Phys. Rev. Lett. 116, 093901 (2016)

Condensed-matter physicists are making advances that could allow for the information carrier of future electronic devices to be the electron's spin or valley index, rather than its charge. Although spin is an intrinsic property, valley refers to the dips in the electronic band structure that, when the electrons cannot easily scatter out, effectively introduce an additional degree of freedom. The valley index therefore emerges from the way that the electron waves travel through a crystal structure. Jiuyang Lu and colleagues have now shown how this concept can be extended to sound waves travelling through a sonic crystal.

Considering a hexagonal array of triangular steel rods immersed in water, Lu et al. predicted that the acoustic valley states would be vortex-like, having both chirality and orbital angular momentum. These states would be excitable using external sounds, but would adhere to selection rules, providing a method for selectively creating acoustic vortices with controllable chirality. Such systems may even exhibit acoustic analogues of Hall effects. So although scalar acoustics lacks an equivalent to spin, the principles of valleytronics provide additional tools for manipulating sound waves.