Magnetoconductance crossover and non-linear Hall effect in MBE-grown Cr-doped Bi$_{2}$Te$_{2}$Se*

Recent predictions have shown that the magnetoconductance (MC) of a topological insulator should show a crossover from negative MC to positive MC when time-reversal symmetry is broken [1]. Observations of this crossover have been reported in 3 QL thick Cr-Bi$_{2}$Se$_{3 }$[2] in the quasi-2D regime where surface hybridization is not negligible. We report magneto-transport results on 15 QL thick Cr-doped and undoped Bi$_{2}$Te$_{2}$Se films grown on Si (111) substrates by MBE. The undoped film exhibits weak anti-localization at low temperatures. In the Cr-doped film, a crossover from negative to positive MC is observed versus temperature at T=12 K. SQUID measurements show that the sample is ferromagnetic in this temperature range. In addition, a non-linear Hall voltage is observed despite the large bulk carrier concentration. This paves the way for further experiments on (Bi,Cr)$_{2}$Te$_{2}$Se in our search for the quantum anomalous Hall effect. [1] H.Z. Lu et al. Phys. Rev. Lett. \textbf{107}, 076801(2011). [2] M.Liu et al. arXiv 1103.3353(2011).