Ordering Phenomena in Transition Metal Oxides

Nesting and pseudo-gap formation in superconducting cobaltites   

The recently found superconductivity in the layered cobaltite Na_xCoO₂ yH₂O with a triangular lattice of Co³⁺/Co⁴⁺ sites has motivated enormous interest. In this system long range Neel ordering with T_N=20 K, a metal-insulator transition with T_MI=50K and superconductivity with T_c=4.6 K are observed as function of composition. There exists evidence for electronic correlations in transport experiments and for pronounced nesting of the hexagonal Fermi surface. Further complications arise due to order/disorder phenomena related to the partly occupied Na sites. Raman experiments on the cobaltites show a continuum of scattering related to electronic mass fluctuations that are enhanced by nesting. In superconducting single crystals this continuum is renormalized evidencing the formation of a gap for temperatures below T*=150 K. This effect is proposed to be due to electronic phase separation by charge ordering of the Co³⁺/Co⁴⁺ sites. Further weak phonon anomalies support this evidence for phase separation that is only weakly coupled to the lattice.

Work in collaboration with:
X. Zhang, D.P. Chen, C.T. Lin, B. Keimer, MPI-FKF, Stuttgart;
K.Y. Choi, 2. Physikalisches Inst., RWTH Aachen, Aachen;
V. Gnezdilov, B. I. Verkin Inst. for Low Temperature Physics, Kharkov;
H. Sakurai, E. Takayama-Muromachi, K. Takada, T. Sasaki, Superconducting Materials Center & Advanced Materials Lab., NIMS, Tsukuba;
F.C. Chou, Center for Materials Science and Engineering, MIT, Cambridge.

Work supported by DFG SPP1073, NATO PST.CLG.9777766 and INTAS 01-278.