Article

Destruction of the small Fermi surfaces in NaxCoO2 by disorder.

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6032, USA.
Physical Review Letters (Impact Factor: 7.73). 08/2006; 97(1):016404. DOI: 10.1103/PhysRevLett.97.016404
Source: arXiv

ABSTRACT We show using density functional calculations that the small e'g Fermi surfaces in NaxCoO2 are destroyed by Na disorder. This provides a means to resolve the prediction of these sections in band structure calculations with their nonobservation in angle resolved photoemission experiments.

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    ABSTRACT: We study theoretically and experimentally the dependence of the electron-phonon interaction in NaxCoO2 on the sodium concentration x. For the two oxygen phonon modes found in Raman experiments, A1g and E1g, we calculate the matrix elements of the electron-phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon frequency we compare the calculated and experimentally observed doping dependence of the A1g mode. Furthermore, due to the momentum dependence of the electron-phonon coupling for the E1g symmetry we find no renormalization of the corresponding phonon frequency which agrees with experiment. Our results shed light on the possible importance of the electron-phonon interaction in this system.
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    ABSTRACT: We report a systematic study on NaxCoO2 (0.3≤x≤0.72) using angle-resolved photoemission spectroscopy (ARPES), with a focus on the Fermi surface topology of the metallic samples. Within this doping range, we observed only one large hexagonal Fermi surface around the Γ-point, which comes from the a1g band. Contrary to most band calculations, our results show that the eg' bands never cross the Fermi level to form small Fermi surface pockets near the K-point. The enclosed area of the a1g Fermi surface is found to be consistent with the Luttinger theorem within the doping range in this study. We also found that, at x = 1/3, the Fermi surface coincides with the zone boundary of the commensurate ordering, indicating the importance of charge fluctuations in this material.
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    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 results 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 to 315 meV in terms of the energy difference. In this work, we perform density-functional theory calculations of this one-electron splitting Delta=gammaa1g-gammaeg' for the full two-layer compound, accounting for the effects of Na ordering, interplanar interactions and octahedral distortion. We find that gammaa1g-gammaeg' 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 that disproportionately affects the extended a1g orbital. We discuss also the effects of octahedral compression and multiorbital filling on the value of Delta 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.
    Physical review. B, Condensed matter 01/2008; 78. · 3.77 Impact Factor

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