Separation of quantum oscillations from bulk and topological surface states in metallic Bi$_2$Se$_{2.1}$Te$_{0.9}$*

Shubnikov-de Haas (SdH) oscillations in metallic Bi$_2$Se$_{2.1}$Te$_{0.9}$ are studied in magnetic fields up to 35 Tesla. It is demonstrated that two characteristic frequencies determine the quantum oscillations of the conductivity. Angle dependent measurements and calculations of the Berry phase show that the two frequencies $F_1$ and $F_2$ describe oscillations from surface and bulk carriers, respectively. At low magnetic fields, only SdH oscillation from topological surface states can be detected whereas at high magnetic field the bulk oscillations dominate. The origin of the separation of bulk and surface SdH oscillations into different magnetic field ranges is revealed in the difference of the cyclotron masses $m_c$. The bulk $m_c$ is nearly three times larger than the surface cyclotron mass resulting in a stronger attenuation of the bulk oscillation amplitude upon decreasing magnetic field. This makes it possible to detect and characterize the surface SdH oscillations in the low-field range.