$π$ Spin Berry Phase in a Quantum-Spin-Hall-Insulator-Based Interferometer: Evidence for the Helical Spin Texture of the Edge States

Abstract: Quantum spin Hall insulator is characterized by the helical edge states, with the spin polarization of electron being locked to its direction of motion. Although the edge-state conduction has been observed, unambiguous evidence of the helical spin texture is still lacking. Here, we investigate the coherent edge-state transport in an interference loop pinched by two point contacts. Due to the helical character, the forward inter-edge scattering enforces a $\pi$ spin rotation. Two successive processes can only produce a nontrivial $2\pi$ or trivial $0$ spin rotation, which can be controlled by the Rashba spin-orbit coupling. The nontrivial spin rotation results in a geometric $\pi$ Berry phase, which can be detected by a $\pi$ phase shift of the conductance oscillation relative to the trivial case. Our results provide a smoking gun evidence for the helical spin texture of the edge states. Moreover, it also provides the opportunity to all-electrically explore the trajectory-dependent spin Berry phase in condensed matter.