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In-plane thermoelectric properties of heavily underdoped high-temperature superconductor Bi2Sr2CaCu2O8+δ

We have measured and analyzed the in-plane thermoelectric parameters from 25 to 210 K in single-crystal samples of Bi2(Sr, La)2CaCu2O8+δ and Bi2(Sr, Ca, La)3Cu2O8+δ, which are known as heavily underdoped high-temperature superconductor Bi2Sr2CaCu2O8+δ. Although the resistivity and thermopower are nearly the same as those of the thermoelectric layered cobalt oxides in this temperature range, the thermal conductivity is twice as large, and the dimensionless figure of merit ZT retains a small value of 0.025 at 210 K. A possible value upper limit for ZT in the layered copper oxides is discussed based on the present results.