The origin of a1g and eg' orderings in NaxCoO2
Abstract
It has often been suggested that correlation effects suppress the small eg' Fermi surface pockets of NaxCoO2 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 from -200 meV to 315 meV in therms of the energy difference. In this work, we perform density functional theory calculations of this one-electron splitting = εa1g-εeg' for the full two-layer compound, Na2xCo2O4, accounting for the effects of Na ordering, interplanar interactions and octahedral distortion. We find that ε a1g-ε eg' is negative for all Na fillings and that this is primarily due to the strongly positive Coulomb field created by Na+ ions in the intercalant plane. This field disproportionately affects the a1g orbital which protrudes farther upward from the Co plane than the eg' orbitals. We discuss also the secondary effects of octahedral compression and multi-orbital filling on the value of as a function of Na content. Our results indicate that if the eg' pockets are indeed suppressed that can only be due to nonlocal correlation effects beyond the standard DMFT.