Scientists made electrons move faster than sound inside graphene

Electrons in bilayer graphene have been driven faster than their own speed of sound, creating a sharp electronic shockwave. The study shows a controlled, chip-scale way to reach this extreme flow regime.

The team carved a microscopic nozzle into bilayer graphene and saw a sudden jump in electrical potential, the signature of a shock.

That jump appears when the electron flow crosses about 270 miles per second (435 kilometers per second), the electronic sonic speed for their conditions.

Electrons behave like liquid

Electrons can behave like a liquid when they collide more with one another than with impurities or the crystal lattice. Clear evidence for this viscous behavior first appeared in graphene nearly a decade ago.

In this work the electrons entered compressible flow, a regime where density can change strongly along the path.

Compressible flow lets shocks form, because signals in the fluid cannot propagate upstream once the flow outruns its own sound speed.

Cory R. Dean of Columbia University led the device effort and measurements. His group built and tested the structures that let electrons accelerate and then slam into a shock.

The key difference here is speed. Earlier hydrodynamic experiments stayed safely below the electronic sound speed, so the flow was smooth and nearly incompressible.

How graphene speeds electrons

The device is an electronic de Laval nozzle, a converging then diverging passage that accelerates a fluid through a narrow throat.

This geometry was a theoretical prediction for turning electron liquids supersonic and creating an electronic shock.

As current increases, the drift velocity climbs to the local sound speed at the throat. Past that point, the flow keeps accelerating downstream, and then it relaxes abruptly in a thin region, the shock.

Engineers describe speed using the Mach number, the ratio of flow speed to sound speed in the same medium.

When the Mach number rises above one, upstream information cannot catch up, and discontinuities such as shocks can appear.

Bilayer graphene helps in two ways. It allows clean, hydrodynamic flow at low carrier density, and it offers a relatively low electronic sound speed when the stray capacitive coupling to gates is minimized.

Detecting invisible shock

To spot the shock, the team used Kelvin probe force microscopy (KPFM), a scanning probe technique that maps local electrical potential with high sensitivity. KPFM records how the potential changes, point by point, while current flows.

The maps revealed an arc-shaped band of flattened potential downstream of the throat. That flat zone signals a sudden conversion of directed flow energy into heat, which is exactly what a shock does.

This feature matches a hydraulic jump, a sudden change in flow state marked by a sharp boundary.

In classical thin films the jump shows up as a height change, while here it appears as a jump in electrochemical potential.

KPFM appears repeatedly throughout the experiments, because it can separate geometry-specific effects from contact artifacts. It also pinpoints where along the nozzle the shock sits for a given current and carrier density.

Closer look at the evidence

Transport measurements in graphene showed a discontinuous rise in differential resistance at the same drive where KPFM saw the flat band.

This transport jump in graphene electrons shifted with gate voltage in a way consistent with crossing the local sonic point.

The location of the flat band shifted when the source and drain were swapped. That behavior matches a shock that forms on the downstream side of the throat and tracks the flow direction.

A perpendicular magnetic field washed out the KPFM signature. Hydrodynamic flow is sensitive to magnetic bending of trajectories, so this loss of the feature reinforces the shock interpretation.

Simulations of compressible electron hydrodynamics reproduced the observed arc and its movement with current.

The modeling also constrained the viscosity and momentum relaxation needed to support a stable shock in such a small channel.

Future of graphene electrons

Supersonic electron flow brings strongly nonlinear behavior into simple, solid-state devices.

Classic theory long predicted that hydrodynamic electron systems might host instabilities that generate tunable terahertz radiation.

There is also a basic physics payoff. Shocks offer a clean way to probe how electronic pressure, viscosity, and energy loss change with density and temperature inside a quantum material.

KPFM will likely continue to be central, because it images the local potential without needing delicate multi-terminal geometries.

That direct view should help separate genuine hydrodynamic effects from ballistic or ohmic transport.

Why this result stands out

The device shows a single, strong signature that lines up across three checks. It appears in transport, in spatially resolved KPFM, and in simulations tuned to experimental parameters.

The nozzle geometry is simple but powerful. Small changes in density or current can push the Mach number over one at the throat, then the electronic liquid has only one way to slow down, a shock.

The approach relies on high material quality and careful control of stray capacitances. That combination kept the electronic sound speed within reach of the applied currents.

By building a repeatable platform, the team moved shocks from prediction to measurement. That shift opens a pathway to devices that use compressible electron flow as a working resource rather than a nuisance.

The study is published in arXiv.

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