Na Induced Correlations in NaₓCoO₂

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Increasing experimental evidence is building which indicates that signatures of strong correlations are present in the Na rich region of NaₓCoO₂ (ie. x≈0.7) and absent in the Na poor region (ie. x≈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 within LDA+DMFT. The Na potential is shown to be a key element in understanding correlations in this material. Furthermore, LDA calculations for the realistic Na ordering predict a binary perturbation of the Co sites which correlates with the Na₁ sites (ie. Na sites above/below Co sites).