Discovery of switchable weak topological insulator state in quasi-one-dimensional bismuth iodide

Abstract

The major breakthroughs in the understanding of topological materials over the past decade were all triggered by the discovery of the Z${}_2$ topological insulator (TI). In three dimensions (3D), the TI is classified as either "strong" or "weak", and experimental confirmations of the strong topological insulator (STI) rapidly followed the theoretical predictions. In contrast, the weak topological insulator has so far eluded experimental verification, since the topological surface states emerge only on particular side surfaces which are typically undetectable in real 3D crystals. Here we provide experimental evidence for the WTI state in a bismuth iodide, $\beta$-Bi4I4. Significantly, the crystal has naturally cleavable top and side planes both stacked via van-der-Waals forces, which have long been desirable for the experimental realization of the WTI state. As a definitive signature of it, we find quasi-1D Dirac TSS at the side-surface (100) while the top-surface (001) is topologically dark. Furthermore, a crystal transition from the $\beta$- to $\alpha$-phase drives a topological phase transition from a nontrivial WTI to the trivial insulator around room temperature. This topological phase, viewed as quantum spin Hall (QSH) insulators stacked three-dimensionally, and excellent functionality with on/off switching will lay a foundation for new technology benefiting from highly directional spin-currents with large density protected against backscattering.

18 pages, 4 figures

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• Topological electronic structure and spin texture of quasi-one-dimensional higher-order topological insulator Bi4Br4
• Room-Temperature Topological Phase Transition in Quasi-One-Dimensional Material Bi${}_4$I${}_4$
• Experimental realization of two-dimensional weak topological insulators
• Transport response of topological hinge modes in $\alpha$-Bi${}_4$Br${}_4$
• Trapped state at a dislocation in a weak magnetomechanical topological insulator
• Accessing the spectral function of in operando devices by angle-resolved photoemission spectroscopy
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• Epitaxial Growth of Quasi-One-Dimensional Bismuth-Halide Chains with Topological Non-Trivial Edge States
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• Quasi-One-Dimensional Higher-Order Topological Insulators
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• Weak topological insulating phases of hard-core-bosons on the honeycomb lattice
• Controlling crystal cleavage in Focused Ion Beam shaped specimens for surface spectroscopy
• K${}_2$CoS${}_2$: A new two-dimensional in-plane antiferromagnetic insulator
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• Mode-Shell correspondence, a unifying phase space theory in topological physics -- part II: Higher-dimensional spectral invariants
• Observation of Dirac-like surface state bands on the top surface of BiSe
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• Intertwined topological phases in TaAs2 nanowires with giant magnetoresistance and quantum coherent surface transport
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• Phonons Drive the Topological Phase Transition in Quasi-One-Dimensional Bi${}_4$I${}_4$