Condensed Matter Physics Seminar

Superconductivity, Charge Order and Anomalous Magnetism in Na Cobaltates

Henri Alloul
Physique des Solides
Université Paris-Sud, Orsay, France
Thursday, May 24, 2007
Higgins 235, 3pm

In 2003, superconductivity at 4.5K has been discovered in the watered cobaltate Na0.35CoO·1.3H2O. This compound has some analogies with the cuprates as the conductivity occurs in the CoO2 planes and the electron doping can be modified by changing the Na content. Also, magnetic phases have been evidenced for some values of x. An originality of the cobaltates is the frustration linked with the triangular lattice of CoO2planes and the expected possibility to stabilize different Co charge states with low spin configurations Co³⁺, S=0 or Co⁴⁺, S=1/2, in the high crystal field of this compound. 

After a general introduction on these materials, we shall present SQUID, NMR [1] and µSR data [2] taken on samples synthesized and characterized by X ray crystallography in LLB, Saclay. The ²³Na and ⁵⁹Co NMR data allow us to evidence that for x=0.35, in the anhydrous and the hydrated SC phase the Co charge is uniform. For most higher x a Na atomic ordering is detected together with  a Co charge order. Although Co³⁺  sites are clearly identified by ⁵⁹Co NMR, except for x=0.5 [3], the data allows us to demonstrate that the other Co sites have a charge which is definitely lower than Co⁴⁺, near Co^3.5+ in all the range from  x=0.67 to x=0.75 [4]. Such a charge disproportionation, which is compatible with the metallicity of these systems leads to their original magnetic properties, which we have investigated in some detail with the powerful local possibilities offered by NMR techniques.

The diversity of electronic properties encountered for different x values demonstrates that cobaltates are new original illustrations of the richness of the physics of correlated elctron systems.

[1] I. Mukhamedchine, H. Alloul, G. Collin et N. Blanchard, Phys. Rev. Letters 93,167601 (2004),  94, 247602 (2005).
[2]  P. Mendels et al , Phys. Rev. Letters 94, 136403 (2005).
[3] J. Bobroff, G. Lang, H. Alloul, N. Blanchard and G. Collin, Phys. Rev. Letters 96, 107201 (2006) .
[4] I. Mukhamedchine, H. Alloul, G. Collin et N. Blanchard, condmat /0703561.