Observation of Chiral character deep in the topological insulating regime in Bi${}_{1-x}$Sb${}_x$

Bi${}_{1-x}$Sb${}_x$ is a topological insulator (TI) for $x\approx 0.03$–$0.20$. 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 longitudnal magnetoresistance (LMR)] and a violation of the Ohm’s law (non-linear $I-V$). We report the unexpected discovery of a Chiral anomaly 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 $x = 0.03$\mathrm{.}Surprisingly,thestrongestWeylphaseisfoundat$x = 0.16$ 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${}_{1-x}$Sb${}_x$ points to a gap in our understanding of the detailed electronic structure evolution in this alloy system.