New Way to Control Particle Motions on 2-D Materials

Researchers at MIT and other institutions have found a new phenomenon in the behavior of a kind of quasiparticles called plasmons as they move along tiny ribbons of two-dimensional materials such as graphene and TMDs (transition metal dichalcogenides), which have a hexagonal structure resembling chicken wire. The team found that these plasmons can be separated into two different streams moving in opposite directions at the edges of the ribbons, like traffic on a two-lane highway, without the need for strong magnetic fields or other exotic conditions.

The new research was carried out by MIT associate professor of mechanical engineering Nicholas X. Fang, recent PhD graduate from that department Anshuman Kumar, and four other researchers from the University of Wisconsin at Milwaukee, Hong Kong Polytechnic University, and the University of Minnesota. The work was reported in a paper in the journal Physical Review B.

Other groups had previously observed such separated flows, Fang says, but that previous work required the use of powerful magnetic fields. Instead, the new process relies largely on optical effects, he says, using beams of circularly polarized light.

The findings are based on exotic states of matter that can occur in two-dimensional materials that, unlike graphene, have a characteristic known as a bandgap, necessary for devices such as transistors or solar cells (and also in graphene that is modified to have a bandgap). These states of matter are based on quantum physics phenomena such as Berry curvature, which occur in configurations known as massive Dirac systems. Although such systems are a hot area of research these days, the researchers say this particular class of phenomena, involving surface electromagnetic properties known as surface plasmons, has been relatively unexplored until now.

Clustering in “valleys”

In the new work, the team showed that shining beams of circularly polarized light onto the graphene ribbons causes electrons in the material to cluster into two different “valleys” in the electronic band structure. The peculiar symmetry properties of this system gives rise to a phenomenon called Berry curvature, which can be thought of as an artificial magnetic field.

Under these conditions, these valleys correspond to motions of the plasmons — which are a kind of oscillation of electron density in the material — in opposite directions on the two edges of the material. The graphene ribbons are just 50 nanometers (billionths of a meter) in width.

This effective magnetic field can be measured by sending in a second polarized beam, whose transmission can then be detected so that the changes in its polarization give a direct measurement of the effects taking place in the surface plasmons.

“This is exciting,” Fang explains, because it opens up a whole new approach to both manipulating the electromagnetic behavior of such systems and measuring the results of these manipulations.

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