Also called Bi₄I₄, bismuth bromide chains
A crystal where the conducting channel is an edge of an edge. Ordinary topological insulators conduct on every surface; Bi4Br4 is a higher-order topological insulator, where the surfaces are gapped and current instead runs along the hinges where two surfaces meet – one-dimensional channels inside a three-dimensional crystal, built by stacking bismuth-halide chains. Because the stacking sets the topology, the related Bi4I4 can be pushed between different topological phases.
Three layers of Bi4Br4, seen along the chains. Each layer is turned by 180° against the one below and slides across the chains, so two layers make up the repeat and c leans away from the layers: 9.57 Å between layers. Cell from von Benda, Simon and Bauhofer, Zeitschrift für anorganische und allgemeine Chemie 438, 53 (1978); COD 1528272.
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A textbook higher-order topological insulator: the bulk is gapped, the surfaces are gapped, and the topology shows up as one-dimensional hinge modes protected by the crystal symmetry rather than by time reversal alone. Because the building block is a chain, the interchain and interlayer couplings – both weak and dispersion-bound – control the gap and the topological index, so predictions depend on getting the stacking and the van der Waals corrections right.
The evidence lives on the small facets: measure the side surface, not the easy cleave plane, and use spatially resolved photoemission. Report the stacking polytype, since it determines the phase, and check for stacking faults. In transport, the hinge contribution scales with perimeter rather than area, which is the cleanest way to separate it from the bulk.
Not a device material. Its interest is scientific: a demonstration that topology can confine conduction to a line inside a crystal, which is the geometry spintronics keeps asking for.
The newest items tagged Bi4Br4, from the news feed updated 5 Oct 2026.
Charge density wave (CDW) implies a periodic modulation of the charge density. Typically observed in metallic systems, CDWs arise from Fermi surface instabilities, resulting in the total or partial gapping of the Fermi surface. Here, we present experimental evidence for a CDW state emerging in a band insulator which…
Three-dimensional (3D) topological ferroelectric (FE) insulators, in which topological and FE orders naturally coexist, enable field-controlled spintronic devices. In this work, we predict a new structure of bismuth monohalides Bi4Br4 and Bi4I4, denoted γ phase, and demonstrate that it is an ideal 3D topological FE…
Three-dimensional (3D) topological ferroelectric (FE) insulators, in which topological and FE orders naturally coexist, enable field-controlled spintronic devices. In this work, we predict a new structure of bismuth monohalides Bi4Br4 and Bi4I4, denoted γ phase, and demonstrate that it is an ideal 3D topological FE…
The quasi-one-dimensional material Bi4I4 hosts two crystallographically similar polymorphs that realize distinct topological insulating phases separated by a first-order structural transition near room temperature. This transition occurs without a change in space group, arising instead from a subtle rearrangement of…
Topological insulators host exotic quantum phenomena such as the quantum spin Hall (QSH) effect, which enables dissipationless one-dimensional edge conduction. Realizing such states at room temperature and on a macroscopic scale is essential for energy-efficient electronics and quantum technologies, yet remains a…
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