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Quantum Coherent Transport of 1D Ballistic States in Second-Order Topological Insulator Bi4Br4

Jules Lefeuvre1,*, Masaru Kobayashi2, Gilles Patriarche3, Nathaniel Findling3, David Troadec4, Meydi Ferrier1, Sophie Guéron1, Hélène Bouchiat1, Takao Sasagawa2 et al.

Richard Deblock1,†

  • *Contact author: jules.lefeuvre@universite-paris-saclay.fr
  • †Contact author: richard.deblock@universite-paris-saclay.fr
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Phys. Rev. X 16, 011031 – Published 20 February, 2026

DOI: https://doi.org/10.1103/2bqq-t7ml

Abstract

We investigate quantum transport in micrometer-sized single crystals of Bi4Br4, a material predicted to be a second-order topological insulator. 1D topological states with long phase coherence times are revealed via the modulation of quantum interference with magnetic field and gate voltage. In particular, we demonstrate the existence of Aharonov-Bohm interference between 1D ballistic states several micrometers long, that we identify as phase-coherent hinge modes on neighboring step edges at the crystal surface. These Aharonov-Bohm oscillations are made possible by a disordered phase-coherent contact region, the existence of which is confirmed by scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy (STEM-EDX) of FIB lamellae. Their coherent nature modulates the transmission of the 1D edge states, leading to weak antilocalization and universal conductance fluctuations with surprisingly large characteristic fields and a strongly anisotropic behavior. These complementary experimental results provide a comprehensive, coherent description of quantum transport in Bi4Br4 and establish the material as a second-order topological insulator with topologically protected 1D ballistic states.

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Popular Summary

Ballistic conduction in one dimension is excessively rare in nature, yet it is a predicted feature of second-order topological insulators which host conducting channels along the physical hinges of 3D crystals. We investigated quantum transport in micrometer-size single crystals of Bi4Br4. Ballistic topological states with long phase coherence times are revealed by quantum interference modulated by magnetic field and gate voltage. This is confirmed by the absence of Hall resistance and the presence of highly nonlocal conduction. These results provide a comprehensive description of transport in Bi4Br4 and establish that its hinges act as perfectly protected one-dimensional wires. Our work demonstrates the potential of this material for low-power electronic devices and provides a robust platform for further exploring the unique properties of high-order topological phases.

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Phys. Rev. X 16, 011031– Published 20 February, 2026
  • Received 19 February 2025
  • Revised 20 June 2025
  • Accepted 28 October 2025
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