Electrons inside graphene have been pushed to supersonic speeds

This groundbreaking achievement allows electrons to reach supersonic speeds, resulting in the formation of shockwaves never before seen in their movement.

While electrical currents are often likened to river flows, their behavior is significantly dissimilar. As electrons traverse materials, they collide with atoms, unlike water droplets that primarily interact with each other. In a previous experiment in 2016, scientists succeeded in making electrons flow akin to a viscous liquid in the incredibly thin sheet of carbon known as graphene. Recently, Cory Dean and his team at Columbia University in New York achieved a remarkable feat by inducing electrons in graphene to enact a hydraulic jump.

One could observe a hydraulic jump while washing dishes. When you turn on the faucet, you create a turbulent boundary between the fast and slower water beneath it. According to Doug Natelson from Rice University in Texas, who was not involved in the study, “It’s like a sonic boom occurring in your kitchen sink.”

Reproducing the electron equivalent of a hydraulic jump was a complex task. The research team designed a microscopic nozzle using two layers of graphene, a modern adaptation of the 19^th century “de Laval nozzle” commonly seen in rocket engines. This nozzle narrows in the center, allowing fluids to maintain their speed and even accelerate upon exiting the constricted area, resulting in a shockwave.

To detect this hydraulic jump—previously undocumented for electrons—the researchers adjusted a specialized microscope to examine voltage at multiple points throughout the nozzle instead of the traditional method of measuring current at its ends, explains team member Abhay Pasupathy from Columbia University.

Natelson emphasizes the skill needed to create high-quality graphene structures, ensuring that electrons are densely packed together to enter this remarkable regime. The tiny scale of the graphene nozzle made it particularly challenging to observe this phenomenon, a feat noted by Thomas Schmidt from the University of Luxembourg.

With a method established for achieving such rapid electron flow, the researchers are now poised to explore fundamental questions regarding electrically charged shockwaves. Dean mentions the ongoing debate around whether the hydraulic jump emits radiation, potentially leading to breakthroughs in creating generators for infrared and radio waves. “All experimentalists we talk to are brainstorming ways to detect this emission, while theorists remain skeptical about its possibility. There’s much more to discover,” he concludes.