A new spin on silicon and graphene

At last year’s March Meeting in Denver, Ian Appelbaum gave a ten-minute talk about how he had injected spin-polarized electrons into a piece of silicon, transported them micrometres and then detected a spin-polarized current at the other end. It was just one of thousands of talks given that year.

But then Appelbaum published his results in Nature and this year he has been invited back to speak for 30 minutes — which he did today in a packed session that focused on spin injection in silicon.

The ultimate goal of Appelbaum’s research is to find practical ways to make “spintronic” devices, which in principle, could use the spin of the electron to process information much more efficient ways than coventional electronics.

To make a spintronic device, you need a material through which electrons can flow without losing their spin polarization — and it would be nice if that material was compatible with chip-making processes. Silicon fits the bill on both accounts, but is also has several drawbacks — it is difficult to inject spin-polarized electrons into the material; and once they are there it is difficult to measure their polarization.

Working at the University of Delaware Appelbaum’s team were the first to overcome these problems and you can find out how here.

I spoke with Appelbaum before his talk about how the fledgling field of silicon spin injection was shaping up. He described his breakthrough as a “clarification of the technologies that are needed”, and added that at least one more year of work by his team and others was needed before it would be possible to take a broader view of where the field was going.

Also speaking at the session was Berry Jonker of the Naval Research Lab. While Appelbaum detected spin polarization electrically, Jonker has worked out ways to detect it using light — something that is not usually possible thanks to silicon’s poor optical properties. Jonker finished his talk by declaring “There is a bright future for silicon spintronics”.

The future could also be bright for spintronics based on pieces of graphene — which are tiny flakes of carbon just one atom thick. It turns out that graphene shares many of silicon’s spin-friendly properties including weak spin-orbit and hyperfine interactions.

Speaking at a session on graphene, Bart van Wees of the University of Groningen, described a similar experiment to Appelbaum’s — but with graphene as the conductor. The experiment revealed that graphene is a good conductor of spin — but nowhere as good as silicon. The Groningen team found that spin polarization decays after the electrons travelled about 2 micrometres — a tiny distance compared to silicon. Indeed, Appelbaum told me that he hopes to transmit spins through a centimetre of silicon by the end of the year.

Van Wees described this shortcoming as a “mystery”.

Graphene has earned a reputation as a “wonder material” thanks to its outstanding electrical, thermal and mechanical properties. It’s comforting to know that graphene has been beaten by humble silicon when it comes to spintronics — at least for now!