Demonstrating a weak topological insulator in bismuth iodide
Synchrotron techniques provide experimental evidence for the WTI state
Topological insulators are one of the most exciting discoveries of the 21st century. They can be simply described as materials that conduct electricity on their surface or edge, but are insulating in their interior bulk. Their conductive properties are based on spin, a quantum mechanical property, and this suppresses the normal scattering of electrons off impurities in the material, or other electrons, and the amount of energy that is consequently lost to heat. In contrast to superconductors, topological insulators can work at room temperature, offering the potential for our current electronics to be replaced with quantum computers and ‘spintronic’ devices that would be smaller, faster, more powerful and more energy efficient. Topological insulators are classified as ‘strong’ or ‘weak’, and experimental confirmations of the strong topological insulator (STI) rapidly followed theoretical predictions. However, the weak topological insulator (WTI) was harder to verify experimentally, as the topological state emerges on particular side surfaces, which are typically undetectable in real 3D crystals. In research recently published in Nature, a team of researchers from Japan used synchrotron techniques to provide experimental evidence for the WTI state in a bismuth iodide crystal.
Bismuth iodide and nano-ARPES
The quasi-one-dimensional (1D) bismuth iodide crystals α-Bi4I4 and β-Bi4I4 have very similar structures, differing only in their stacking sequences along the c-axis. This small difference in structure leads to a substantial difference in the resistivity of the two phases, in both absolute magnitude and temperature dependence. At room temperature first-order transitions occur between the two crystal phases, with the more resistive α-phase forming preferentially when the sample is slowly cooled.
The WTI state
The researchers obtained a microscopic intensity map for a tiny cleavage surface, using nARPES before angle-resolved measurements
Beamline I05 was well suited for this material. The nARPES setup allowed us to shine a light on part of a cleaved side surface, to solely investigate the surface electronic structure of the (100) plane
Mr Noguchi, lead author of the paper
They then observed a quasi-one-dimensional Dirac topological surface state at the side surface (the (100) plane), while the top surface (the (001) plane) is topologically dark with an absence of topological surface states. Their results visualised the WTI state realized in β-Bi4I4, and showed that a crystal transition from the β-phase to the α-phase drives a topological phase transition from a non-trivial WTI to a normal insulator at room temperature.
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Noguchi R et al. A weak topological insulator state in quasi-one-dimensional bismuth iodide. Nature 566:518–522 (2019). DOI:10.1038/s41586-019-0927-7.
An introduction to ARPES
Learn more about how Angle-Resolved Photo-Emission Spectroscopy (ARPES) At Diamond's I05 Beamline works: