Quantum transport properties of β-B i4 I4 near and well beyond the extreme quantum limit

We have investigated the magnetotransport properties of β-Bi4I4 bulk crystal, which was recently theoretically proposed and experimentally demonstrated to be a topological insulator. At low temperature T and magnetic field B, a series of Shubnikov-De Haas (SdH) oscillations is observed on the magnetoresistivity (MR). Detailed analysis reveals a light cyclotron mass of 0.1me, and the field angle dependence of MR reveals that the SdH oscillations originate from a convex Fermi surface. In the extreme quantum limit (EQL) region, there is a metal-insulator transition occurring soon after the EQL. We perform scaling analysis, and all the isotherms fall onto a universal scaling with a fitted critical exponent ζ≈6.5. The enormous value of critical exponent ζ implies this insulating quantum phase originated from strong electron-electron interactions in high fields. However, in the far end of EQL, both the longitudinal and Hall resistivity increase exponentially with B, and the temperature dependence of the MR reveals an energy gap induced by the high magnetic field, signifying a magnetic freeze-out effect. Our findings indicate that bulk β-Bi4I4 is an excellent candidate for a three-dimensional topological system for exploring EQL physics and relevant exotic quantum phases.