Metal--Insulator Transition in Ca-Doped Sr_14-xCa_xCu₂₄O₄₁ Systems Probed by Thermopower Measurements

High quality Sr_14-xCa_xCu₂₄O₄₁ single-crystals are successfully grown by floating-zone technique, and the transport properties are studied. The temperature dependence of resistivity along the c-axis direction is semiconductor-like for x≤10 and it can be fitted by the thermal activation equation ρ=ρ₀exp(Δ/k_BT) with k__B being the Boltzmann constant and Δ the activation energy. A break in the slope of thermopower (S) versus the inverse
temperature (1/T) corresponding to the formation of charge-density waves (CDW) is first observed for x≤6. The temperature dependence of thermopower becomes metallic for x≥8 while the resistivity is still semiconductor-like. We propose that the insulation behaviour of the resistivity in the Ca doping range 8≤ x≤11 could result from the localization of the charge carriers due to the disorder induced by Ca doping and a revised electronic phase diagram is derived based on our observations.