Doping dependence of normal-state transport properties in La- and Pb-doped Bi2Sr2CuOy

We report measurements of the electrical resistivity, thermoelectric power, and Hall coefficient on La- and Pb-doped Bi2Sr2CuOy compounds as a function of temperature. Both insulating and overdoped nonsuperconducting-metal samples have been obtained. Analysis of the electrical resistivity in the insulating region suggests that the conduction is governed by a variable-range-hopping mechanism in the low-temperature region. As the system changes from a superconductor to an overdoped nonsuperconducting-metal, the resistivity undergoes a change from a linear temperature dependence to a power-law temperature dependence with exponent n∼1.5. This n∼1.5 behavior occurs over a wide temperature range. Remarkable changes associated with the insulator-superconductor- nonsuperconducting-metal transition are also observed both in the thermoelectric power and the Hall coefficient. A significant difference is that the thermoelectric power becomes negative at the higher doping level, while the Hall coefficient remains positive. We explain the experimental results from a two-carrier model by assuming that the Cu 3dx2-y2 electrons undergo a change from a localized state to a partially delocalized state with an increase in the number of dopant O 2p holes.