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Sounding off about valleytronics

Photograph of a valley in Glacier National Park in the US

Valley state: real-life landscapes can be as beautiful as their condensed-matter counterparts. (CC BY-SA BorisFromStockdale)

By Hamish Johnston

Condensed matter is a physicist’s paradise because of the seemingly endless number of ways that atoms can be rearranged to create systems with new and exciting behaviours. A great example of this is the emerging field of “valleytronics”, which is concerned with a property of electrons that emerges in some semiconductors and 2D materials such as graphene.

The eponymous valley is a local minimum in the conduction band of a solid that “traps” electrons into a specific momentum state. Things get interesting when a material has two valleys that result in two distinct momentum states. In some materials these states resemble the quantum-mechanical property of spin: an electron can be in one of two spin states (up or down) and it can also be in one of two momentum states. As a result, this property is sometimes referred to as valley pseudospin.

Just as spintronics seeks to develop devices in which information is stored and transported using the spin states of electrons, valleytronics is focused on using the pseudospin. In 2013, for example, Jan Isberg and colleagues at Uppsala University in Sweden invented a way of creating, transporting and detecting valley-polarized electrons in diamond.

Now, as if to illustrate my point about the richness of condensed-matter systems, physicists in China have unveiled a new acoustic metamaterial that has “valley vortex states”. Developed by Zhengyou Liu and colleagues at Wuhan University, the material is a sonic crystal, which has a band structure for sound waves that resembles that of a semiconductor. The team says that its vortex states could be useful for rotating tiny particles or creating tiny “micromotors”.

You can read all about this wonderful new material in “Valley vortex states in sonic crystals”, which is published in Physical Review Letters.

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