Electrons inside graphene were pushed at supersonic speeds

For the first time, researchers caused electrons to flow so fast that they became supersonic, creating a shock wave.

The electrical currents that flow through our devices share a name with river currents, but they are actually quite different. When electrons pass through materials, they collide with atoms, hindering their movement, while water droplets in rivers typically collide with each other. Despite this, in 2016, researchers managed to make electrons flow like a viscous liquid inside graphene, an extremely thin carbon material. Now, Cory Dean of Columbia University in New York and his colleagues made graphene’s electrons do something very different: The particles flowed so quickly that they performed a hydraulic jump.

You might experience a hydraulic jump when washing dishes. When you run a faucet, the messy ring-shaped boundary separating fast- and slow-flowing water that forms in the sink below is just that. “In a way, it’s like a supersonic boom happening in your kitchen sink,” says Doug Natelson of Rice University in Texas, who was not involved in the experiment.

The engineering of the electronic version was less straightforward. Researchers created a microscopic nozzle from two layers of graphene to form a version of the “Laval nozzle”, designed in the 19^th century and is commonly used in the design of rocket engines. It is a tube pinched in the middle such that if a liquid reaches supersonic speed inside the constriction, it continues to accelerate instead of slowing down as it exits. This results in the fluid forming a shock wave.

But researchers had to find a way to detect this hydraulic jump, which had never been observed with electrons before. Team member Abhay Pasupathy, also at Columbia University, explains that instead of measuring the flow of electron current between the two ends of the device, as is common, they adapted a type of microscope to map the electron voltage at many different points across the nozzle.

Natelson says it takes art and finesse to make the graphene structures pristine enough that the electrons are truly “cheek to cheek,” that is, to squeeze them close enough to enter this more dramatic regime. Given that the graphene nozzle was microscopically small, it is also technically impressive that the team was able to solve the jump, says Thomas Schmidt of the University of Luxembourg.

Now that they know how to make electrons flow so quickly, researchers have an opportunity to answer some long-standing questions about electrically charged shock waves. Dean says it’s an ongoing debate whether the hydraulic jump comes with a release of radiation that could eventually be used to build new infrared and radio wave generators. “Every experimentalist we talk to about this topic is thinking about ways to detect this emission. Every theorist says there’s no chance it’s emitting anything. There’s a question there about what’s really going on,” he says.