Quantum Criticality Reveals a Topological Semimetal Phase in CeRu₄Sn₆

A quantum state of matter has emerged in a material in which physicists believed it could not exist, prompting a reassessment of the rules governing electron behaviour in certain materials.

The finding, by an international research team, may contribute to progress in quantum computing, more efficient electronics, and improved sensing and imaging technologies.

The phase is known as a topological semimetal phase. It had been predicted theoretically at low temperatures in cerium-ruthenium-tin, CeRu₄Sn₆, and has now been confirmed experimentally.

Quantum criticality in CeRu₄Sn₆

At temperatures extremely close to absolute zero, CeRu₄Sn₆ enters quantum criticality: a point at which a material sits on the verge of changing phase. In these intensely cold conditions, quantum fluctuations take over, making the material more like a pool of waves than a mist of particles.

The unexpected aspect of the study is that quantum criticality can produce states previously considered to depend on interactions between particles, including electrons acting as distinct charge carriers.

"This is a fundamental step forward," says physicist Qimiao Si, from Rice University in the United States.

"Our work shows that powerful quantum effects can combine to create something entirely new, which may help shape the future of quantum science."

In physics, topology concerns the geometry of a material's structure. Certain topological states can safeguard particle properties from the disruptive effects of neighbouring particles jostling one another.

Normally, studying topological states involves assembling these properties into particle-like maps, which is not something a material was expected to possess while under quantum criticality.

Quantum criticality and topology each offer different advantages in materials. Combining them could create a new category of materials with highly sensitive quantum responses alongside dependable stability.

Topological Hall effect without a magnetic field

After cooling CeRu₄Sn₆ to near absolute zero and applying an electric charge, the researchers detected the Hall effect in the electrons conducting current through the material. Put simply, the electrical current curved sideways.

The team says this provided an unambiguous indication of topological effects. Ordinarily, the Hall effect needs a magnetic field to divert electrons, yet no magnetic field was used here. Instead, an intrinsic feature of the material itself was determining the current's route.

"This was the key insight that allowed us to demonstrate beyond doubt that the prevailing view must be revised," says physicist Silke Bühler-Paschen from the Vienna University of Technology.

Moreover, the topological effect was most powerful where the material was least stable in terms of its electron patterns. The quantum-critical fluctuations were, in fact, stabilising the newly identified phase.

Exploring the newly identified quantum state

Considerably more research is needed. The scientists intend to investigate whether this quantum state also occurs in other materials, which would show how broadly applicable it is.

They also plan to examine the topology found in this case in greater detail, as well as the exact conditions needed for it to arise.

"The findings address a gap in condensed matter physics by demonstrating that strong electron interactions can give rise to topological states rather than destroy them," says Si.

"Additionally, they reveal a new quantum state with substantial practical significance."

"Knowing what to search for allows us to explore this phenomenon more systematically," he adds.

"It's not just a theoretical insight, it's a step toward developing real technologies that harness the deepest principles of quantum physics."

The research was published in Nature Physics.