Visualizing the evolution of surface localization in the topological state of Bi2Se3 by circular dichroism in laser-based angle-resolved photoemission spectroscopy

We use angle-resolved photoemission spectroscopy with a circularly polarized 7-eV laser, and find that the angle-resolved pattern of circular dichroism (CD) for the topological surface state of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ is dramatically changed with surface aging. Shortly after sample cleaving, the CD pattern exhibits a $\mathrm{sin}(\ensuremath{\phi}$)-like modulation ($\ensuremath{\phi}$ is the Fermi surface angle) reminiscent of the helical spin texture. Intriguingly, this pattern gradually changes with time, and eventually turns into a $\mathrm{sin}(2\ensuremath{\phi})$-like modulation on the aged surface. These two patterns are also revealed in quantum well states with different quantum numbers, indicating that the CD pattern is a valid measure to identify the surface confinement of electrons. Our experiments thus demonstrate the time evolution of electron localization in the topological surface state.