Y. Lysogorskiy, O. Nedopekin, S. Krivenko, D. Tayurskii, B. Minisini
Puzzling properties of sodium cobaltates Na${}_x$CoO${}_2$ become an issue of numerous recent theoretical and experimental investigations because of very high thermopower and intricate phase diagram of this material. Experiments indicated Na${}_{2/3}$CoO${}_2$ to have a kagome-like cobalt sublattice, which implies additional complexity of this strongly correlated system. In the present work we have employed the ab-initio GGA+U modeling method to investigate spin states of Na${}_{2/3}$CoO${}_2$, using an exact crystal cell established in the experiment. Stability of sodium imprint for concentration $x=2/3$ proposed in the publications has been obtained. An AFM-A type magnetic ordering has been recognized as a ground state. Moreover, a competition between low spin state (LS) and intermediate spin state (IS) has been established to depend on on-site Hubbard $U$ correction parameter. LS state has been identified as a ground state of Na${}_{2/3}$CoO${}_2$.
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http://arxiv.org/abs/1204.3101
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