Transport properties of new Pb-based Topological Insulators*

A topological insulator (TI) has a gapped insulating bulk and a gapless metallic surface. So far, such materials as Bi$_{1-x}$Sb$_{x}$, Bi$_{2}$Se$_{3}$, and TlBiSe$_{2}$ are known to be TIs. Recently, several theoretical predictions have been made for new TI materials. In this work, we focus on the Pb-based ternary chalcogenides as new candidate TIs. We have grown a number of single crystals in the systems of Pb-Bi-Se, Pb-Bi-Te, Pb-Sb-Te and Pb-(Sb,Bi)-Te. After selecting single-phase samples, we measured the transport properties to check for their bulk-insulating nature. It was found that Pb(Sb$_{x}$Bi$_{1-x}$)$_{2}$Te$_{4}$ shows a change in the carrier type at around x= 0.55 as inferred both by the themopower and by the Hall effect, but the temperature dependences of the resistivity remained metallic in all the samples studied. We discuss the prospect of making a bulk-insulating material in the Pb-based TIs.