Na rich part of the phase diagram, the high density of off-plane Na dopants (or dilute Na vacancies) increases the tendency toward carrier localization in the Co plane, which competes with in-plane ferromagnetic (FM) correlations described by a renormalized Stoner theory and leads to an inhomogeneous FM state, exhibiting nonmagnetic Co 3+ patches, AF correlated regions, and FM clusters with AF domains. We argue that the newly discovered x=11/13 phase associated with Na vacancy order can be described by the interplay between the sodium potential and the strong correlation, which gives rise to the coexistence of FM itinerant carriers and local moments.