Variation ofTcand transport properties of the Co-dopedBi2Sr2Ca1−xYxCu2O8system

We report the effect of the substitution of the $3d$ transition metal element Co for Cu on the resistivity, Hall coefficient, and thermopower in the normal state of the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ system over a wide range of carrier concentration. The carrier concentration is varied by partial replacement of Ca by Y ions. The superconducting transition temperature ${(T}_c)$ is found to decrease considerably and a significant enhancement of the thermopower is observed with increasing Co doping. Both $T_c$ and thermopower show linear dependence on Co concentration. The temperature dependence of the thermopower for the underdoped samples shows a weak anomaly above the broad peak. This anomaly may be assigned to the opening of a normal-state gap in the density of states. A scaling behavior for the normal-state thermopower for optimum and underdoped samples has been established from our studies. We have also shown that the thermopower of all underdoped cuprates can be scaled into a single universal functional form.