Scanning tunneling spectroscopic studies of Dirac fermions and impurity resonances in the surface-state of a strong topological insulator Bi$_{2}$Se$_{3}$

Scanning tunneling spectroscopic studies of MBE-grown Bi$_{2}$Se$_{3}$ epitaxial films on Si (111) revealed surface-state (SS) characteristics of Dirac fermions and signatures of strong impurity resonances. The impurity resonances in this three-dimensional strong topological insulator (3D-STI) occurred near the Dirac energy (E$_{D})$ and diverged as the Fermi level (E$_{F})$ approached E$_{D}$. They were also highly localized within a region of radius $\sim $ 0.2 nm, beyond which the SS spectra of the 3D-STI recovered quickly, suggesting robust topological protection against non-magnetic impurities. Similar spectral characteristics and separations between E$_{F}$ and E$_{D}$ were also observed in the MBE-grown Bi$_{2}$Se$_{3}$ films on CdS. For sufficiently thin samples, opening of an energy gap due to wave-function overlap between the surface and interface layers was observed. The Rashba-like spin-orbit splitting further gave rise to spin-preserving quasiparticle interferences. Finally, the effect of different impurities (e.g. Cr and Mn) on the SS spectra of Bi$_{2}$Se$_{3}$ as a function of magnetic field will be reported. This work was supported by FENA and DARPA.