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Quantum Oscillations in a Topological Insulator Bi$_2$Te$_2$Se with Large Bulk Resistivity (6 $Ω$cm)

We report the observation of prominent Shubnikov-de Haas oscillations in a Topological Insulator, Bi$_2$Te$_2$Se, with large bulk resistivity (6 $Ω$cm at 4 K). By fitting the SdH oscillations, we infer a large metallicity parameter $k_F\ell$ = 41, with a surface mobility ($μ_s\sim$ 2,800 cm$^2$/Vs) much larger than the bulk mobility ($μ_b\sim$ 50 cm$^2$/Vs). The plot of the index fields $B_ν$ vs. filling factor $ν$ shows a $\frac12$-shift, consistent with massless, Dirac states. Evidence for fractional-filling states is seen in an 11-T field.