Physicists in the US have proposed a way to make an “electron cloak” – an object that is invisible to electrons. Inspired by cloaks that hide objects from light or sound waves, the electron cloak would be made of a tiny structure that is about the same size as the wavelength of electrons it is hiding from. Although the design has not yet been tested in the lab, it could be used to make novel electronic devices and perhaps even help develop better thermoelectric materials for improved energy harvesting and conversion.
Researchers have already succeeded in making “invisibility cloaks” that hide objects from electromagnetic waves. Such cloaks are made from “metamaterials”, which are artificial structures with special optical properties such as negative indices of refraction. These structures are arranged in such a way that incoming waves flow smoothly around the cloak, meeting up on the other side as if the cloak was not there. The same principle has also been applied to make cloaks that are invisible to sound waves.
Core and shell
Thanks to quantum mechanics, electrons behave like waves, and now new calculations by Gang Chen and colleagues at the Massachusetts Institute of Technology (MIT) suggest that cloaks for electrons could be made. The researchers have put forward a practical design that would be made of nanoparticles that comprise an inner core and an outer shell. The core–shell nanoparticle could then be embedded in a host semiconductor, so that it does not disturb the flow of electrons.
Electrons normally travel as waves over a certain distance before scattering destroys their wave phases. Over this so-called coherent transport length, the particles exhibit characteristic wave behaviour, such as amplitude superposition (or interference).
Reflecting electron waves
“In our electron-cloak design, the core–shell nanoparticles essentially provide multiple interfaces where electron waves are reflected,” explains team member Bolin Liao. “Through careful tuning of the interfaces, the multiple reflected waves from the interfaces can destructively interfere with each other and cancel the total reflection almost perfectly. The electron waves with the ‘correct energy’ can thus travel through the nanoparticle structure without being reflected, as if there was nothing in their way.” The nanoparticle structures are about the same size as the wavelength of electrons themselves – around 10 nm in the MIT study.
Such electron cloaks may find use in applications where high electron mobility is required, such as in semiconductor electronics, says Chen. “We might also be able to design novel electronic switches that go from the visible (‘open’ structure) and invisible (‘closed’) states,” he says. “What is more, the electron scattering versus energy profile of the structures, which varies greatly, could benefit applications that call for strong energy-dependent scattering mechanisms, like those at work in thermoelectric devices.”
The team is now busy putting its theories into practice, by trying to make real core–shell nanoparticle electron cloaks. “We are also looking at extending our idea to lower-dimensional structures,” adds team member Mona Zebarjadi.
The current work is detailed in Physical Review Letters.