Regensburg 2007 – wissenschaftliches Programm

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TT: Fachverband Tiefe Temperaturen

TT 13: Symposium “Superconductivity and Magnetism in Lamellar Cobaltates”

TT 13.2: Hauptvortrag

Dienstag, 27. März 2007, 14:30–15:00, H18

Phase diagram and in-plane spin fluctuation in the novel superconducting system Na_xCoO₂ * y H₂O — •Kazuyoshi Yoshimura — Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

Recent studies on the physical and chemical properties of the bilayer hydrated (BLH) triangular lattice superconductor Na_xCoO₂ * y H₂O have revealed the fact that the superconductivity would occur near the ferromagnetic electron correlations. We newly synthesized the non-superconducting and superconducting BLH Na_xCoO₂ * y H₂O compounds by the systematic sample preparation with time-controlling duration effect in high humidity atmosphere. As a result, we succeeded in constructing the electronic phase diagram as a function of the ⁵⁹Co NQR frequency, and found that superconducting phase exists very closely to the non-superconducting magnetic BLH phase in the vicinity of quantum critical point of magnetic correlations. These results strongly suggests that the superconductivity in BLH Na_xCoO₂ * y H₂O originates from the ferromagnetic fluctuation within the CoO₂ layer. This result is also supported by the results of 1/T₁ of ²³Na NMR which can be explained by a part of the A-type fluctuations observed in the mother compound Na_xCoO₂. We will give a review of the physical and chemical properties of the exotic and anisotropic superconducting BLH compound, Na_xCoO₂ * y H₂O.

This work was done as collaborations with Hiroto Ohta, Chishiro Michioka, Yutaka Itoh, Yoshihiko Ihara and Kenji Ishida from Kyoto University.