Physicists Use Light to Reveal Hidden Quantum Motion Inside a Wigner Crystal

In a Wigner crystal, electrons do not move independently. Instead, strongly interacting electrons confined to a two-dimensional layer arrange themselves into a repeating lattice, similar to atoms in an ordinary crystal, but held in place only by their mutual repulsion rather than by any surrounding material. Physicists have studied this state of matter for decades, but measuring how its electrons move collectively and react to disturbances has proven difficult. Researchers at the University of Basel and the Technical University of Munich (TUM) have now found a way to do it using light, in a study published in Nature Physics.

The team, led by Professor Tomasz Smolenski at the University of Basel, studied a single atomic layer of tungsten diselenide cooled to a few degrees above absolute zero. By shining light on the material and analyzing what bounced back, they identified previously unseen optical signals linked to the collective behavior of the crystal's electrons. These signals arise when the ordered electrons interact with excitons - particle-like excitations created in the material by light - forming hybrid quasiparticles called Wigner crystal polarons.

"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," said first author Lujun Wang, who carried out the experiments with PhD student Ferdinand Menzel in Smolenski's group. The strength of the signals also depends on how strongly the electrons interact with one another, making the method useful for studying other strongly correlated systems, in which many interacting particles together determine a material's properties.

To explain the observations, theorists led by Professor Michael Knap at TUM built a model describing how the Wigner crystal polarons form. "These signals carry information not only about how the electrons are arranged, but also about their quantum dynamics," said TUM PhD student Fabian Pichler. The findings show that atomically thin materials can serve as a platform for observing the collective movement of electrons within ordered quantum states.