An international group of physicists has demonstrated an electron spin-splitting effect in a semiconductor that is far larger than has ever been seen before. The large Rashba effect – the phenomenon of spin splitting with an applied electric field instead of a magnetic field – could herald the room-temperature operation of spintronic devices.
Spintronics is expected to be one of the next revolutions in computing. The idea is to fabricate devices that operate using not just an electron’s charge, but also its spin. Because the spin of an electron can be switched more quickly than charge can be moved round, these spintronic devices should operate faster and at lower temperatures than their electronic counterparts.
Electron spins are tiny magnetic moments, so to manipulate them a magnetic field is needed. As magnetic fields are difficult to control on the small scales typical in computing, physicists tend to exploit the so-called spin–orbit interaction. In this phenomenon, an electron moving in an electric field “sees” a magnetic field, which interacts with the electron’s spin.
Towards a room-temperature Rashba effect
In an external electric field, this leads to the so-called Rashba effect – a splitting of the spin-up and spin-down states in energy and momentum that is crucial for proposed spintronic devices. In the design for spin transistors, for example, electrons of a single spin are injected and then – under an applied electric field – have their spins rotated. But the Rashba effect in well-established semiconductors, such as silicon and gallium arsenide, is so small that electrons have to travel large distances – perhaps several microns – before any spin rotation is noticeable. Such distances require ultrapure materials and low temperatures to ensure that the electrons are not knocked off course.
Now, Phil King and colleagues at the University of St Andrews in the UK, together with other researchers in Europe and China, have come up with a material that could make the Rashba effect, and spintronics in general, feasible at room temperature. Bismuth selenide – the researchers’ material of choice – is unusual in that its inner bulk structure behaves as a semiconductor while its surface behaves as a metal. Such materials, known as topological insulators, have been around for decades but it is only in recent years that their unique behaviours have been discovered.