The origin of strong correlations and superconductivity in Na$_x$CoO$_2$

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Khaliullin, G.
Chaloupka, J.
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We propose a minimal model resolving a puzzle of enigmatic correlations observed in sodium-rich Na$_x$CoO$_2$ where one expects a simple, free motion of the dilute $S=1/2$ holes doped into a band insulator NaCoO$_2$. The model also predicts singlet superconductivity at experimentally observed compositions. The model is based on a key property of cobalt oxides -- the spin-state quasidegeneracy of CoO$_6$ octahedral complex -- leading to an unusual physics of, {\it e.g.}, LaCoO$_3$. We show that correlated hopping between $t_{2g}$ and $e_g$ states leads to the spin-polaron physics at $x\sim 1$, and to an extended s-wave pairing at larger doping when coherent fermionic bands are formed.
Comment: 7 pages, 6 figures, extended version as published in PRB
Keywords
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Superconductivity
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