Magnetotransport Studies on Topological Insulators

The present work describes the magnetotransport studies of topological-surface state (SS) in metallic topological compounds. We have chosen three different classes of topological insulators, Bi2Se2.1Te0.9 (BST), Bi2Te3 (BT), and Sb2Te2Se (STS) for our study. The BST sample shows metallic behavior and has p-type bulk charge carriers. From the angle dependence of quantum oscillations and our Berry phase calculations, we have proved the existence of topological SS in the metallic BST sample. Based on the frequency analyses at high field, up to 35 T, the SS quantum oscillations dominate at low magnetic field and the surface to bulk state cross-over of oscillations takes place at higher magnetic field. The physical origin of the SS in the BST metallic sample can be understood as the low position of the Fermi energy measured from the Dirac point; however, it still cuts the two valence band maxima in the band structure explaining the bulk metallic property. We have found the existence of weak antilocalization (WAL) in the three BT metallic single crystals with different bulk charge carriers. From the angle dependence of the WAL, we found that the topological SS dominates in the samples having lower bulk carriers. From our Hikami-Larkin-Nagaoka analyses, we have found a larger number of conduction channels and a smaller phase coherence length in the samples having more bulk carriers as compared to those having less bulk carriers. Similar magnetoresistance measurements have been carried out in the p-type metallic STS sample in high- field up to 31 T. The angle dependence of quantum oscillations and the Berry phase calculations also show the dominance of topological SS. The physical origin of topological SS in the STS sample can be understood the same way as in the BTS sample.