Quantum spin Hall insulators show a lot of promise for spintronic technology, which exploits the spin of electrons as well as their usual charge. They could form devices that run on very low power or contain super-fast processors, and they could even be fundamental ingredients in future quantum computers. Getting them to work often involves stabilising heavy elements on suitable substrates, but even when they do work, the elements are prone to degrade in the air outside a clean laboratory. Now, a team led by Ralph Claessen and Simon Moser at the University of Würzburg in Germany have employed photoemission spectroscopy at the Diamond I05 and I09 beamlines to show that a layer of graphene suspended above a quantum spin Hall insulator can protect it, without disturbing the precious electronic structure beneath. Published in Nature Communications, the result paves the way for practical applications of the technology.
Quantum spin Hall insulators are topological, meaning that properties of their bulk materials determine novel behaviour taking place along their edges, or surfaces. There are several different types of topological devices, but the special nature of quantum spin Hall insulators results from their being made very thin, so that they are effectively two-dimensional. When the materials are in this form, their electrons are too constrained to travel on the inside, but find themselves with a free pass on the outside – so long as those with a ‘spin up’ momentum zip along one edge, and those with a ‘spin down’ momentum zip in an opposite direction along the other. This split in current can form the basis of a computational device, but based on a novel concept– use spin instead of the electron's electrical charge – which is potentially very easy to manipulate.
One of the best aspects of topological materials is that their special conduction states are intrinsic to the architecture, so they are not disturbed by defects, temperature fluctuations or any of the other hazards that usually disturb electron flow. Still, the phenomenon can disappear if the material completely changes, for example if it oxidises in air. This is what can happen to heavy elements such as mercury, bismuth or indium, which are often best suited to being two-dimensional insulators.
Jonas Erhardt and Cedric Schmitt of the University of Würzburg, the first authors of the latest research say:
In the three-dimensional materials typically used for electronics this is rarely a problem. In that case, oxidation occurs only for a few surface layers and can even act as a protective shield for the metallic layers underneath. But when there’s only one layer in the first place, we can’t sacrifice it for protection.
Coating the layer with something else might seem like an obvious solution, but most coatings change the architecture, destroying the quantum spin Hall effect. The trick Erhardt, Schmitt and colleagues have found is to employ another atom-thick material – in this case graphene, which consists of a single layer of carbon atoms. (In fact, the researchers grow the graphene on a substrate first, before sandwiching or ‘intercalcating’ a monolayer of indium in between.) The graphene interacts only very weakly, via so-called van-der-Waals forces, and so keeps the topological electronic structure intact – as the researchers demonstrated by high-resolution angle-resolved photoemission spectroscopy (ARPES) at I05, and X-ray standing wave (XSW) photoemission spectroscopy at I09.
Some types of spintronics have already found practical applications. Modern hard disc drives employ a spintronic phenomenon known as giant magnetoresistance, in which the scattering, or resistance, of electrons is determined by their up or down spin. Other types of spintronics have failed to make headway, however, in part because the materials they use do not fare well in everyday circumstances.
The latest findings suggest a new way forwards. Erhardt and Schmitt say,
For the first time, we can explore the quantum material outside of its birthplace in an ultra-high vacuum. Our work paves the way for practical applications, with potential advances in spintronic transistors, memory devices and other low-power technologies that capitalize on the dissipationless, spin-locked edge currents unique to the quantum spin hall effect.
The catch is that graphene itself is metallic, and so the researchers suspect that further tests might reveal it short-circuiting the spin currents along the edges. For that reason, they are now planning to try the same technique of coating but with a different, non-metallic monolayer – boron nitride.
To find out more about the I05 and I09 beamlines please contact the Principal Beamline Scientists Cephise Cacho at cephise.cacho@diamond.ac.uk and Tien-Lin Lee at tien-lin.lee@diamond.ac.uk
Cedric Schmitt et al. Achieving environmental stability in an atomically thin quantum spin Hall insulator via graphene intercalation Nature Communications 15 (2024) 1486: DOI:10.1038/s41467-024-45816-9
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