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Ideal weak topological insulator and protected helical saddle points

Ji Seop Oh1,2, Tianyi Xu3, Nikhil Dhale3, Sheng Li3, Chao Lei4, Chiho Yoon3, Wenhao Liu3, Jianwei Huang2, Hanlin Wu3 et al.

Makoto Hashimoto5, Donghui Lu5, Chris Jozwiak6, Aaron Bostwick6, Eli Rotenberg6, Chun Ning Lau7, Bing Lv3,*, Fan Zhang3,†, Robert J. Birgeneau1,8,‡, and Ming Yi2,§

  • 1Department of Physics, University of California, Berkeley, California 94720, USA
  • 2Department of Physics and Astronomy, Rice University, Houston, Texas 77024, USA
  • 3Department of Physics, The University of Texas at Dallas, Richardson, Texas 75080, USA
  • 4Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA
  • 5Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
  • 6Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
  • 7Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA
  • 8Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

  • *Corresponding author: blv@utdallas.edu
  • †Corresponding author: Zhang@utdallas.edu
  • ‡Corresponding author: robertjb@berkeley.edu
  • §Corresponding author: mingyi@rice.edu

Phys. Rev. B 108, L201104 – Published 7 November, 2023

DOI: https://doi.org/10.1103/PhysRevB.108.L201104

Abstract

We report the design and observation of an ideal weak topological insulator in a quasi-one-dimensional bismuth halide, Bi4I1.2Br2.8. Via angle-resolved photoemission spectroscopy, we identify that Bi4I1.2Br2.8 hosts topological surface states on the (100)′ side surface in the form of two anisotropic π-offset Dirac cones while topologically dark on the (001) top surface. The results fully determine a unique side-surface Hamiltonian and thereby identify two pairs of nondegenerate helical saddle points. The fact that both the surface Dirac and helical saddle points are in the global bulk band gap of 195 meV suggests the potential of this system to be a fertile ground for topological many-body physics.

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