Electrons inside graphene have been pushed to supersonic speeds

For the first time, researchers made electrons flow so fast that they reached supersonic speeds, creating a shock wave.

The currents of electricity passing through our devices have the same name as river currents, but in reality they are very different. When electrons pass through materials, they collide with atoms, impeding their movement, and water droplets in rivers most often collide with each other. However, in 2016, researchers were able to make electrons flow like a viscous fluid inside the extremely thin carbon material graphene. Now, Corey Dean from Columbia University in New York and his colleagues made electrons inside graphene do something completely different—the particles moved so fast that they made a hydraulic jump.

You may encounter a hydraulic jump while washing dishes. When you run the faucet, the sink underneath forms a messy ring-shaped border separating fast and slow flowing water. “In some ways it's like a sonic boom coming from your kitchen sink,” says Doug Natelson at Rice University in Texas, which was not involved in the experiment.

Developing the electronic version was less straightforward. Researchers have created a microscopic attachment from two layers of graphene form a version of the “de Laval nozzle” conceived in 19^th centuries and is commonly used in rocket engine designs. This is a tube that is pinched in the middle so that if the liquid reaches supersonic speed inside the constriction it continues to accelerate rather than slowing down as it exits. This ends with the liquid creating a shock wave.

But the researchers had to find a way to detect this hydraulic jump, which had never before been observed in electrons. Team Member Abhay pasupati, Columbia also says that instead of measuring the electron flow between the two ends of the device, as is usually the case, they adapted a type of microscope to map the electron voltage at many different points in the nozzle.

Natelson says there is an art and subtlety to making graphene structures pure enough that the electrons can actually be cheek-to-cheek, that is, squeezed close enough to enter that space. more dramatic mode. Given that the graphene nozzle was microscopically small, it was technically impressive that the team was able to pull off the jump, he says. Thomas Schmidt at the University of Luxembourg.

Now that they know how to make electrons move so fast, researchers have a chance to answer some long-standing questions about electrically charged shock waves. Dean says there is ongoing debate about whether the hydraulic jump releases radiation that could be used to create new infrared and radio wave generators. “Every experimentalist we discuss this with is thinking about how this radiation can be detected. Every theorist says it can't emit anything. The question arises as to what is actually happening,” he says.