Generally, electrons flow freely through most metals. After encountering any obstacle, the charged particles experience friction and scatter randomly like colliding billiard balls.
However, in some exotic materials, electrons flow single-mindedly on purpose. The electrons in these materials become locked to the material’s edge and flow in one direction.
Electrons in this edge state flow frictionlessly, and glide through the obstacles very easily.
In a new study, MIT physicists observed edge states in a cloud of ultracold atoms. For the first time, they have captured images of ultracold atoms flowing freely, without friction, in an exotic “edge state.”
The results of this study could pave the way towards super-efficient, lossless transmission of energy and data.
In 1980, physicists observed the phenomenon called the Quantum Hall effect by experimenting with layered materials. The electrons in these materials were confined to two dimensions. These experiments were conducted in ultracold conditions and under a magnetic field. After sending a current through the materials, the flow of electrons was found to be accumulated on one side.
To explain this phenomenon, physicists suggested that the edge states carry the hall currents throughout the materials. They suggested that, under a magnetic field, electrons in a current could be pushed to the edges of the material. There, they would flow and gather in a way that could explain what they had observed.
Co-author Richard Fletcher, an assistant professor of physics at MIT, said, “The way charge flows under a magnetic field suggests there must be edge modes. But to actually see them is quite a special thing because these states occur over femtoseconds and across fractions of a nanometer, which is incredibly difficult to capture.”
Instead of catching electrons in an edge state, the team recreated the same physics in a more significant and observable system. They studied the behavior of ultracold atoms in a carefully designed setup that mimics the electrons’ behavior under a magnetic field.
Electron whirlpools seen for the first time
Martin Zwierlein, the Thomas A. Frank Professor of Physics, said, “In our setup, the same physics occurs in atoms, but over milliseconds and microns. That means that we can take images and watch the atoms crawl forever along the system’s edge.”
In this new study, scientists conducted experiments on a cloud with 1 million sodium atoms. These atoms were corralled in a laser-controlled trap and cooled to nanokelvin temperatures. By controlling the trap, the team was able to spin the atoms.
The trap pulls the atoms inward, but due to centrifugal force, the atoms move outward. Both forces balance each other, and the atom lives in a flat space.
Here’s the third force at play: the Coriolis effect. Due to this force, atoms get deflected when they try to move in queue. As a result, the atoms behave as if they were electrons living in a magnetic field.
Scientists then introduced ‘edge’ into this reality as a ring of laser light. A round wall occurs around the spinning atoms.
The team captured images of this system and observed that when the atoms encountered the ring of light, they flowed along its edge in just one direction.
Zwierlein said, “You can imagine these are like marbles that you’ve spun up really fast in a bowl, and they just keep going around and around the rim of the bowl. There is no friction. There is no slowing down or atoms leaking or scattering into the rest of the system. There is just beautiful, coherent flow.”
Fletcher added, “These atoms are flowing, free of friction, for hundreds of microns. To flow that long, without any scattering, is a type of physics you don’t normally see in ultracold atom systems.”
This flow of atoms remains stable even after placing an obstacle in the way.
“It’s a very clean realization of a very beautiful piece of physics, and we can directly demonstrate the importance and reality of this edge,” Fletcher says. “A natural direction is to now introduce more obstacles and interactions into the system, where things become more unclear as to what to expect.”
Journal Reference:
- Yao, R., Chi, S., Mukherjee, B. et al. Observation of chiral edge transport in a rapidly rotating quantum gas. Nat. Phys. (2024). DOI: 10.1038/s41567-024-02617-7
