The Quantum Spin Hall Effect at Room Temperature in Stanene

Often in the physical sciences, the effects that pioneers in the field seem most excited about can take a while to percolate down to the public. The field of study can seem obscure and esoteric to many. There’s a story that, when the first flight by balloon took place in Paris on the eve of the French Revolution, Benjamin Franklin – one of the early scientists to study electricity – was asked “What good is this?” of the new innovation. He quipped in response: “What good is a newborn baby?” The newborn babies of the subsequent decades changed the world.

It’s with this in mind that you should consider the new darling of theoretical condensed matter physics – the quantum spin Hall effect. The original Hall effect refers to the production of a voltage across a conductor when a magnetic field is applied in a direction perpendicular to a current – in basic terms, the magnetic force deflects the current according to the Lorentz force law acting on the electrons, which results in a potential difference.

The quantum Hall effect can be observed in 2D electron systems – in low temperatures and strong magnetic fields, the conductance due to this perpendicular movement of electrons can take on new values. Due to the quantum nature of the states, the conductance itself takes on discrete values – it can jump between them in phase transitions. You can imagine the quantum hall effect as resulting in motion of the electrons along the edges of a semiconductor. Unfortunately, the requirement for extremely high magnetic fields and low temperatures precludes many applications for the QH effect.

The quantum spin hall effect is a similar – but also subtly different – state which can arise in certain materials which are often called topological insulators. What good is this newborn baby? Topological insulators are electrically insulating on the inside, but along a thin layer on the outside of the TI, they are perfect conductors. Amazingly, in these edge modes, the electrons are not scattered by impurities or defects in the material – resulting in something like superconductivity. Naturally, perfect transmission of charge carriers would be a very useful application for electronics and nanocircuitry, where any dissipation of energy can be the limiting factor that prevents circuits from getting smaller and having high conductivity.

The properties of electrons in 2D materials may well have seemed like an abstract concern – of interest to students and theoreticians, perhaps, but ultimately just an interesting consequence of the laws of physics without much practical utility. But in 2004, graphene was isolated and characterised at the University of Manchester, and it set off a scramble to synthesise and characterise a new class of 2D materials. Quantum-mechanical effects mean that many of them have fascinating and potentially useful electronic properties. Stanene – the analogue of graphene that consists of a lattice made of tin atoms, rather than carbon – is no exception.

Stanene was first theoretically predicted in 2011, when the materials scientists involved noted that it was likely to be a 2D topological insulator. As of 2018, several different research groups have been able to grow stanene on substrates using molecular beam epitaxy, where individual atoms of tin are gradually deposited onto a substrate at very high temperatures and low pressures. Crucially, stanenalphae may maintain its TI behaviour up to temperatures of 100C. This means that its superconducting edge states may remain superconducting well above room temperature, and into the operating range of circuitry. This is in contrast to bulk material superconductors, which as yet still require temperatures of over a hundred degrees below 0C to phase-transition into their superconducting states (even for HTSCs.).

The key to this property of stanene is its high spin-orbit coupling – unlike carbon atoms and graphene, where the spin-orbit coupling is comparatively small. This also gives rise to useful potential applications in spintronics. Traditional semiconductor electronics and computer chip design makes use of the electron charge to store and transfer information, and thus to perform calculations. Spintronics, in a quantum computer, would potentially manipulate the electron’s spin instead, enabling different types of calculations to be performed. The large spin-orbit coupling of tin atoms in stanene means that there is a significant gap in energy levels depending on the orientation of the electron’s spin, which is what renders it useful for these spintronic applications – as well as forming field effect transistors (FETs), which might be compatible with existing silicon devices and circuitry.

The edges of topological insulators could prove to be better conductors than any nanowires that can be fabricated out of copper or traditional conductors; breakthroughs in this field are likely to be necessary to allow Moore’s Law to continue down to the atomic scale, and the limits of conventional physics. Recently, multi-layer alpha-tin stanene has been synthesised – and a two-band superconductor was discovered, with a doubling of the transition temperature.

Professor Guy Le Lay of Aix-Marseille University has been at the forefront of the synthesis of 2D materials for a number of years; when stanene was first isolated on a silver substrate by an international collaboration, he displayed considerable enthusiasm for the potential applications of the room-temperature QSH effect.

The QSH effect is rather delicate, and most topological insulators only show it at low temperatures. However, stanene is predicted to adopt a QSH state even at room temperature and above, especially when functionalized with other elements. In the future, we hope to see stanene partnered up with silicene in computer circuitry. That combination could drastically speed up computational efficiency, even compared with the current cutting-edge technology.

Professor Guy Le Lay, Aix-Marseille University

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