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 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 Δ= ε_a1g-ε_e_g' for the full two-layer compound, Na2xCo2O4, accounting for the effects of Na ordering, interplanar interactions and octahedral distortion. We find that εa_1g-εe_g' 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 a_1g orbital which protrudes farther upward from the Co plane than the e_g' orbitals. We discuss also the secondary effects of octahedral compression and multi-orbital filling on the value of Δas a function of Na content. Our results indicate that if the e_g' pockets are indeed suppressed that can only be due to nonlocal correlation effects beyond the standard DMFT.
Anomalous magnetic ordering in PrBa_2Cu_3O_{7-y} single crystals: Evidence for magnetic coupling between the Cu and Pr sublattices
在无Al的O_{7-y}单晶中，与Pr反铁磁有序有关的磁化率chi_{ab}（T）随温度变化的扭折，在磁场H中场冷（FC）后消失||AB平面。在H的FC之后，chi_c（T）的扭结保持不变||c轴。作为一种可能的解释，位于ab平面的Cu磁矩的冻结是由H中的FC引起的||ab阻碍了它们的重取向，并且由于Pr和Cu（2）子晶格之间的耦合，阻碍了Pr^{3+}矩的有序化。对H和Pr^{3+}，都发现了场诱导的相变和Pr^{3+}有序温度的场依赖性||c和H|| AB.
The Origin of the Charge Ordering and Its Relevance to Superconductivity in $θ$-(BEDT-TTF)$_2$X: The Effect of the Fermi Surface Nesting and the Distant Electron-Electron Interactions
