Electron waves refract negatively

Graphene subs for metamaterials to elicit backward bending

negative refraction graph

BENDING ELECTRONS  Light refracts, or bends, as it moves from one material to another (top left). It can bend more drastically when exposed to metamaterials (bottom left). Now researchers have achieved the same extreme bending with electrons (right), using graphene covered with gold to focus electrons to a point.

G.-H. Lee, G.-H. Park and H.-J. Lee/Nature Physics 2015; E. Otwell

Nearly a decade after getting waves of light to bend backward, physicists have done the same with electrons.

Electrons coursing through a sheet of carbon atoms exhibited negative refraction, bending at angles not seen in nature, physicists report September 14 in Nature Physics. By exploiting this unusual bending, the researchers created a lenslike device to focus the electrons to a tiny point. The new technique could help physicists learn how to manipulate electrons in the tight confines of miniaturized electronic devices, where the particles often behave like waves.

Waves bend via refraction when they cross the interface between two materials. Light moving from air to glass, for example, bends toward an imaginary line perpendicular to the glass surface but keeps moving in the same general direction.

Intricate engineered structures called metamaterials bend light differently, causing waves to take a sharp turn toward the direction from which they came. This negative refraction gives physicists unprecedented control over light. They can focus light with powerful lenses or steer it around objects by crafting invisibility cloaks (SN: 7/15/06, p. 42).

Electrons, though usually thought of as particles, can also be waves. Hu-Jong Lee, a condensed matter physicist at Pohang University of Science and Technology in South Korea, and colleagues bent electrons backward using ultrathin carbon sheets known as graphene. The researchers placed electrodes and a patch made of gold atop a graphene sheet sandwiched by pieces of boron nitride. Lee’s team observed that electrons spreading apart after streaming from one electrode suddenly bent toward each other once they entered the patched region of graphene, a clear signal of negative refraction.

Vladimir Fal’ko, a condensed matter theorist at Lancaster University in England who proposed this setup for negative refraction in 2007, says the feat is impressive because it doesn’t require the precise engineering required to build metamaterials for light. “What metamaterial engineers have to do by hand, the graphene lattice has already done for us,” he says.

While the negative refraction of electrons doesn’t lend itself to exciting applications like invisibility cloaks, Lee says it’s important to learn how to manipulate electrons when they’re acting like waves. Unlike particles, waves interfere with each other after scattering off walls or slipping through narrow gaps. Engineers will need to account for this behavior if they want to keep shrinking the sizes of transistors and other electronics components.

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