Superconductivity breakthrough: ‘Edge state’ atoms flow without friction

MIT scientists have twisted atoms into an unusual “state” for the first time, allowing them to flow completely without friction. The breakthrough could lead to better superconductor materials.

As electrons pass through different materials, they encounter different levels of resistance. Basically, insulators allow little to move, semiconductors allow some, conductors allow more, and superconductors allow complete freedom to move without opposition. Therefore, superconductor materials can be used for high-speed data transmission, while the strong electric field they produce can facilitate high-speed transport.

The problem is, studying the movement of electrons is a tricky business, because these particles are so small and move so fast. So for a new study, the MIT team found a way to trick atoms, larger and slower, into the same behavior.

In particular, the researchers studied a type of superconductivity called edge states. In some materials, electrons do not move freely throughout the material but are confined to the edges, where they flow without any friction. Even when obstacles are placed in their path, they dive around them without effort, instead of jumping like they used to.

For electrons, these states occur over femtoseconds (quadrillionths of a second), and nanometer distances, which are difficult to grasp. But atoms make that more visible.

“In our group, the same physics happens with atoms, but over milliseconds and microns,” said Martin Zwierlein, co-author of the study. “That means we can take pictures and watch the atoms crawl endlessly along the edge of the system.”

The researchers trapped a cloud of about a million sodium atoms in a laser trap, at a temperature just above zero, and spun them in a circle at high speed.

“The trap is trying to pull the atoms in, but there’s a centrifugal force trying to pull them out,” said Richard Fletcher, co-author of the study. “These two forces are balanced, so if you’re an atom, you think you’re living on a flat surface, even though your world is spinning. There’s also a third force, the Coriolis effect, which if they try to move in a line, they deviate. So these massive atoms now behave as if they were electrons living in a magnetic field.”

Then, they produced the edge – a ring of laser light that formed a kind of wall around the outside. When the atoms touched the ring, they were found to stick to it, flowing freely along the boundary in one direction.

After that, the researchers generated more speed bumps to see how the atoms reacted. They shone a light inside the ring, and indeed the atoms went on undisturbed.

“We’re deliberately sending out this big, obnoxious slab, and the atoms have to dry up,” Fletcher said. But instead, what you see is that they magically find their way around it, get back to the wall, and go on their merry way.

Everywhere, the behavior of atoms matches the way electrons behave at sharp points, making it directly visible for the first time. Scientists can now use this model to test new concepts and learn more, which can help lead to better superconductors.

“It’s a neat realization of a really cool piece of physics, and we can show the importance and validity of this point,” Fletcher said. “The natural order is to introduce more constraints and interactions into the system, where things are more ambiguous about what to expect.”

The research was published in the journal Natural Physics.

Source: MIT