Physicists from the Laboratoire de Physique des Solides (CNRS/Université Paris-Saclay) have revealed coherent, ballistic one-dimensional electronic states along the edges of the topological crystal Bi₄Br₄, as evidenced by quantum interference over several micrometers.
Topological insulators are fascinating materials: their bulk is insulating, but their edges conduct electricity in a way protected by fundamental quantum properties. A new class, higher-order topological insulators, takes this concept even further: in these systems, conduction is confined not to surfaces, but to the crystal edges, in the form of one-dimensional electronic states.
In this work, researchers studied the material Bi₄Br₄, a promising candidate for this type of topological phase. By measuring electrical transport in small exfoliated crystals, they revealed ballistic and coherent electronic states over several micrometers. In other words, electrons can travel without scattering over remarkably long distances.
A key result is the observation of Aharonov-Bohm-type quantum interferences, a direct signature of the electrons’ phase coherence. These interferences reveal that the conducting states propagate along the crystal edges and can interact with each other, forming quantum loops sensitive to magnetic fields.
Surprisingly, the electrical contacts, though imperfect, play an essential role: they introduce a disordered yet coherent region that allows these interference effects to be revealed. This situation highlights a rich physics, combining one-dimensional ballistic transport and mesoscopic quantum phenomena.
These results clearly establish Bi₄Br₄ as a higher-order topological insulator and open perspectives for the use of these robust states in quantum devices, particularly in dissipationless electronics or quantum computing.
Contributors:
Jules Lefeuvre, Masaru Kobayashi, Gilles Patriarche, Nathaniel Findling, David Troadec, Meydi Ferrier, Sophie Guéron, Hélène Bouchiat, Takao Sasagawa, Richard Deblock
Affiliations:
Laboratoire de Physique des Solides (CNRS / Université Paris-Saclay)
Centre de Nanosciences et de Nanotechnologies (CNRS / Université Paris-Saclay)
Université de Lille – IEMN
Institute of Science Tokyo
Funding:
JSPS KAKENHI, JST CREST, ERC (BALLISTOP), Renatech network, CPER IMITECH, Métropole Européenne de Lille, European Union
References
J. Lefeuvre et al., Quantum Coherent Transport of 1D Ballistic States in Second-Order Topological Insulator Bi₄Br₄, Physical Review X (2026)
HAL: https://hal.science/hal-04970280