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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 3-D 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.
The quasi-one-dimensional (1-D) 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.
Research - Team - Photoemission - Spectroscopy - ARPES
The research team used laser-based angle-resolved photoemission spectroscopy (ARPES) measurements with high energy and momentum resolutions to determine the electronic structures of α-Bi4I4 and β-Bi4I4. They observed a superposition of the ARPES signals from the (001) and (100) planes in these experiments, because the...
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