Electrons, negatively charged subatomic particles that are found in all atoms, are known from our high school physics as the almost massless, excitable particles that move around as a result of energy absorption.
Taking this further, electrons are what is known as ‘free agents.’
This means that they can move through metals in whatever direction they like; this seemingly random movement includes collisions and changes of direction if they do encounter an obstacle.
However, sometimes – if they encounter other types of material – the electrons may form a line following the edge of that material, and travel in that smooth line. This is referred to as the ‘edge state’.
The edge state was first discovered in 1980 when physicists noticed that – in specific conditions, including ultracold temperatures and under the forces of a magnetic field – electrons sent in a current through layered materials did not flow through, instead gathering together in precise formations.
Through this research, those scientists concluded that electrons could be directed under magnetic force.
Over forty years late, physicists at MIT have been testing these theories, and have captured images of atoms flowing around a material without any of the friction or deflection that obstacles would normally provide.
The images, published in Nature Physics are groundbreaking, and offer scientists hope in learning to manipulate electrons for purposes including modern tech.
Richard Fletcher, co-author of the study, explains in a statement from MIT that while there is a real future for this research, the images are simply magnificent in their own right:
“You could imagine making little pieces of a suitable material and putting it inside future devices, so electrons could shuttle along the edges and between different parts of your circuit without any loss. I would stress though that, for us, the beauty is seeing with your own eyes physics which is absolutely incredible but usually hidden away in materials and unable to be viewed directly.
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.”
In order to create the conditions required for the edge state, the researchers used a cloud of 1 million sodium atoms, which were enclosed, made ultracold, and spun around in a laser controlled trap.
Light from a laser then formed an edge; the reaction of the atoms was then captured in the images, in which you can see that the atoms flowed along the edge of the ring-shaped laser, as Fletcher describes:
“The trap is trying to pull the atoms inward, but there’s centrifugal force that tries to pull them outward. The two forces balance each other, so if you’re an atom, you think you’re living in a flat space, even though your world is spinning. There’s also a third force, the Coriolis effect, such that if they try to move in a line, they get deflected. So these massive atoms now behave as if they were electrons living in a magnetic field.
“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.”
The team’s research, which has been recently published in Nature Physics, describes how the electrons stayed in this edge state even when obstacles were placed in front of them.
Rather than changing their path due to increased friction or colliding with one another, the sodium atoms – in this magnetized, ultracold state – continued to follow their path.
In the statement, Fletcher goes on to explain how these atoms, and their surprising behaviour, explain further how edge states really work for electrons:
“We intentionally send in this big, repulsive green blob, and the atoms should bounce off it. But instead what you see is that they magically find their way around it, go back to the wall, and continue on their merry 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. A natural direction is to now introduce more obstacles and interactions into the system, where things become more unclear as to what to expect.”
Though there are compelling future implications for this research, for a moment it’s worth following Fletcher’s advice and simply admiring the images.
Isn’t physics beautiful?
If you thought that was interesting, you might like to read about a quantum computer simulation that has “reversed time” and physics may never be the same.