If you’ve had any contact with the concept of ‘digital devices’ (as in theory of, not the use of) you’ve heard it explained like ‘switches’ (i.e. gates) that are either ON or OFF, zeroes or ones – the binary code – that sort of thing. Information is stored or processed based on a sequence of such ‘switches’ for example as bits and bytes. Most of the computing we’re familiar with, the personal computer in particular, is based on ‘switches’ built with silicon semiconductors. These have served us very well, becoming ever more powerful and less expensive. But there are limits, and the manufacturing process is approaching them; so for the last decade or so the race has been on to develop new ways to perform digital operations.
One of those ways is the exploration of what is called spintronics. “Spin” in this case is a property of quantum mechanics. It’s about the rotation (angular momentum) of elementary particles (quarks for example), or the composite spin of elementary particles (such as for a proton or neutron). Experiments have discovered that this rotation has a property of uniform direction – spin up, or spin down. It’s not difficult to understand that up or down can be stand-ins for ON or OFF – the binary of digital coding.
The big difference is that in a spintronic device, once the direction of the spin is set, requires no energy (electrical power) to keep it that way. Spintronic devices are also faster than traditional semiconductor devices. The difficulty, no surprise, is how to ‘set’ the spin and in what material. This is where graphene comes in.
Graphene is a sheet of pure carbon one atom thick (making it two-dimensional) with the atoms arranged in a honeycomb pattern. Graphene has the distinction of being something relatively tangible (it can actually be made by ‘peeling’ ordinary graphite, as in a lead pencil, with scotch tape), yet it has some very exotic properties that scientists are hustling to exploit – for example, with spin.
Now normally graphene as a flat sheet has its particles (essentially in this case, electrons) spinning every-which-way (random). However, researchers at the Niels Bohr Institute, Nanoscience Center (University of Copenhagen, Denmark) collaborating with colleagues in Japan discovered that if graphene is shaped into a tube only a few nanometers in diameter (essentially a carbon nanotube with walls one atom thick) the spin of the electrons is strongly influenced by the motion of the electrons as they are forced to move around the nanotube. The electrons all move in one direction around the tube, and the spin synchronizes. The effect is robust, working on any number of electrons and even on graphene that has imperfections. The effect can be controlled, turned on and off, which makes this approach a candidate for using graphene in spintronic computer applications. The results were published in the journal Nature Physics [Gate-dependent spin–orbit coupling in multielectron carbon nanotubes].
A very different approach to controlling the spin of graphene particles was taken by researchers at the City University of Hong Kong and The University of Science and Technology (Hefei, China). The publication, in Applied Physics Letters [Spin current generation by adiabatic pumping in monolayer graphene] is described in no simple terms:
It involves using spin splitting in monolayer graphene generated by ferromagnetic proximity effect and adiabatic (a process that is slow compared to the speed of the electrons in the device) quantum pumping. They can control the degree of polarization of the spin current by varying the Fermi energy (the level in the distribution of electron energies in a solid at which a quantum state is equally likely to be occupied or empty), which they say is very important for meeting various application requirements.
Got that? In short (apologies for oversimplification), they use the magnetic properties of graphene (ferromagnetic) to induce the particles to split direction of movement (spin splitting), which sets up an adiabatic quantum pump, a bit of quantum mechanics that literally ‘pumps’ or forces the particles to spin in a specific direction.
Obviously this is a good deal more complicated than simply ‘rolling a graphene nanotube,’ but as these two approaches go further down the line with experimentation and especially as they gear up for real-world applications, it remains to be seen which is the more reliable, controllable, and cost effective. In either case, it’s a demonstration that graphene can be used to achieve spintronic effects. As the optimistic PR releases always say, this opens the path to many applications.
[Here’s a previous SciTechStory’s post: A First: Spintronics made visible]