Collaborative DFG-Project

Superconductivity, Magnetism and
colossal thermopower in NaxCoO2 yH2O

Collaboration: L.Alff (TU Darmstadt), P. Lemmens (TU Braunschweig),
B. Keimer (MPI-FKF), H.-U. Habermeier (MPI-FKF)




Sodium cobaltate is a material with a large variety of interesting properties that originate in strong electron correlation effects. While its large Seebeck coefficient and low resistivity makes it a candidate for thermoelectric device applications, the layered sodium cobalt oxyhydrate NaxCoO2 · yH2O (x < 0.35, y < 1.3) has recently been shown to be superconducting with TC ≈ 4.5 K.
   The phase diagram of this superconductor with a triangular cobalt-oxygen lattice resembles that of the high-temperature superconductors (HTSC) with square copper-oxygen layers. The study of this new material has thus far been restricted to single crystals or polycrystalline ceramic materials.
   Only very recently, the first thin films of sodium cobaltate have been successfully prepared. The work proposed here will take advantage of the unique opportunities offered by the synthesis of high-quality thin films. The phase purity and surface quality of epitaxial thin films grown by pulsed laser deposition (PLD) are far superior to those of ceramic samples. High-quality films can be fabricated over a wider range of Na concentrations than single crystals. This will enable a comprehensive characterization of the optical properties by infrared and Raman scattering experiments over the entire phase diagram. Moreover, water-intercalated superconducting thin films have recently been prepared by the applicants, opening up the tantalizing possibility of carrying out phase-sensitive Josephson interference experiments to determine the pairing symmetry, as well as a search for pseudogap phenomena well known from HTSC.
   Altogether, these studies will provide deepened insight into a new superconducting system where coupled spin and charge dynamics are expected to play an essential role for superconductivity.

Further activities of the applicants on Cobaltates:

  • Intl. Workshop "Mobile Fermions and Bosons on frustrated lattices", ITFRUS07, 10.-13.1.2007 at MPI-PKS in Dresden with two sessions related to cobaltates.

  • Symposium TT13 on "Lamellar Cobaltates" within the German Physical Society (DPG) Meeting 2007 in Regensburg, 27.3.2007.



The lamellar cobaltates NaxCoO2 y H2O are a unique system with electronic correlations and competing interactions. As function of Na content and hydration different magnetic, charge ordered and superconducting phases appear.

A very large thermopower with application relevant figure of merit is observed. The effect of hydration is twofold: The crystal structure is elongated along the c axis and doping is induced by the formation of hydronium states H3O+.

Hydration is performed at ambient conditions leading to changes of resistivity. This compound is a rare example where local electronic correlations are modified by room temperature soft chemistry. This is based on the charge states of Co3+/Co4+ in distorted O octahedra.

Raman scattering is used to probe local charge correlations via the phonon frequency of an out-of-plane mode and electronic scattering.

The electronic scattering forms a continuum that shows a pseudo gap similar to observations in high temperature superconductors.

Presently we systematically investigate the properties of well characterized thin films and single crystals as function of doping in an in-situ doping setup.


Publications within the project:

1) Magnetic Properties of the Layered Cobaltite NdBaCo2O5.50, A. Jarry, H. Luetkens, Y.G. Pashkevich, M. Stingaciu, E. Pomjakushina, K. Conder, P. Lemmens, and H.-H. Klauss, Physica B, in print (2008).

2) Spin state and orbital moments across the metal-insulator-transition of REBaCo2O5.5 investigated by XMCD, M. Lafkioti,  E. Goering,  S. Gold, G. Schütz, S. N. Barilo, S.V. Shiryaev, G. L. Bychkov, V. Hinkov, P. Lemmens, J. Deisenhofer, A. Loidl,
New Journ. Phys. in print (2008).

3) Microscopic Evidence of Spin-State Order and Spin State Phase Separation in Layered Cobaltites RBaCo2O5,5 with R=Y, Tb, Dy, and Ho, H. Luetkens, M. Stingaciu, Yu. G. Pashkevich, K. Conder, E. Pomjakushina, A. A. Gusev, K. V. Lamonova, P. Lemmens, and H.-H. Klauss, Phys. Rev. Lett. 101, 017601 (2008).

4) Electron-phonon interaction in the lamellar cobaltate NaxCoO2, A. Donkov, M. M. Korshunov, I. Eremin, P. Lemmens, V. Gnezdilov, F. C. Chou, C. T. Lin,
Phys. Rev. B 77, 100504(R) (2008).

5) Oxygen Isotope Effect on the Magnetic Phase Transition Temperatures of the Layered Cobaltite HoBaCo2O5.47, H. Luetkens, M. Stingaciu, Y.G. Pashkevich, P. Lemmens, E. Pomjakushina, K. Conder, D. Cheptiakov, H.-H. Klauss, Journ. Magn. Mag. Mat. 310, 1566 (2007).

6) Comment on ”Raman spectroscopy study of NaxCoO2 and superconducting NaxCoO2 · yH2O”, P. Lemmens, P. Scheib, Y. Krockenberger, L. Alff, F. C. Chou, C. T. Lin, H.-U. Habermeier, and B. Keimer, Phys. Rev. B 75, 106501 (2007).

7) Sodium Cobaltates: Crystal Growth, Structure, Thermoelectricity, and Superconductivity,  C.T. Lin, D.P. Chen, A. Maljuk, and P. Lemmens,
Journ. of Cryst. Growth. 292, 422-428 (2006).

8) Anomalous electronic Raman scattering in NaxCoO2 ∙yH2O, P. Lemmens, K.-Y. Choi, V. Gnezdilov, E.Ya. Sherman, D.P. Chen, C.T. Lin, F.C. Chou, and B. Keimer,
Phys. Rev. Lett 96, 167204 (2006).



Impressum - Back to the Webpages of Peter Lemmens -  10.03.2007