Thermoelectric power of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathit{y}}$

The effects of oxygen nonstoichiometry and substitution of composition elements on the thermoelectric power of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Y}}_{\mathit{x}}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$ were investigated. Experiments studying the Seebeck coefficient S for the ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$ sample sintered in air yielded negative values for S in the temperature range from 310 K down to ${\mathit{T}}_{\mathit{c}}$. However, the Seebeck coefficient for ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{0.8}$ ${\mathrm{Y}}_{0.2}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$ was negative around room temperature, then became positive around 270 K, and remained positive down to ${\mathit{T}}_{\mathit{c}}$. Also, the Seebeck coefficients for the ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$ sample reduced in Ar and for the ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{0.8}$ ${\mathrm{Y}}_{0.2}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$ sample sintered in air exhibited a similar temperature dependence. From these results, it was considered that the characteristic behavior of the Seebeck coefficient was controlled by the Cu valence. To explain these results, it was considered that the Fermi-surface geometry for the midgap states was two dimensional. When the Fermi level is located near the van Hove singularity point, the states both above and below the van Hove singularity point may contribute to S. This model agreed with Seebeck coefficient measurements. From the results on S for antiferromagnetic insulating ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{0.2}$ ${\mathrm{Y}}_{0.8}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$ and ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{YCu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$ sintered in air, it was considered that the electrical conduction of these compounds was by a variable-range-hopping mechanism.