New research shows that electrons passing through a narrow constriction in a piece of metal can move much faster than expected, and that they move faster if there are more of themï¿½a seemingly paradoxical result. In this illustration, the orange surface represents the potential energy needed to get an electron moving, and the ï¿½valleyï¿½ at center represents the constricted portion. CREDIT: Courtesy of the researchers.
A group of physicists from MIT and Israel have discovered that when electrons are subjected to specialized conditions, they have the ability to speed through a narrow opening in a metal piece more easily than traditional theory suggests.
Leonid Levitov, an MIT physics professor, stated that such a “superballistic” flow is identical to the behavior of gases that flow through a constricted opening, but it occurs in a quantum-mechanical electron fluid. Levitov is the senior author of a paper reporting the discovery that will be published this week in the
Proceedings of the National Academy of Sciences.
In such constricted passageways, the more the electrons move through a metal section narrowing to a point or the more the gases pass through a tube, the stronger is the superballistic flow - put differently, huge numbers of electrons, or huge numbers of gas molecules, move faster than smaller numbers of electrons or molecules passing through the same bottleneck.
This behavior appears to be paradoxical. It is as if a crowd trying to pass through a doorway at the same time finds that people in the crowd can pass through faster than a single person passing through alone and undisturbed.
However, for almost 100 years, researchers have an idea that this is precisely the case with gases passing through a minute opening, and Levitov stated that this behavior can be described by means of simple, fundamental physics.
In the case of a passageway of a fixed size, when fewer gas molecules pass, they can move in straight lines unobstructed. That is, if the molecules move at random, most of them will rapidly hit the wall and bounce off, losing a portion of energy to the wall and consequently slowing down every time they hit the wall.
However, in the case of a huge number of molecules, most of the molecules hit the other molecules more often than hitting the walls. As the total energy of the two colliding particles is conserved, collisions with other molecules become “lossless,” preventing the overall slowdown.
Molecules in a gas can achieve through ‘cooperation’ what they cannot accomplish individually.
Leonid Levitov, Physics Professor, MIT
According to Levitov, when the density of molecules entering the passageway increases, “
You reach a point where the hydrodynamic pressure you need to push the gas through goes down, even though the particle density goes up.” Put simply, though sounding strange, the crowding results in the molecules speeding up.
Currently, the researchers describe that an identical phenomenon regulates the behavior of electrons when they crowd through a narrow metal piece, which makes them to travel in a fluid-like manner.
Consequently, electrons have the ability to flow through an adequately narrow, point-like bottleneck in a metal at a rate that is far beyond the fundamental limit considered, that is, Landauer’s ballistic limit. This is the reason why the researchers have coined the term “superballistic” flow for the new effect. This characterizes a considerable reduction in the metal’s electrical resistance, but this reduction falls short of the value needed to produce zero resistance in superconducting metals by a great amount. Nevertheless, in contrast to superconductivity that necessitates exceptionally lower temperatures, the new phenomenon can even occur at room temperature, and thus can be easily implemented for applications in electronic devices.
Indeed, in reality, the phenomenon increases with increase in temperature. Levitov stated that unlike superconductivity, superballistic flow “
is assisted by temperature, rather than hindered by it.”
Levitov further stated that using this mechanism, “
we can overcome this boundary that everyone thought was a fundamental limit on how high the conductance could be. We’ve shown that one can do better than that.”
He added that although the paper is absolutely theoretical, other research groups have experimentally proven its basic predictions even earlier. Although the speedup noticed in the flow of gas in the analogous case can be tenfold or even higher, it is a big question whether such advancements can be achieved in the case of electrical conductance. Even modest decrease in resistance in specific electronic circuits can prove to be highly significant.
This work is careful, elegant, and surprising - all the hallmarks of very high-quality research. In science, I feel phenomena that confound our intuitions are always useful in stretching our sense of what is possible. Here, the idea that more electrons can fit through an aperture if the electrons deflect each other rather than traveling freely and independently is quite counterintuitive, in fact the opposite of what we’re used to. It’s especially intriguing that Levitov and co-workers find that the conductance in such systems follows such a simple rule.
David Goldhaber-Gordon, Professor of Physics, Stanford University
Goldhaber-Gordon added that although this research was theoretical, “
Testing Levitov’s simple and striking predictions experimentally will be really exciting and plausible to achieve in graphene. ... Researchers have imagined building new types of electronic switches based on ballistic electron flow. Levitov’s theoretical insights, if validated experimentally, would be highly relevant to this idea: Superballistic flow could allow these switches to perform better than expected (or could show that they won’t work as hoped).”
The lead author of the paper, Haoyu Guo, is a junior who had just reached MIT as a second-year transfer student from Peking University when he started working on this research - a remarkable achievement for an undergraduate who just arrived, stated Levitov. Guo worked on the research in part through MIT’s Undergraduate Research Opportunities Program (UROP).
Ekin Ilseven from MIT and Gregory Falkovich, a professor of physics from the Weizmann Institute in Rehovot in Israel were also part of the research team.