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.

Continue reading…

Following this year’s Nobel prize in physics to Andre Geim and Konstantin Novoselov, the relevance of graphene hardly needs to be stated. Graphene-based devices have a real potential owing to the material’s unique electronic properties. If graphene, which is metallic, is cut into small pieces it becomes semiconducting and could be used as a transistor. The problem is however the edges of such small graphene devices. These perturb the operation of graphene transistors, and this is the reason one has to be cautious when it comes to immediate relevance for applications.

To figure out what exactly happens with atoms at the edges of graphene, Kazu Suenaga and Masanori Koshino from Japan’s National Institute of Advanced Industrial Science and Technology imaged and characterized the electronic properties of single atoms at the edge of graphene with a high-resolution scanning electron microscope. Their findings on how these atoms bond with each other are published this week in Nature.

When it comes to transistors that are smaller than anything that could be done with silicon, graphene is one of the materials of choice. As transistors shrink so much they consist of only a few atoms the electric currents that atomic bonds have to carry can become huge. Only a few materials can sustain this and graphene would be perfect for it. “The bonds between the carbon atoms in graphene are very strong and can carry exceptionally high currents,” Andre Geim told me once when researching a feature on future computing technologies. Moreover, electrons can travel through graphene for long distances, easily comparable to the distance between the source and drain electrical contacts of a transistor. “Your electrons would move between source and drain without scattering,” says Geim.

Characterising individual atoms of graphene. The atoms depicted in red, blue and green colours represent atoms at different positions on a graphene sheet. Their energy loss spectra mirror their different properties. Reprinted by permission from Nature doi: 10.1038/nature09664 (2010).

While all this is true for the centre of graphene sheets, the edges are a different matter. There, electrons scatter and all these nice benefits of graphene are diminished. And the smaller the transistors get the more edges there are in relation to the rest of the surface.

The atoms at the edges of graphene have of course been imaged many times before. What the researchers have now achieved is that they are also able to measure their energy absorption. This leaves a spectral fingerprint on how these atoms are bonded to their neighbours, depending on their position in the atomic structure. The identification of novel electronic states is one of the key findings of their study according to Suenaga. “No one else has ever seen the peaks we report in this work.”

Continue reading…