Metallicity and positive magnetoresistance induced by Pb substitution in a misfit cobaltate crystal.

We have synthesized single crystals of the misfit-layered cobalt oxide, [Bi1.5Pb0.5Sr2O4-d][CoO2](1.86), with quadruple NaCl-type layers, using a flux method, and measured their transport properties. From structural refinements, it is found that the modulation in the BiO layer observed in [Bi1.74Sr2O4-d](RS)[CoO2](1.82) is suppressed by Pb substitution. The in-plane resistivity, thermopower, and Hall coefficient are 4.3 m Omega cm, 101 mu V K-1, and 1 x 10(-2) cm(3) C-1 at 300 K, respectively; these are consistent with those of the misfit-layered cobalt oxides. All of these values are smaller than those of [Bi1.74Sr2O4][CoO2](1.82), indicating that the carrier concentration is larger than that of the undoped crystal. Moreover, the low T upturn of resistivity observed for most of the cobalt misfit oxides is replaced by a metallic behavior, following a T-2 dependence, indicating strong correlations in the Pb-doped crystal. Also, the magnetoresistance, usually negative for misfit oxides, is replaced by a small positive magnetoresistance. Doping with Pb is thus an efficient way to suppress the low T localized behavior usually observed for misfits.