STM Study Of The Topological Insulators

Topological insulator is a new class of quantum material, which possesses an insulating gap in the bulk and gapless states on surface. These surface states are comprised of an odd number of massless Dirac cones in which crossing of two dispersion branches with opposite spins is fully protected by the time-reversal-symmetry at the Dirac point. Topological insulators may provide new route to generating novel phases and quasi-particles, and may find applications in spintronics and quantum computing.In 2009, a new family of topological insulators, including Bi₂Se₃, Bi₂Te₃ and Sb₂Te₃, was identified theoretically and experimentally. However, the most intriguing property of time-reversal-symmetry that protects topological surface states hasn't been confirmed. In this thesis, by growing atomically flat thin films with molecular beam epitaxy, we have systematically investigated the electronic properties and topological features of Bi₂Se₃ and Bi₂Te₃ by using scanning tunneling microscopy/spectroscopy (STM/STS). The main results are summarized as follows:1. Landau quantization of the topological surface states in Bi₂Se₃ was observed by STM/STS measurement under magnetic field. The occurrence of discrete Landau levels (LLs) and the suppression of LLs by surface impurities of Ag strongly support the 2D nature of the topological states. In particular, we discovered the zeroth Landau level, which is predicted to give rise to the half-quantized Hall effect for the topological surface states. The magnetic field dependence of LLs is consistent with the cone structure of the surface states. The formation of LLs also implies the high mobility of our film samples so that quantum Hall effect can be expected.2. By high energy resolution dI/dV mapping, we directly observed the standing waves on the surface of topological insulator Bi₂Te₃. The interference fringes are caused by the scattering of the topological states off Ag impurities and step edges on the Bi₂Te₃ (111) surface. By studying the voltage-dependent standing wave patterns, we determined the energy dispersion E(k), which confirms the Dirac cone structure of the topological states. We further show that, different from the conventional surface states, the backscattering of the topological states by nonmagnetic impurities is completely suppressed. The absence of backscattering is a distinct manifestation of the time-reversal-symmetry, which offers a direct proof of the topological nature of the surface states.3. We also studied the influence of magnetic impurities (Fe and V), which may break the time-reversal-symmetry, on topological states in Bi₂Se₃ and Bi₂Te₃. Adsorption of V atoms on Bi₂Te₃ leads to the Zeeman splitting, suggesting existence of magnetic anisotropy owing to the crystal field. We observed standing waves around the magnetic impurities, and their behavior is the same as those for the nonmagnetic impurities. The results provide useful information for understanding of topological surface states.