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.
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 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.
To find out more about the I05 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Cephise Cacho: cephise.cacho@diamond.ac.uk.
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.
Learn more about how Angle-Resolved Photo-Emission Spectroscopy (ARPES) At Diamond's I05 Beamline works:
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
Copyright © 2022 Diamond Light Source
Diamond Light Source Ltd
Diamond House
Harwell Science & Innovation Campus
Didcot
Oxfordshire
OX11 0DE
Diamond Light Source® and the Diamond logo are registered trademarks of Diamond Light Source Ltd
Registered in England and Wales at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom. Company number: 4375679. VAT number: 287 461 957. Economic Operators Registration and Identification (EORI) number: GB287461957003.
Science