Electrons in Graphene Accelerate to Supersonic Speeds

Researchers have achieved an unprecedented feat: accelerating electrons to supersonic speeds, generating shock waves.

The flow of electricity through devices resembles the flow of a river, yet they differ greatly. Electrons collide with atoms as they traverse matter, while water droplets in a river frequently collide with one another. In 2016, scientists managed to make electrons flow like a viscous liquid in the ultrathin carbon material, graphene. Recently, Cory Dean and his team at Columbia University in New York have taken this further, introducing electrons into graphene, which resulted in a hydraulic jump due to the high speed of particle flow.

Picture a jump in water pressure while doing the dishes. When you turn on a faucet, you experience a similar phenomenon, with a chaotic ring-like border forming in the sink beneath, separating fast and slow flows. “In a way, it’s akin to a sonic boom happening in your kitchen sink,” remarks Doug Natelson from Rice University, who was not involved in the study.

Designing the electronic version was a complex task. The researchers crafted a microscopic nozzle using two layers of graphene, emulating the “de Laval nozzle,” a design from the 19th century often utilized in rocket engines. This nozzle is tapered in the center, allowing fluid to maintain acceleration and produce a shock wave upon exit if it reaches supersonic speeds within the constriction.

However, detecting the hydraulic jump posed a challenge, as it had never been observed with electrons before. Team member Abhay Pasupathy explains that instead of measuring electrons’ flow as usual, they utilized a specialized microscope to map the voltage at various points along the nozzle.

Natelson notes the intricate process of refining the graphene structure to ensure the electrons could “puff it in the cheek,” meaning they had to compress it sufficiently to enter this more dramatic phenomenon. The team’s achievement in resolving the hydraulic jump is technically remarkable, given the minuscule size of the graphene nozzle, according to Thomas Schmidt at the University of Luxembourg.

Now that they can accelerate electrons to such speeds, researchers aim to explore long-standing inquiries concerning charged shock waves. Dean mentions an ongoing debate about whether hydraulic jumps emit radiation that could potentially be harnessed for new infrared or radio generators. “Every experimenter we’re discussing with is figuring out how to detect this emission. Conversely, there’s a prevailing opinion among theorists that no emissions occur. There remains uncertainty about what is truly happening,” he concludes.