Spin-Momentum Locking Induced Anisotropic Magnetoresistance in Monolayer WTe₂

Monolayer WTe₂ is predicted to be a quantum spin Hall insulator (QSHI), and its quantized edge transport has recently been demonstrated. However, one of the essential properties of a QSHI, spin-momentum locking of the helical edge states, has yet to be experimentally validated. Here, we measure and observe gate-controlled anisotropic magnetoresistance (AMR) in monolayer WTe₂ devices. Electrically tuning the Fermi energy into the band gap, a large in-plane AMR is observed and the minimum of the in-plane AMR occurs when the applied magnetic field is perpendicular to the current direction. In line with the experimental observations, the theoretical predictions based on the band structure of monolayer WTe₂ demonstrate that the AMR effect originates from spin-momentum locking in the helical edge states of monolayer WTe₂. Our findings reveal that the spin quantization axis of the helical edge states in monolayer WTe₂ can be precisely determined from AMR measurements.