Researchers at the University of Basel and the Technical University of Munich have found a new way to examine how electrons move together inside one of the most elusive forms of matter, the Wigner crystal. By using light to probe this delicate quantum state, the physicists uncovered properties that had previously been extremely difficult to observe.
When electrons are restricted to a two-dimensional plane and interact strongly enough, they can begin behaving very differently from ordinary electrons. Instead of moving independently, they arrange themselves into a repeating pattern similar to the orderly structure of atoms in a conventional crystal.
This unusual arrangement is called a Wigner crystal. Scientists have studied it for decades because the crystal structure is created by interactions between the electrons themselves rather than by the underlying structure of the material containing them.
While researchers have already detected Wigner crystals in a variety of physical systems, understanding what happens inside them has been much harder. Scientists have struggled to directly investigate how their electrons move collectively, interact with one another, and react to outside disturbances.
Using Light to Probe a Wigner Crystal
In a study published in Nature Physics, experimental researchers led by Professor Tomasz Smoleński at the University of Basel examined a single atomic layer of tungsten diselenide that had been cooled to only a few degrees above absolute zero.
The team illuminated the material and carefully analyzed the light reflected from it. Those measurements revealed previously unseen optical features that contain information about the collective behavior of electrons inside the Wigner crystal.
The signals emerge from interactions between the ordered electrons and excitations created in the material by light, known as excitons. Together, these components form hybrid quasiparticles called Wigner crystal polarons. These quasiparticles serve as highly sensitive optical probes that can reveal both the crystal itself and the collective motion occurring within it.
“Our measurements show that light can do more than simply detect the presence of this exotic state — it can reveal how the state behaves internally,” says first author Dr. Lujun Wang from the University of Basel, who carried out the experiments together with Ferdinand Menzel, a PhD student in Smoleński’s group.
“This gives us a powerful new tool for studying collective excitations of electronic crystals that would otherwise be extremely difficult to access,” adds Smoleński.
Electron Interactions Shape the Optical Signals
The researchers also discovered that the strength of the interactions between electrons influences the optical signatures they observed. That connection could make the signals especially useful for investigating strongly correlated systems, where the behavior of the material emerges from interactions among many particles rather than from individual particles acting on their own.
To account for the experimental findings, a theoretical team led by Professor Michael Knap at the Technical University of Munich (TUM) developed a model describing the formation of Wigner crystal polarons. Their work explains how these quasiparticles arise through the coupling of optically generated excitons with the collective movement of electrons in the crystal.
A New View of Strongly Correlated Quantum Matter
“What is particularly exciting is that these signals carry information not only about how the electrons are arranged, but also about their quantum dynamics,” explains Fabian Pichler, a PhD student at TUM. “This allows us to connect the experimental observations directly to the underlying many-body physics.”
The findings suggest that atomically thin materials could provide an especially useful platform for observing how electrons move collectively within ordered quantum states. By making these hidden dynamics easier to study, the approach could help scientists develop a deeper understanding of strongly correlated matter and the complex behavior that emerges when many particles interact.