The origin of a$_{1g}$ and e$_g$' orderings in Na$_x$CoO$_2$

@article{Pillay2008TheOO,
  title={The origin of a\$\_\{1g\}\$ and e\$\_g\$' orderings in Na\$\_x\$CoO\$\_2\$},
  author={Devina Pillay and Michelle D. Johannes and Igor I. Mazin and Ole Krogh Andersen},
  journal={Physical Review B},
  year={2008}
}
It has often been suggested that correlation effects suppress the small e_g' Fermi surface pockets of NaxCoO_2 that are predicted by LDA, but absent in ARPES measurements. It appears that within the dynamical mean field theory (DMFT) the ARPES can be reproduced only if the on-site energy of the eg' complex is lower than that of the a1g complex at the one-electron level, prior to the addition of local correlation effects. Current estimates regarding the order of the two orbital complexes range… 
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