Visualizing spin-vortex evolution of a topological insulator*

Charge carriers on the surface of a topological insulator are predicted to form a spin-vortex in momentum space with the direction of spin rotation determined by whether the carriers are electron-like or hole-like. We show that the angular momentum of photon is extremely sensitive to the spin of carriers by performing time-of-flight based angle-resolved-photoelectron spectroscopy (TOF-ARPES) with photons of different helicity. We demonstrate the first reciprocal space volumetric mapping of the vectorial spin-texture of the surface states of $Bi_2 Se_3$ and directly observe spin-vortex evolution from electron-like to hole-like states and the departure from perpendicular momentum-spin locking.