Electronics operate by exploiting the properties of the carriers of electric charge in materials, which are due to the detailed interactions between electrons and the crystal lattice formed by atoms. Conventional semiconductor electronics use the flow of electric charge, and spintronics are based on the spins of the charge carriers. Another possible type of electronics, known as valleytronics, involves channeling the charge carriers into "valleys" of set momentum in a controlled way.

Two papers in Nature Nanotechnology describe the control of valleytronics using light. Both experiments involve the semiconductor molybdenum disulphide (MoS 2 ), whose crystal structure creates two momentum valleys that are not quite symmetric, in a similar way to the asymmetry between the right and left hands. By using polarized light to manipulate the charge carriers, the researchers nudged carriers preferentially into one valley. These results point the way to possible new devices that exploit the interaction between light and the spin of the charge carriers, such as fast optical switches.

Just as the atoms in a solid dictate the structure of the crystal lattice, both electrons interacting with each other and the atoms dictate the conduction properties of the material. These properties are the basis for the band structure. The conduction band is the set of energy and momentum values a charge carrier can have that lets it move freely through the material. Metals have many electrons in the conduction band while semiconductors have electrons in the conduction band only under certain conditions, such as an external voltage source.

The charge carriers can be electrons (as is the case with metals), but in many materials, they are quasiparticles, particle-like excitations created by the interactions. In the case of MoS 2 (as with many other semiconductors), the charge carriers are holes, places where an electron is absent. These holes have mass, positive electric charge, and spin, which determine how they interact and flow through the material.

The lattice structure of MoS 2 looks like a hexagon from above, but not all the atoms lie in the same plane. This means the lattice has two pieces, which are mirror images of each other. Each involves two sulfur atoms and one molybdenum atom. Each piece of the lattice contributes to the conduction band, and they both contain a "valley" in the energy spectrum. Valleys of this kind can trap holes, although note the trapping is not into a specific place, but into a preferred momentum—meaning they channel the flow of charge in a particular way.

This isn't so unusual yet. Graphite (sheets of carbon atoms in a hexagonal pattern) also has this kind of structure. (Molybdenum disulphide, like graphite, is used as a lubricant, since in both cases the layers of the hexagonal sheets are not bound tightly together.) However, while the two valleys corresponding to the two sublattices in MoS 2 have the same energy, they correspond to different momentum values. Additionally, they interact with the spins of the holes in opposite ways. Under ordinary circumstances, both valleys are equally populated.

The two research teams directed polarized light onto a single layer of MoS 2 . Polarization is akin to the spin of charged particles, so the polarized photons interacted with the spin of the holes, pushing them preferentially into one of the two valleys. The effect is known as valley polarization. They managed to keep the holes in one valley for more than a nanosecond, which is stable enough for many applications, as well as being a proof-of-principle experiment for later work. Trapping holes in a valley in this way allows for transport of charge (the usual electric current) and spin simultaneously. The effect vanished entirely when the researchers used two layers of MoS 2 , since second layer overlaps the first in such a way to eliminate the left-hand/right-hand asymmetry present in the single layer.

Valleytronics is a very new field: the first paper laying out the principles dates to 2002, with a scattering of research in the following years. (There isn't even a Wikipedia article on the subject as of right now, which tells you how esoteric it still is). As a result, proving valley polarization to be workable is the main challenge, and applications are still in the hazy future. However, these new results certainly show valleytronics is a real subject for study, and the use of the relatively common material MoS 2 means practical devices may eventually become a reality.

Nature Nanotechnology, 2012. DOI: 10.1038/nnano.2012.95 (About DOIs).

Nature Nanotechnology, 2012. DOI: 10.1038/nnano.2012.96.