Experimental demonstration of the topological surface states protected by the time-reversal symmetry

In this work, we use a low temperature scanning tunneling microscope (STM) to observe the standing waves formed by the nontrivial surface states of topological insulator Bi$_{2}$Te$_{3}$. Molecular beam epitaxy was used to grow Bi$_{2}$Te$_{3}$(111) film on Si(111) substrate. The interference fringes are caused by the scattering of the topological states off Ag impurities and step edges on the Bi$_{2}$Te$_{3}$(111) surface. By studying the voltage-dependent standing wave patterns, we determine the energy dispersion E(k), which confirms the Dirac cone structure of the topological states. We show that the backscattering of the topological states by nonmagnetic impurities is completely suppressed, which is very different from the conventional surface states. The absence of backscattering is a spectacular manifestation of the time-reversal symmetry, which offers a direct proof of the topological nature of the surface states.