Researchers at TU Wien have uncovered a material where electrons no longer act like distinct particles, yet it still exhibits topological properties thought to require such behavior. This discovery in the compound CeRu₄Sn₆ challenges long-held assumptions in quantum physics. The findings suggest topological states are more universal than previously believed.
Physicists have traditionally described electrons as tiny particles moving through materials, a model that underpins explanations of electric currents and advanced concepts like topological states of matter. These states, which earned a Nobel Prize in 2016, were assumed to rely on electrons having well-defined positions and velocities. However, a study from TU Wien reveals this particle picture can break down entirely while topological features persist.
The material in question, CeRu₄Sn₆—a compound of cerium, ruthenium, and tin—was examined at temperatures just above absolute zero. There, it displays quantum-critical behavior, fluctuating between two states without settling on one. "Near absolute zero, it exhibits a specific type of quantum-critical behavior," notes Diana Kirschbaum, the study's lead author. "The material fluctuates between two different states, as if it cannot decide which one it wants to adopt. In this fluctuating regime, the quasiparticle picture is thought to lose its meaning."
Despite this, experiments detected a spontaneous anomalous Hall effect in the material, where charge carriers deflect without an external magnetic field—a hallmark of topological properties. This effect was strongest amid the largest fluctuations and vanished when suppressed by pressure or magnetic fields. "The topological effect is strongest precisely where the material exhibits the largest fluctuations," Kirschbaum adds. "When these fluctuations are suppressed by pressure or magnetic fields, the topological properties disappear."
Prof. Silke Bühler-Paschen, from TU Wien's Institute of Solid State Physics, highlights the surprise: "This was a huge surprise. It shows that topological states should be defined in generalized terms." Collaborators at Rice University, including Lei Chen and Prof. Qimiao Si, developed a theoretical model linking quantum criticality to an emergent topological semimetal phase.
The discovery implies particle-like behavior is not essential for topology, which arises through more abstract mathematical distinctions. It opens new avenues for finding topological materials in quantum-critical systems, potentially advancing quantum data storage and sensors. The results appear in Nature Physics (2026).