Novel electronic states in sodium cobaltates Na_xCoO₂

– Role of strong correlation, geometrical frustration, and Na dopant order

Ziqiang Wang

Boston College

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 recent experiments and argue that strong correlation, geometrical frustration, and Na dopant order play essential roles in understanding the unconventional behaviors of the charge, spin, and orbital degrees of freedom of the Co 3d electrons. We show that strong correlation renormalizes the crystal field splitting and the bandwidth. This causes inter-orbital carrier transfer and results in a single band description of the low-energy electronic structure consistent with ARPES experiments. On this basis, we study the electronic phases using a minimal one-band t-U-V ionic Hubbard model on the triangular lattice that takes into account the electrostatic potential of the off-plane Na dopants. We find a class of charge and spin density ordered states by the mechanism of alleviated frustration via inhomogeneity and describe a theory of the unexpected “x=0.5 phase” consistent with neutron scattering, NMR, quantum oscillation and transport experiments. In the Na rich part of the phase diagram, the sodium vacancies increase the correlation effects. We argue that the newly discovered “x=11/13 phase” associated with Na vacancy order can be described by a useful notion of “superMottness”, where strong correlation effects on the superlattice structure give rise to coexistence of FM itinerant carriers and local moments. Finally, we propose a class of nodal chiral superconductors and discuss their topological properties in connection to the superconducting state observed in the hydrated cobaltates.