Yonsei University Researchers Uncover Direct Evidence of Wigner Crystals and Electronic Rotons

Elusive electronic rotons, detected for the first time, reveal the formation of Wigner crystallites in a two-dimensional electron liquid

SEOUL, South Korea, March 27, 2025 /PRNewswire/ --Yonsei University researchers have provided new experimental evidence for electronic rotons and their connection to Wigner crystallization in a two-dimensional system. Using angle-resolved photoemission spectroscopy on alkali-metal-doped black phosphorus, their studies demonstrate that these rotons play a role in triggering the self-organization of electrons into ordered arrays. These findings contribute to the understanding of strongly correlated electron phenomena and the behavior of novel quantum materials.

For decades, researchers have explored how electrons behave in quantum materials. Under certain conditions, electrons interact strongly with each other instead of moving independently, leading to exotic quantum states. One such state, first proposed by Nobel laureate Eugene Wigner, is the Wigner crystal—a structured electron arrangement caused by their mutual repulsion. Although widely theorized, experimental proof has been rare.

Researchers at Yonsei University have provided evidence of Wigner crystallization and the associated electronic rotons. In a study published in volume 634 of the journal Nature, on October 16, 2024, Prof. Keun Su Kim and his team used angle-resolved photoemission spectroscopy (ARPES) to analyze black phosphorus doped with alkali metals. Their data revealed aperiodic energy variations, a hallmark of electronic rotons. Crucially, as they decreased the dopant density within the material, the roton energy gap shrank to zero. This observation confirmed a transition from a fluid-like quantum state to a structured electron lattice, characteristic of Wigner crystallization.

"Electrons are known to behave like waves, moving freely in solids. However, Wigner proposed that at low densities, strong repulsion between electrons could immobilize them into a crystal-like structure. Here, we identified flakes of such Wigner crystals by detecting anomalously aperiodic signals in ARPES data from alkali metals on black phosphorus," explains Prof. Kim, lead researcher of the study.

The researchers found that the energy patterns of the electrons were irregular, showing a dip at a certain momentum. This irregular pattern, not seen in regular crystalline solids, strongly indicated the presence of electronic rotons. By looking at how the electrons were arranged, using a technique called structure factor analysis, they confirmed that the electrons had a short-range order, which is important for forming these Wigner crystals.

Using structure factor analysis, the researchers found that as the Wigner crystal formed, the electrons became more evenly spaced. This showed that tiny, ordered groups of electrons—like mini-crystals—were forming within the material.

This discovery is a major step in understanding how electrons behave in strongly correlated systems. The Wigner crystal has been proposed as a key to understanding the mechanism of high-temperature superconductivity and superfluidity. "Once we can understand high-temperature superconductivity, it will totally change our real life. Phones and computers will never overheat no matter how long one uses them. There will be no energy loss in electricity transmission, lowering energy costs. Lastly, we will see revolutionary changes in transportation, like magnetic levitation trains becoming more affordable," says Prof. Kim.

While this study focuses on fundamental physics, its implications extend far beyond. By deepening our understanding of quantum materials, this work moves us closer to one day achieving room-temperature superconductors, which could transform energy, electronics, and transportation.

Reference
Title of original paper: Electronic rotons and Wigner crystallites in a two-dimensional dipole liquid
Journal: Nature
DOI: 10.1038/s41586-024-08045-0 

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