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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 Delta= epsilon_a1g-epsilon_e_g for the full two-layer compound, Na2xCo2O4, accounting for the effects of Na ordering, interplanar interactions and octahedral distortion. We find that epsilon a_1g-epsilon e_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 Delta as 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.
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 s
Magnetism of layered cobaltites Na$_x$CoO$_2$ with $x$ = 0.6 and 0.9 has been investigated by a positive muon spin rotation and relaxation ($mu^+$SR) spectroscopy together with magnetic susceptibility and specific heat measurements, using single crys
The single electron tunneling spectroscopy on superconductor Na$_{x}$CoO$_2$$cdot$ yH$_2$O and its starting compound Na$_{x}$CoO$_2$ has been studied with point-contact method. The spectra of Na$_{x}$CoO$_2$ have two types of distinct shapes at diffe
The idea that surface effects may play an important role in suppressing $e_g$ Fermi surface pockets on Na$_x$CoO$_2$ $(0.333 le x le 0.75)$ has been frequently proposed to explain the discrepancy between LDA calculations (performed on the bulk compou
The two-dimensional t-J model on a frustrating lattice is studied using mean-field variational theories with Gutzwiller approximation. We find that a superconducting state with broken time-reversal symmetry (d+id state) is realized in the parameter r