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New types of electronic orderings detected in cobaltates

New types of electronic orderings detected in cobaltates

For a few years, many teams of the laboratory have been interested in the way electrons may move in triangular planes of cobalt (ref. abstract of J. Bobroff). Surprisingly, electrons do not always distribute homogeneously among the different Co sites, but may occupy preferentially particular sites. This was demonstrated by NMR in Na0.67CoO2 (Fig. 1a), where metallic electrons form a "kagomé" lattice, while others are localized [see H. Alloul et al., EPL 85, 47006 (2009)].


It was generally believed that this type of ordering only exists in Na cobaltates, where Na ions intercalated above or below the Co planes may trap electrons on the Co sites directly located at their vertical. We have shown that this phenomenon is more general and also takes place in the so-called "misfits" cobaltates, where there is no Na, but other planes, this time rectangular, intercalated between the Co planes. We have measured by Angle Resolved Photoemission (ARPES) the electronic structure of these compounds, i.e. the energies and velocities of electrons in different directions of the space. We have observed that this structure is affected by the periodicity (usually incommensurate) of these rectangular planes [1]. Moreover, above a certain value of the number of electrons in the Co planes, we observe that all electrons do not take part to the metallic structure. This value corresponds very well to that where electronic orders of the type sketched in Fig. 1a are observed in Na cobaltates. This suggests that tendency to charge ordering is intrinsic to Co planes and that the intercalated planes only dictate the structure of the observed order


Why are these electronic orders stable and how do they influence metallic properties of these phases ? ARPES also allows to quantify how strongly electrons interact with each others, which is a key point to understand the nature of these systems. We have analysed these effects in "misfits" cobaltates [2] and we have demonstrated that photoemission spectra (Fig. 1b) imply very strong correlations between these electrons and that these correlations increase with the number of electrons. This is a surprising result as theory would rather predict the opposite behavior. The nature of these strong correlations remains to be understood as well as if they are the origin or the consequences of the new types of electronic orderings observed in these compounds.

[1] New electronic orderings observed in cobaltates under the influence of misfit periodicities, A. Nicolaou, V. Brouet, M. Zacchigna, I. Vobornik, A. Tejeda, A. Taleb-Ibrahimi, P. Le Fèvre, F. Bertran, C. Chambon, S. Kubsky, S. Hébert, H. Muguerra and D. Grebille, Europhysics Letters 89, 37010 (2010)


[2] Experimental study of the incoherent spectral weight in the photoemission spectra of the misfit cobaltate [Bi2Ba2O4][CoO2]2, A. Nicolaou, V. Brouet, M. Zacchigna, I. Vobornik, A. Tejeda, A. Taleb-Ibrahimi, P. Le Fèvre, F. Bertran, S. Hébert, H. Muguerra and D. Grebille, Physical Review Letters 104, 056403 (2010)


Fig. 1:

  • (a) Structure of a Co triangular plane in Na0.67CoO2. The blue points correspond to metallic Co sites and the yellow and brown points to Co3+ sites, which have trapped one electron. From H. Alloul et al., EPL 85, 47006 (2009).
  • (b) Shape of the dispersion of the metallic band in one misfit cobaltate (see ref. [2]). Colors correspond to the number of electrons detected for one energy value (y axis) in one particular point of space (x-axis). The orange-red zones correspond to maxima. The way the electrons distribute as a function of these two parameters is characteristic of their properties (velocity etc.). The break of the dispersion in two separated parts at low energies (E<0.1eV) and higher energies is characteristic of strong electronic correlations.


Contact: Véronique Brouet