Na-induced correlations in NaxCoO2

Increasing experimental evidence is building which indicates that signatures of strong correlations are present in the Na-rich region of NaxCoO2 (i.e., x 0.7) and absent in the Na-poor region (i.e., x approximate to 0.3). This is unexpected, given that NaCoO2 is a band insulator and CoO2 has an integer-filled open shell, making it a candidate for strong correlations. We explain these experimental observations by presenting a minimal low-energy Hamiltonian for the cobaltates and solving it using density functional theory and dynamical mean-field theory. The Na potential is shown to be a key element in understanding correlations in this material. Furthermore, density functional theory calculations for the realistic Na ordering predict a binary perturbation of the Co sites which correlates with the Na-1 sites (i.e., Na sites above or below Co sites).