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Title: Destruction of the Small Fermi Surfaces in Na xCoO 2 by Disorder

Abstract

We show using density functional calculations that the small e0g 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.

Authors:
 [1];  [2]
  1. ORNL
  2. University of California, Davis
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
931273
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Letters; Journal Volume: 97; Journal Issue: 1
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; SODIUM OXIDES; COBALT OXIDES; FERMI LEVEL; DENSITY FUNCTIONAL METHOD; PHOTOEMISSION; ORDER-DISORDER TRANSFORMATIONS; ELECTRONIC STRUCTURE; fermi; disorder

Citation Formats

Singh, David J, and Kasinathan, Deepa. Destruction of the Small Fermi Surfaces in NaxCoO2 by Disorder. United States: N. p., 2006. Web. doi:10.1103/PhysRevLett.97.016404.
Singh, David J, & Kasinathan, Deepa. Destruction of the Small Fermi Surfaces in NaxCoO2 by Disorder. United States. doi:10.1103/PhysRevLett.97.016404.
Singh, David J, and Kasinathan, Deepa. Sun . "Destruction of the Small Fermi Surfaces in NaxCoO2 by Disorder". United States. doi:10.1103/PhysRevLett.97.016404.
@article{osti_931273,
title = {Destruction of the Small Fermi Surfaces in NaxCoO2 by Disorder},
author = {Singh, David J and Kasinathan, Deepa},
abstractNote = {We show using density functional calculations that the small e0g 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.},
doi = {10.1103/PhysRevLett.97.016404},
journal = {Physical Review Letters},
number = 1,
volume = 97,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • We discuss the electronic structure of NaxCoO2 from the point of view of first principles electronic structure calculations. The band structure contains low spin Co ions, with average charge 5+x leading to a nearly full Co t2g manifold. The bands corresponding to this manifold are narrow and separated from the O 2p bands and from the eg bands, which are also narrow. There are two main sheets of Fermi surface, a large section derived from ag symmetry states, and small hole pockets. We find significant effects due to Na disorder on these small sections, with the result that they shouldmore » be localized. This is discussed in relation to recent photoemission experiments. For comparison, we present a virtual crystal band structure of beta-SrRh2O4. Like NaxCoO2 it shows a large crystal field gap between narrow t2g and eg manifolds, but because of its stoichiometry is a semiconductor rather than a high carrier density metal.« less
  • Cells using polyethylene oxide as a sodium ion conducting electrolyte, P2 phase Na[sub x]CoO[sub 2] as the positive electrode and either sodium or sodium/lead alloy as the negative electrode were assembled, discharged, and cycled. Na[sub x]CoO[sub 2] intercalates sodium over a range of x = 0.3--0.9, giving theoretical energy densities of 1,600 Wh/liter (for sodium) or 1,470 Wh/liter (for sodium/lead alloy). Cells could be discharged at rates up to 2.5 mA/cm[sup 2] corresponding to 25% depth of discharge and typically were discharged and charged at 0.5 mA/cm[sup 2] (100% depth of discharge) or approximately 1--2 C rate. Over one hundredmore » cycles to 60% utilization or more, and two hundred shallower cycles at this rate have been obtained in this laboratory. Experimental evidence suggests that the cathode is the limiting factor in determining cycle life and not the Na/PEO interface as previously thought. Estimates of practical energy and power densities based on the cell performance and the following configuration are presented: 30--45 w/o electroactive material in the positive electrode, a twofold excess of sodium, 10 [mu]m separators, and 5 [mu]m current collectors composed of metal coated plastic. On the basis of these calculations, practical power densities of 335 W/liter for continuous discharge at 0.5 mA/cm[sup 2] and up to 2.7 kW/liter for short periods of time should be attainable. This level of performance approaches or exceeds that seen for some lithium/polymer systems under consideration for electric vehicle applications, but with a lower anticipated cost.« less
  • The Raman spectra of the parent compound Na{sub x}CoO{sub 2} (x = 0.75) and the superconducting oxyhydrates Na{sub x}CoO{sub 2} {center_dot} yH{sub 2}O with different superconducting temperatures (T{sub c}) have been measured. Five Raman active phonons around 195 cm{sup -1} (E{sub 1g}), 482 cm{sup -1}, 522 cm{sup -1}, 616 cm{sup -1} (3E{sub 2g}), and 663 cm{sup -1} (A{sub 1g}) appear in all spectra. These spectra change systematically along with the intercalation of H{sub 2}O and superconducting properties. In particular, the Raman active phonons (A{sub 1g} and E{sub 1g}) involving the oxygen motions within the Co-O layers show up monotonous decreasemore » in frequency along with superconducting temperature T{sub c}. The fundamental properties and alternations of other active Raman phonons in the superconducting materials have also been discussed.« less
  • We report a systematic angle-resolved photoemission study on Na{sub x}CoO{sub 2} for a wide range of Na concentrations (0.3 {le} x {le} 0.72). In all the metallic samples at different x, we observed (i) only a single holelike Fermi surface centered around {Gamma} and (ii) its area changes with x according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The e{prime}{sub g} band and the associated small Fermi surface pockets near the K points predicted by band calculations are found to sink below the Fermi energy in a manner almost independentmore » of the doping and temperature.« less