Light Makes an Electron Crystal Vibrate—and Can Melt Its Order

Electrons usually move through materials, but under sufficiently low temperatures and densities, their mutual electrical repulsion can force them into an orderly arrangement known as a Wigner crystal. Now, researchers have found a way to observe the vibrations of one of these fragile electron lattices and manipulate it with light. The team included scientists from the University of Maryland, ETH Zurich and other institutions. In a study published in Nature Physics, they examined a single atomically thin layer of tungsten diselenide, or WSe₂, enclosed between insulating layers of hexagonal boron nitride. Graphite gates controlled the electron population, while cooling the device to 5 Kelvin—about −268°C—allowed the electron crystal to form when the density was low enough. To investigate the crystal, the researchers illuminated the material and measured the wavelengths of reflected light. Some of the light produced excitons, pairs consisting of an electron and a hole. These excitons disturbed nearby electrons in the crystal, creating a combined state that the researchers call a Wigner polaron. The resulting optical signals revealed not only the excitons but also the vibrations, or phonons, of the electron lattice. According to senior author You Zhou, the team initially noticed these additional features unexpectedly while studying the material’s optical response. The researchers also used circularly polarized light to influence the spins of the electrons. This approach allowed them to initialize the spins in a particular direction without applying an external magnetic field. Increasing the optical power produced a large population of excitons and made the Wigner crystal less stable. Measurements showed that its vibrational energy decreased, indicating that the lattice had softened and was more likely to lose its ordered structure. The exact reason for this optical melting remains uncertain. One possibility, suggested in the report, is that excitons screen the electrical interactions holding the crystal together, although the study does not establish that mechanism conclusively. The researchers say the method could provide a new way to study and control electronic, optical and spin-related properties in strongly interacting materials. They also suggest that similar optical techniques may be useful for investigating Wigner crystals in other materials.