Materials from Na_xCoO₂ class, which structurally resemble layered copper oxides based high temperature superconductors, represent new class of materials, which for doping level 0.28<x<0.37 become superconducting [1]. The structure of the materials consists of Na and CoO₂ layers, where Co atoms create a triangular lattice. Transfer of an electron from Na atom to CoO₂ layer causes change of magnetic S=1/2 Co⁴⁺ ion in nonmagnetic Co³⁺ ion. Consequently, concentration of Na ions characterizes both electron doping level and magnetic dilution level. This unique situation combined with geometrical frustration of Co lattice is suggested to be responsible for appearance of new quantum phases [2], which have not been experimentally observed yet. In addition, the possibility of “charge separation” [3] or phase separation on mesoscopic level controlled only by Coulomb repulsion [4-6] is probable.    

[1] K. Takada, et al., Nature 422 (2003) 453.
[2] G. Misguich and C. Lhuillier, cond-mat/0310405.
[3] J. L. Gavilano, et al., Phys. Rev. B 69 (2004) 100404R.
[4] P. Carretta, M. Mariani, C. B. Azzoni, M. C. Mozzati, I. Bradaric, I. Savic, A. Feher, J. Šebek, Phys. Rev. B 70 (2004) 024409.
[5] T. Motohashi, et al., Phys. Rev. B 67 (2003) 064406.
[6] M. Vojta, E. Dagotto, Phys. Rev. B 59 (1999) R713.

A. Feher, A. Zorkovská
J. Šebek, E. Šantavá (Inst. of Phys. AS CR, Praha), I. Bradaric, I. Savic (Belgrade, Serbia and Montenegro), P. Carretta (Dipartimento di Fisica “A. Volta” e Unitá INFM di Pavia, Pavia, Italy)