Thermoelectric power for the overdoped superconductors (Yb,Ca)(Ba,Sr)2Cu3Oz

Abstract

We have made a measurement of Seebeck coefficients for overdoped superconductors, (${\mathrm{Yb}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$) (${\mathrm{Ba}}_{0.8}$ ${\mathrm{Sr}}_{0.2}$ ${)}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{z}}$. The hole concentration was controlled by cation substitution and/or oxygen nonstoichiometry. For the sample where x=0.3, as the oxygen content (z) increased, the magnitude of the Seebeck coefficient at room temperature (${\mathit{S}}_{300\mathrm{}\mathrm{K}}$) decreased from positive several tens of μV/K for z=6.10 down to minus tens of μV/K for z=6.78, passing through the zero value. For the sample where ${\mathit{S}}_{300}$ K∼0 μV/K, the ${\mathit{T}}_{\mathit{c}}$ was the highest. For the samples with a high oxygen content of z=6.83±0.06 and various Ca contents, ${\mathit{S}}_{300\mathrm{}\mathrm{K}}$’s were negative, and, as x increased, both ${\mathit{T}}_{\mathit{c}}$ and ${\mathit{S}}_{300\mathrm{}\mathrm{K}}$ decreased. This indicated that these samples were in the region of overdoped holes. Combining these results with those for the samples with various values of z, we could conclude then that ${\mathit{S}}_{300\mathrm{}\mathrm{K}}$ decreased monotonically with increasing hole concentration in the ${\mathrm{CuO}}_2$ plane, from positive values in the region of underdoped holes to negative values in the region of overdoped holes. This composition dependence of Seebeck coefficients may reflect the change in the electronic picture from the strongly correlated system to the weakly correlated Fermi-liquid metal.