At just a few degrees above the coldest temperature possible, electrons in a special material have revealed a secret scientists have chased for decades. Researchers at the University of Basel in Switzerland and the Technical University of Munich in Germany have found a new way to watch Wigner crystals move and behave—using nothing but light.
Wigner crystals are exotic states of matter where electrons abandon their usual independence and arrange themselves into orderly patterns, like soldiers falling into formation. Unlike ordinary crystals, which get their structure from atoms, these crystals form purely because electrons repel each other and settle into the most organized arrangement possible. Scientists first predicted they should exist nearly a century ago, but probing their inner workings has remained nearly impossible.
Now, Dr. Lujun Wang and his colleagues have cracked that problem. Working with a single atomic layer of tungsten diselenide—impossibly thin, just one atom thick—the team cooled the material to temperatures colder than outer space and then shone light on it. What bounced back told them something remarkable: new signals appeared that expose how the electrons inside a Wigner crystal move together as one.
"When light hits these electrons, it creates excitations that couple with their collective motion, producing hybrid particles we call Wigner crystal polarons," explains Professor Tomasz Smoleński, who led the experimental team. These polarons act like tiny detectives, carrying information about the crystal's internal behavior back to the scientists' detectors.
Ferdinand Menzel, a Ph.D. student working with Smoleński, helped carry out the delicate experiments. On the theory side, Professor Michael Knap's group at TUM developed the mathematical framework explaining why these signals appear at all. Ph.D. student Fabian Pichler helped connect what the light revealed to the underlying "many-body physics"—the complex math describing how countless particles influence each other simultaneously.
The discovery, published in the journal Nature Physics, does more than satisfy scientific curiosity. The optical signals change depending on how strongly the electrons interact, which means researchers can now use light to explore materials where particles behave in synchronized, correlated ways. This could eventually help scientists understand superconductors, materials that conduct electricity with zero resistance, and other quantum phenomena that behave in strange ways.
"What is particularly exciting is that these signals carry information not only about how the electrons are arranged, but also about their quantum dynamics," Pichler noted. The team found that atomically thin materials offer an ideal window into these quantum states. By making the invisible visible, this new optical technique gives physicists a powerful tool for studying electronic crystals that would otherwise remain locked behind a wall of inaccessibility.