Dirac-Surface-State-Modulated Bilinear Magnetoresistance in Topological Insulators*

Topological insulators (TIs) are a class of quantum materials in which the interior is insulating but electrons can travel through the two-dimensional (2D) conducting surfaces known as Dirac surface states. The Dirac surface states harbor a helical spin texture and thus provide a good platform for potential spintronic applications. Prior experiments have shown that the helical spin texture of the Dirac surface states can be resolved using bilinear magnetoresistance (BMR) measurements. The observation of BMR in previous studies was attributed to the presence of the hexagonally warped Fermi surface at the Dirac surface state in samples with heavy doping. Here, we fabricated a series of (Bi_1-xSb_x)₂Te₃ samples with different x and systematically studied their BMR. When the chemical potential is near the Dirac point and the warping effect of the Dirac surface state disappears, the BMR reaches a maximum rather than vanishes. This observation is further confirmed in a dual-gate-tuned device. Our work indicates that BMR has more complex origins than the warping effect of the Dirac surface state alone.