All That Matters

Last week I blogged about the potential of using the magnetic properties of an electron, its spin, for novel electronics. And already this week we have come a step further towards spin electronics through the demonstration of a spin-based transistor device!

In spin electronics, it is the spin of the electron and not its electrical charge that could be used for computing. Indeed, this could be done entirely without electrical currents, and would be more energy efficient as it is easier to switch a spin than carry an electric current. In such devices, the spin can assume two orientations, which can be used to represent the 1 and 0 of computer bits.

The fundamental unit of a computer is the transistor. So what about the equivalent for spin electronics, the spin transistor? Well, the concept of a transistor that only switches an electron’s spin instead of charge was proposed 20 years ago by Supriyo Datta and Biswajit Das, but was never realized. The problem has been to control the spin of an electron in a clear and efficient way by electrical voltages while it is in transit through a nanoscale device.

The Spin Hall effect transistor. (c) Science 330, 1801 (2010)

Work by Jörg Wunderlich from the Hitachi Laboratory in Cambridge, Tomas Jungwirth from the Institute of Physics in Prague and the University of Nottingham in the UK, and their colleagues now published in Science comes the closest yet to the Datta-Das spin transistor: they present a spin Hall effect transistor.

Unlike the Datta-Das transistor, which basically is the concept of the conventional transistor transferred to spin electronics, the spin Hall effect transistor is a little more elaborate. The researchers excite electrons with a predefined spin (yellow cylinder in the figure). As the electrons travel from there to the other end of the device they scatter and get diverted either to the left or the right, depending on the direction their spin is pointing at. If the electrons all have spins pointing in the same direction, as in the experiment, they all get deflected in the same direction. This creates a Hall voltage along a crossbar (R_H in the figure), even though no electric current flows in this device.

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Go to any condensed-matter physics meeting, and topological insulators are prominent on the agenda, and talks on the topic attract large audiences. For good reason. Topological insulators promise nothing less than a revolution in electronics. Even though as their name suggests topological insulators are electrically insulating, they are conducting on their surface. And unlike regular conductors, these surface currents flow without the electrons being thrown off the track by most (albeit not all) scattering effects from impurities. This is one of their key features that ultimately may lead to smaller and faster electronic devices.

Even though first experimental breakthroughs have been achieved since 2006 in two-dimensional (thin films) of HgTe with similar properties, the tell-tale surface currents haven’t been observed in three-dimensional topological insulators such as the widely studied Bi₂Se₃ and Bi₂Te₃. So far, samples have not reached a sufficient purity and researchers had to make do with indirect characterisation experiments rather than direct measurements of electrical transport. This has now changed. In a study published in today’s issue of Science, Robert Cava, Nai Phuan Ong and colleagues from Princeton University report on the first experiments demonstrating electron conduction on the surface of Bi₂Te₃.

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