Scanning Tunnelling Spectroscopic Studies of Dirac Fermions in Graphene and Topological Insulators

¹ Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA
² Department of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA
³ Department of Physics and Astronomy, University of California, Riverside, CA 92521, USA
⁴ Department of Electrical Engineering, University of California, Los Angeles, CA 90095, USA
⁵ Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA

Abstract

We report novel properties derived from scanning tunnelling spectroscopic (STS) studies of Dirac fermions in graphene and the surface state (SS) of a strong topological insulator (STI), Bi₂Se₃. For mono-layer graphene grown on Cu by chemical vapour deposition (CVD), strain-induced scalar and gauge potentials are manifested by the charging effects and the tunnelling conductance peaks at quantized energies, respectively. Additionally, spontaneous time-reversal symmetry breaking is evidenced by the alternating anti-localization and localization spectra associated with the zero-mode of two sublattices while global time-reversal symmetry is preserved under the presence of pseudo-magnetic fields. For Bi₂Se₃ epitaxial films grown on Si(111) by molecular beam epitaxy (MBE), spatially localized unitary impurity resonances with sensitive dependence on the energy difference between the Fermi level and the Dirac point are observed for samples thicker than 6 quintuple layers (QL). These findings are characteristic of the SS of a STI and are direct manifestation of strong topological protection against impurities. For samples thinner than 6-QL, STS studies reveal the openup of an energy gap in the SS due to overlaps of wave functions between the surface and interface layers. Additionally, spin-preserving quasiparticle interference wave-vectors are observed, which are consistent with the Rashba-like spin-orbit splitting.