Physicists Capture the First Direct View of Superconducting Electrons Jiggling in Unison

The superconducting electrons were jiggling. Not bouncing randomly like atoms in hot water, but oscillating together, a trillion times per second, like a single coherent wave rippling through the material. For the first time, physicists could actually watch this quantum dance play out.

“This new microscope now allows us to see a new mode of superconducting electrons that nobody has ever seen before,” says Nuh Gedik at MIT. What they were seeing was a superfluid plasmon, a collective wave of electrons moving without friction, captured in exquisite detail by a new type of microscope that operates at terahertz frequencies.

The breakthrough addresses a longstanding frustration in physics. Terahertz light—radiation that oscillates about a trillion times per second—sits in a sweet spot on the electromagnetic spectrum. It’s safe, like visible light and radio waves, yet can penetrate materials like X-rays. More importantly, it oscillates at exactly the right pace to probe how atoms and electrons naturally vibrate inside materials. In theory, it’s the perfect tool for studying quantum phenomena. In practice, there’s been a problem.

Terahertz waves are huge compared to the microscopic samples physicists want to study. The wavelength stretches hundreds of microns long, whilst interesting quantum materials might measure just 10 microns across. “You might have a 10-micron sample, but your terahertz light has a 100-micron wavelength, so what you would mostly be measuring is air, or the vacuum around your sample,” explains Alexander von Hoegen, a postdoc at MIT who led the work. “You would be missing all these quantum phases that have characteristic fingerprints in the terahertz regime.”

The solution came from an unlikely source. Rather than trying to focus terahertz light into a tighter beam—a losing battle against the laws of diffraction—von Hoegen and his colleagues used spintronic emitters, ultrathin metallic layers that produce sharp pulses of terahertz radiation when hit with a laser. The trick was placing their sample directly against this emitter, trapping the terahertz light before it had a chance to spread out. In that confined space, smaller than the wavelength itself, the light could interact with microscopic features that were previously invisible.

To test their microscope, the team turned to BSCCO—bismuth strontium calcium copper oxide, a high-temperature superconductor with a tongue-twisting name that physicists have shortened to “BIS-co.” This material superconducts at relatively balmy temperatures compared to conventional superconductors, and crucially, it remains superconducting even when shaved down to atomically thin layers. The sample they studied was just 28 nanometres thick, roughly eight or nine unit cells stacked together.

When they cooled the BSCCO to 10 kelvin and illuminated it with terahertz pulses, something peculiar happened. The transmitted signal didn’t just pass through cleanly. “We see the terahertz field gets dramatically distorted, with little oscillations following the main pulse,” von Hoegen says. The material was ringing like a bell. “That tells us that something in the sample is emitting terahertz light, after it got kicked by our initial terahertz pulse.”

What they’d found was the superfluid plasmon—a collective oscillation of the superconducting electrons within the ultra-thin BSCCO planes. These electrons, paired up in Cooper pairs and moving without resistance, were sloshing back and forth together. “It’s this superconducting gel that we’re sort of seeing jiggle,” von Hoegen says. Whilst physicists had predicted such modes should exist in two-dimensional superconductors, actually seeing them had proved impossible until now.

By scanning the microscope across their sample, the researchers could map out the plasmon wave’s structure in space. In one rectangular sample, they rotated the terahertz polarisation to align with either the short or long edge. The plasmon frequency shifted depending on orientation—lower when aligned with the long edge, exactly as predicted for a wave confined by the sample’s geometry. They could even track how the plasmon softened as temperature approached BSCCO’s critical point, where superconductivity vanishes. Roughly 10 kelvin below that transition, damping increased sharply as thermal fluctuations began tearing apart the coherent superfluid.

The technique opens new ground for studying two-dimensional quantum materials. The same microscope could probe lattice vibrations, magnetic excitations, and other collective modes that oscillate at terahertz frequencies but have remained hidden from conventional optics. “We can now resonantly zoom in on these interesting physics with our terahertz microscope,” von Hoegen says.

There’s a practical angle as well. “There’s a huge push to take Wi-Fi or telecommunications to the next level, to terahertz frequencies,” von Hoegen notes. Terahertz communications could potentially transmit far more data than today’s microwave-based systems. “If you have a terahertz microscope, you could study how terahertz light interacts with microscopically small devices that could serve as future antennas or receivers.” Testing those devices requires seeing how terahertz radiation behaves at scales smaller than its own wavelength—exactly what this microscope provides.

For now, though, the real prize might be what this reveals about superconductivity itself. By watching how the superfluid plasmon evolves with temperature and momentum, physicists gain direct access to the frequency-dependent and momentum-dependent superconducting transition in two dimensions. It’s a window into how Cooper pairs form, how they move collectively, and what finally breaks them apart. The jiggling, it turns out, tells you quite a lot.

Study link: https://www.nature.com/articles/s41586-025-10082-2