Scientists Discover New State of Matter That Shouldn't Exist — An 'Emergent Topological Semimetal'

Physicists at TU Wien and Rice University have identified a fundamentally new class of quantum state in the material cerium-ruthenium-tin, a discovery that challenges a cornerstone assumption of condensed matter physics and expands the universe of known quantum phenomena. The findings, published January 14, 2026 in the journal Nature Physics, describe what the researchers call an "emergent topological semimetal" — a state in which topological properties arise spontaneously from quantum criticality without the presence of conventional, well-defined quasiparticles, a scenario previously considered theoretically impossible and whose experimental confirmation rewrites textbook understanding of how topology and quantum mechanics interact.

The discovery emerged from experiments conducted by Diana Kirschbaum, a doctoral researcher at TU Wien's Institute of Solid State Physics, under the supervision of Professor Silke Bühler-Paschen. Working at temperatures below one degree above absolute zero — approximately minus 272 degrees Celsius — Kirschbaum and her colleagues observed a spontaneous anomalous Hall effect in cerium-ruthenium-tin, written chemically as CeRu₄Sn₆. The anomalous Hall effect, in which a measurable transverse voltage develops across a material without an external magnetic field, is a hallmark signature of topological behavior in quantum materials. Detecting it in a material that sits precisely at a quantum critical point — a boundary between ordered and disordered quantum phases — where conventional quasiparticles dissolve and become ill-defined was entirely unexpected.

The conventional framework for understanding topological materials relies on well-defined quasiparticles: electrons or their mathematical analogs that behave as elementary particles with fixed masses and charges, and whose mathematical properties can be characterized by topological invariants — numbers that remain unchanged under continuous deformations. At a quantum critical point, these quasiparticles break down, dissolving into collective quantum excitations that resist standard description. The prevailing theoretical wisdom held that topology, which is intimately linked to quasiparticle wave functions, could not survive this breakdown. Kirschbaum's measurements directly refuted that assumption and demanded a new explanation.

The theoretical framework that explains the observation was developed by collaborating physicists at Rice University, led by Professor Qimiao Si and first author Lei Chen. The emergent topological semimetal arises, they show, because quantum criticality itself can generate effective topological invariants even when the quasiparticle description fails. The researchers conclude that "topological states should be defined in generalized terms" — a statement that opens an entirely new class of materials for exploration. Bühler-Paschen described the finding as "opening a door we didn't know was there," adding that the team expects many more emergent topological semimetals will be identified in coming years as researchers reexamine previously overlooked compounds near quantum critical points.

The practical implications span multiple scientific frontiers. Topological materials are of intense interest for next-generation electronics because their protected surface states allow electrons to flow without backscattering, potentially enabling dissipationless conductors that waste no energy as heat. Topological semimetals are also candidates for hosting Majorana fermions — exotic quantum particles that could serve as the basis for inherently error-resistant quantum computers. The discovery that topological states can emerge in the unconventional regime of quantum criticality, without requiring the standard quasiparticle scaffold, dramatically expands the parameter space in which materials scientists should be searching for these coveted properties. The finding suggests that heavy-fermion compounds near quantum critical points — a broad family studied for decades without recognition of their topological character — may be hiding a wealth of exotic quantum phenomena waiting to be uncovered.