When moving through a conductive material in an electric field, electrons tend to follow the path of least resistance—which runs in the direction of that field.
But now physicists at Massachusetts Institute of Technology (MIT) and the Univ. of Manchester have found an unexpectedly different behavior under very specialized conditions—one that might lead to new types of transistors and electronic circuits that could prove highly energy efficient.
They’ve found that when a sheet of graphene is placed atop another 2-D material, electrons instead move sideways, perpendicular to the electric field. This happens even without the influence of a magnetic field—the only other known way of inducing such a sideways flow.
What’s more, two separate streams of electrons would flow in opposite directions, both crosswise to the field, canceling out each other’s electrical charge to produce a “neutral, charge-less current,” explains Leonid Levitov, an MIT professor of physics and a senior author of a paper describing these findings in Science.
The exact angle of this current relative to the electric field can be precisely controlled, Levitov says. He compares it to a sailboat sailing perpendicular to the wind, its angle of motion controlled by adjusting the position of the sail.
Levitov and co-author Andre Geim at Manchester say this flow could be altered by applying a minute voltage on the gate, allowing the material to function as a transistor. Currents in these materials, being neutral, might not waste much of their energy as heat, as occurs in conventional semiconductors—potentially making the new materials a more efficient basis for computer chips.
“It is widely believed that new, unconventional approaches to information processing are key for the future of hardware,” Levitov says. “This belief has been the driving force behind a number of important recent developments, in particular spintronics.”
The MIT and Manchester researchers have demonstrated a simple transistor based on the new material, Levitov says.
“It is quite a fascinating effect, and it hits a very soft spot in our understanding of complex, so-called topological materials,” Geim says. “It is very rare to come across a phenomenon that bridges materials science, particle physics, relativity, and topology.”
In their experiments, Levitov, Geim and their colleagues overlaid the graphene on a layer of boron nitride—a 2-D material that forms a hexagonal lattice structure, as graphene does. Together, the two materials form a superlattice that behaves as a semiconductor.
This superlattice causes electrons to acquire an unexpected twist—which Levitov describes as “a built-in vorticity”—that changes their direction of motion, much as the spin of a ball can curve its trajectory.
Electrons in graphene behave like massless relativistic particles. The observed effect, however, has no known analog in particle physics, and extends our understanding of how the universe works, the researchers say.
Whether or not this effect can be harnessed to reduce the energy used by computer chips remains an open question, Levitov concedes. This is an early finding, and while there is clearly an opportunity to reduce energy loss to heat locally, other parts of such a system may counterbalance those gains. “This is a fascinating question that remains to be resolved,” Levitov says.