R dependence of superconductivity and thermopower in the Bi₂Sr_2-xR_xCuO_y (R = La, Sm and Eu) system

We have grown single crystals of Bi₂Sr_2-xR_xCuO_y with R = La, Sm and Eu for a wide range of R content x and measured resistivity and the thermopower. The relation between the value of thermopower at 290 K, S(290), and the superconducting transition temperature T_c determined simultaneously on the same single crystal clearly shows that T_c at the optimal doping T_c^max strongly depends on the R element. More importantly, we found that the range of S(290) values of samples with a non-zero T_c is narrower when the R element is changed to one with a smaller ionic radius. The narrowing of the range of S(290) values indicates that the range of carrier density where high-T_c superconductivity occurs in this system depends on the R element. We argue that these behaviors are a consequence of the difference in the transfer integrals in the CuO₂ plane accompanied with the structural change caused by replacing the R element to one with a different ionic radius.