Thermoelectric Properties and Power Factor of YBa 2 Cu 3 O 6+x with Rare-earth Dopants
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Thermoelectric properties and power factor of YBa2 Cu3 O61x with rare-earth dopants T. Kawahara, S. Tamura, H. Inai, Y. Okamoto, and J. Morimoto Department of Materials Science and Engineering, National Defense Academy, Yokosuka 239-8686, Japan (Received 18 July 1998; accepted 28 September 1998)
Both the thermoelectric power and the resistivity of the oxygen deficient YBa2 Cu3 O61x samples were measured. The rare-earth-doped samples, such as Sm and Dy, whose magnetic moments in the 13 valence state are different, were studied. Then the power factor of the thermoelectric ability was calculated. The power factor of the Dy-doped samples is smaller than those of the other samples, especially at high temperature. This smallness of the power factor is the main reason why the Dy-doped samples have larger resistivity. We try to analyze the data by the theoretical expression under the variable range hopping conduction model. The expression of the thermoelectric power could be fit for the nondoped and Sm-doped samples, except at low temperature. In this situation, the thermoelectric power increases as temperature increases, and this temperature dependence is good for the thermoelectric materials at high temperature where the resistivity decreases with temperature increasing.
I. INTRODUCTION
Since the discovery of high temperature superconductors (HTSC),1 copper oxide HTSC has been applied to many fields of electronics. Copper oxide HTSC has anomalous electronic properties because this material is located near the Mott-Hubbard metal-insulator (M-I) phase boundary.2 In this system, the effects of their strong Coulomb interaction may play a quite important role. We could not analyze by using the standard band theory which is very useful for semiconductors. There are several research works related to the thermoelectric power in low temperature and high temperature in HTSC materials. The dopants of ferromagnetic impurities were also investigated where the magnetic impurities destroyed the superconducting properties. For example, Co-doped samples have high resistivities in YBa2 Cu3 O61x (YBCO)3 and the Zn doping suppresses the anomalous contribution of electrons to the various physical quantities in La22x Srx CuO4 (LSCO).4 At low temperature, thermoelectric power is understood by the fluctuation model5 in a magnetic field. In zero magnetic field, a small hump was reported which was caused by Van Hove singularity6,7 just above the transition temperature. On the other hand, at high temperature, the normal state spin gap was seen in thermoelectric power at HTSC of YBCO and LSCO,8 and of Bi-based materials.9 Recently, thermoelectric power has been controlled by the oxygen concentration in the oxide to design new materials. High Tc cuprites have unusual characteristics because they are strongly correlated materials and it is 1200
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 4, Apr 1999
Downloaded: 05 Apr 2015
difficult to use the band theory constructed on the w
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