Researchers at TU Wien (Vienna University of Technology) have discovered a new and unexpected state of matter in a quantum material—one that challenges long-held ideas about how matter must behave.
The finding, published in Nature Physics, shows that a key concept in modern physics, known as topology, is far more general than scientists once believed.
In everyday physics, it is often useful to imagine electrons as tiny particles moving through a material, much like balls rolling along a track.
This picture works surprisingly well, even though quantum physics tells us that electrons are actually more like waves, with no precise position.
Many important theories, including those describing electrical currents in metals, are built on this particle-like view.
Even the theory of topological states of matter—which earned the Nobel Prize in Physics in 2016—relies on this idea.
Topological states describe special patterns in how electrons move and interact, and these patterns are extremely robust. Small defects or disturbances do not easily destroy them, making such materials attractive for future technologies like quantum computing and advanced sensors.
But not all materials follow the rules. In some exotic quantum materials, the particle picture completely breaks down.
In these cases, electrons no longer behave like well-defined particles with a clear speed or direction. For a long time, physicists believed that topological states could not exist under these conditions.
The new research shows that this assumption was wrong.
The TU Wien team studied a compound made of cerium, ruthenium, and tin, known as CeRu₄Sn₆.
When cooled to extremely low temperatures—just fractions of a degree above absolute zero—the material enters a strange regime called quantum criticality. In this state, the material constantly fluctuates between different quantum phases, as if it cannot decide what it wants to be. In such an environment, the usual particle-based description of electrons is expected to fail.
Despite this, theoretical work suggested that the material might still host topological properties. At first, the experimental team was skeptical, because the theory seemed to contradict established ideas. But curiosity led them to look more closely.
What they found was striking. At very low temperatures, the material showed a clear signature of topology: a spontaneous Hall effect. Normally, the Hall effect occurs when a magnetic field bends the path of moving charges. In this case, however, the effect appeared without any external magnetic field. Instead, it arose purely from the internal, topological nature of the material.
Even more surprising was where the effect was strongest. The topological behavior peaked precisely in the most fluctuating, least particle-like regime. When these quantum fluctuations were suppressed—by applying pressure or magnetic fields—the topological properties vanished.
This result overturns the traditional view. It shows that topological states do not require electrons to behave like particles at all. In fact, the absence of particle-like behavior may actually help create these states.
The researchers describe this newly observed phase as an “emergent topological semimetal.” Working with theorists at Rice University, they developed a new framework that unites quantum criticality and topology into a single picture.
Beyond its fundamental importance, the discovery offers a practical benefit. Quantum-critical behavior is relatively easy to identify in experiments and appears in many different materials. This means scientists now have a powerful new strategy for finding previously unknown topological materials.
By revealing that an “impossible” state of matter can exist after all, the study opens a new chapter in quantum physics—and hints that many more surprises may still be waiting to be found.
