2-D Material Could Lead the Way to "Valleytronics"

Researchers demonstrate light can be used in place of magnetic fields to induce a shift in electrons

Illustration: MIT
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Earlier this year, we reported on researchers at the Massachusetts Institute of Technology (MIT) who had demonstrated that two-dimensional materials could be an alternative to diamonds in the esoteric world known as “valleytronics.”

Now, once again, researchers at MIT have shown that the 2-D material known as tungsten disulfide (WS2)—which belongs to a class of 2-D crystals known as transition metal dichalcogenides—could lead the way to valleytronics replacing conventional electronics.

Valleytronics essentially moves us away from the use of electrons’ electrical charge as a means for storing information to a scheme where we instead employ the wave quantum number of an electron in a crystalline material to encode data. The term valleytronics refers to the fact that if you plotted the energy of electrons relative to their momentum on a graph, the resulting curve would feature two deep valleys.

Manipulating these two valleys so that one is deeper than the other, would yield a way for the electrons to populate one of the two valleys. The positions into which electrons fall is a way to represent the zeroes and ones in digital computing.

But to get to the point where you could create stored information, you need to create a difference in the energies of the electrons populating the two valleys. The problem is that the electrons naturally want to settle into the lowest energy value, and they can achieve that in either of the two valleys.

You need to find some way to induce a difference in the energies of the two electron valleys. To date, the idea has been to achieve this change through the use of magnetic fields. However, to trigger that change you need a very powerful magnetic field, in the range of hundreds of tesla, to get even the most miniscule change. This limits the technique’s use to the lab.

“We discovered a way to directly control this valley by using light,” said Edbert Jarvis Sie, a MIT graduate student, in a press release.

The MIT researchers were able to achieve a much greater energy shift in the electrons by using a relatively conventional laser pulse with a special polarization.

“Being able to manipulate the valley degree of freedom in two-dimensional transition metal dichalcogenides would enable their application in the field of valleytronics,” said David Hsieh, an assistant professor of physics at Caltech, who was not connected to this research, in a press release. “This experiment makes a large step toward realizing this goal by demonstrating a method to control the energy difference between two valleys in tungsten disulfide for the first time.”

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