Quantum transport properties of β -Bi4I4 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 0.1 m_e , 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.