Anomalous negative longitudinal magnetoresistance in the topological insulating regime in Bi_1−xSb_x

Bi_1−xSb_x is a topological insulator (TI) for x ≈ 0.03–0.20. Close to the Topological phase transition at x = 0.03, a magnetic field induced Weyl semi-metal (WSM) state is stabilized due to the splitting of the Dirac cone into two Weyl cones of opposite chirality. A signature of the Weyl state is the observation of a Chiral anomaly [negative longitudinal magnetoresistance (LMR)] and a violation of the Ohm’s law (non-linear I−V). We report the unexpected discovery of Chiral anomaly-like features in the whole range (x = 0.032, 0.072, 0.16) of the TI state. This points to a field-induced WSM state in an extended x range and not just near the topological transition at x = 0.03. Surprisingly, the strongest Weyl phase is found at x = 0.16 with a non-saturating negative LMR much larger than observed for x = 0.03. The negative LMR vanishes rapidly with increasing angle between B and I. Additionally, non-linear I–V is found for x = 0.16 indicating a violation of Ohm’s law. This unexpected observation of a strong Weyl state in the whole TI regime in Bi_1−xSb_x points to a gap in our understanding of the detailed crystal and electronic structure evolution in this alloy system.