Recent work led by Professor Silke Bühler-Paschen at the Vienna University of Technology has revealed that a class of quantum materials can display well-defined topological behavior even when electrons no longer behave as particles. The study, carried out with international collaborators and published in January 2026, addresses a long-standing assumption in condensed-matter physics: that topological states require electrons to have clear energies and velocities. The findings suggest that this requirement is not as fundamental as once thought.
Kirschbaum, D. M., Chen, L., Zocco, D. A., Hu, H., Mazza, F., Karlich, M., Lužnik, M., Nguyen, D. H., Larrea Jiménez, J., Strydom, A. M., Adroja, D., Yan, X., Prokofiev, A., Si, Q., & Paschen, S. (2026). Emergent topological semimetal from quantum criticality. Nature Physics. https://doi.org/10.1038/s41567-025-03135-w
In most electronic materials, engineers and physicists rely on a simplified description in which electrons are treated as quasiparticles that move through a lattice and scatter in predictable ways. This framework underpins band theory, electrical transport models, and much of modern device design. Even in materials where electron interactions are strong, the quasiparticle picture often remains usable with suitable corrections.
Professor Silke Bühler-Paschen at the Vienna University of Technology stated,
“We now know that it is worthwhile—perhaps even particularly worthwhile—to search for topological properties in quantum-critical materials. Because quantum-critical behavior occurs in many classes of materials and can be reliably identified, this connection may allow many new ’emergent’ topological materials to be discovered.”
There are, however, extreme regimes where this description is expected to fail. These occur near so-called quantum critical points, where a material fluctuates between competing electronic states down to very low temperatures. In such conditions, electrons cannot be assigned a single velocity or lifetime, and the notion of a particle-like charge carrier loses its meaning.
The team at Vienna University of Technology focused on a cerium-based compound, CeRu₄Sn₆, which is known to enter a quantum-critical regime when cooled to within a degree of absolute zero. Earlier theoretical work had suggested that this material might host topological states, but this appeared inconsistent with its strongly fluctuating electronic behavior. Topology is usually defined using quantities that assume well-behaved electronic bands.
To resolve this contradiction, the researchers carried out precision transport measurements at ultralow temperatures. They observed a clear anomalous Hall effect, a transverse electrical response that appears without an applied magnetic field. This effect is widely regarded as a hallmark of topological electronic structure. Strikingly, the signal emerged most strongly in the regime where electronic fluctuations were largest and the quasiparticle picture should be least applicable.
Further experiments showed that when the quantum fluctuations were reduced, either by applying pressure or magnetic fields, the anomalous Hall effect weakened and eventually vanished. This direct link between quantum criticality and topology indicates that the topological properties do not survive despite the loss of particle-like electrons, but rather emerge because of it.
The experimental results are supported by theoretical modeling developed in collaboration with researchers in the United States, which demonstrates how topological distinctions can arise from collective electronic behavior instead of individual particle states. In this framework, topology becomes a more general property of the quantum system, not one tied exclusively to band structures or quasiparticles.
While the material studied is unlikely to be used directly in devices due to the extreme conditions required, the implications are broader. Quantum-critical behavior occurs in many families of correlated materials, some of which are already of interest for electronic and sensing applications. The new work suggests that such materials may host unexpected forms of robust electronic order.
For engineering and applied physics, the study points to a shift in how topological materials might be identified and understood. Instead of searching only for clean, weakly interacting systems, it may be equally productive to explore materials where electronic order breaks down in controlled ways. In these regimes, stability can arise not from classical order, but from the collective structure of quantum fluctuations themselves.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
