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

We have synthesized single crystals of the misfit-layered cobalt oxide, [Bi_1.5Pb_0.5Sr₂O_4-δ][CoO ₂]_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 [Bi _1.74Sr₂O_4-δ]^RS[CoO ₂]_1.82 is suppressed by Pb substitution. The in-plane resistivity, thermopower, and Hall coefficient are 4.3 mΩcm, 101 νVK^-1, and 1 × 10^-2cm³C^-1 at 300K, respectively; these are consistent with those of the misfit-layered cobalt oxides. All of these values are smaller than those of [Bi _1.74Sr₂O₄][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.