Low Temperature Transport Properties of Bi$_{2-x}$Tl$_{x}$Te$_3$ Single Crystals

We show that Tl-doping progressively changes the electrical conduction of Bi$_{2-x}$Tl$_{x}$Te$_3$ ($x=$0$-$0.30) single crystals from $p$-type (0$\le x\le $0.08) to $n$-type (0.12$\le x\le $0.30), which is observed via measurements of both the Seebeck coefficient and the Hall effect performed in the crystallographic \textit{ab}-plane in the temperature range of 2K-300K. The temperature dependent electrical resistivity in the \textit{ab}-plane of Bi$_{2-x}$Tl$_{x}$Te$_3$ maintains its metallic character with the decreasing hole density at low doping levels of 0$\le x\le $0.05. Heavier Tl-doping with 0.08$\le x\le $0.12 drives the electrical resistivity into a prominent non-metallic regime, associated with characteristic metal-insulator-metal transitions upon cooling down from 200K. For even more Tl-doped samples, 0.20$\le x\le $0.30, the system reverts back into the metallic state. Thermal conductivity measurements of Bi$_{2-x}$Tl$_{x}$Te$_3$ single crystals reveal a progressively stronger point defect scattering of phonon with the increasing Tl content. The systematic evolution of transport properties suggests that the Fermi level of Bi$_2$Te$_3$ which initially lies in the valence band (for $x=$0), is gradually shifted, with increasing Tl-doping, toward the top of the valence band (for 0.01$\le x\le $0.05), then into the band gap (for 0.08$\le x\le $0.10), and eventually into the conduction band (for 0.20$\le x\le $0.30).