Thermal and Electrical Properties of γ-Na_xCoO_2(0.70 ≤ x ≤ 0.78)(Cross-disciplinary Physics and Related Areas of Science and Technology)

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We have performed specific heat and electric resistivity measurements of Na_xCoO_2(x = 0.70-0.78). Two anomalies have been observed in the specific heat data for x = 0.78, corresponding to magnetic transitions at T_c = 22K and T_k ≃ 9K reported previously. In the electrical resistivity, a steep decrease at T_c and a bending-like variation at T_b, (= 120K for x = 0.78) have been observed. Moreover, we have investigated the x-dependence of these parameters in detail. The physical properties of this system are very sensitive to x, and the inconsistent results of previous reports can be explained by a small difference in x. Furthermore, for a higher x value, a phase separation into Na-rich and Na-poor domains occurs as we previously proposed, while for a lower x value, from characteristic behaviors of the specific heat and the electrical resistivity at the low-temperature region, the system is expected to be in the vicinity of the magnetic instability which virtually exists below x = 0.70.
社団法人日本物理学会の論文
2004-09-15