Anisotropically electrical property of Anderson insulator state in Bi2Ca2-xSrxCo2Oy (x=0.0, 0.5, 1.0, 1.5, 2.0) single crystals

Journal of Alloys and Compounds(2020)

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In this work, Bi2Ca2-xSrxCo2Oy (x = 0.0, 0.5, 1.0, 1.5, 2.0) crystals have been grown by the flux method, and their electrical properties were characterized. The analysis of temperature-dependent-resistance indicate that in-plane resistance ρab of Bi2Ca2-xSrxCo2Oy shows the Mott variable-range-hopping mechanism, while out-of-plane resistance ρc follows the thermally activated behavior, suggesting the Anderson insulator state in these crystals. The extracted localization length ξ of wave-function of all Bi2Ca2-xSrxCo2Oy samples is increased with more Sr-concentration. Quantitatively, ξ increases from 18.1 Å in Bi2Ca2Co2Oy to 29.6 Å in Bi2Sr2Co2Oy. The variable-range-hopping or thermally activated behavior is dependent on comparison between localization length ξ of samples and average distance between two neighboring defects along ab-plane or c-axis. The increased ξ from Bi2Ca2Co2Oy to Bi2Sr2Co2Oy is attributed to increased carrier concentration (equivalent to Fermi energy EF), which is also proved by X-ray photoemission spectroscopy. This work shows the delicate effect of isovalence-element replacement on the electrical conductivity of Bi2AE2Co2Oy systems, which may be instructive on how to manipulate electrical conductivity in the layered thermoelectric oxide compounds.
Misfit cobaltites,Crystal growth,Anisotropically electrical transport,Anderson localization
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