Topological semimetal phase quantum state found in CeRu4Sn6

A quantum state of matter has been identified in a material where physicists had assumed it could not occur, prompting a reassessment of the rules thought to govern how electrons behave in certain solids.

The finding, reported by an international collaboration, could guide progress in quantum computing, boost electronic efficiency, and support more capable sensing and imaging methods.

The newly observed state is described as a topological semimetal phase. It had been predicted to emerge at low temperatures in a compound made from cerium, ruthenium and tin (CeRu₄Sn₆), and experiments have now confirmed that prediction.

Quantum criticality in CeRu₄Sn₆

When cooled to extremely low temperatures, CeRu₄Sn₆ reaches quantum criticality - the point at which a material sits on the verge of a phase change, and conditions are so cold that quantum fluctuations dominate, making the system behave more like a pool of waves than a haze of particles.

The twist is that quantum criticality can generate states that were thought to be characterised by particle interactions, including the familiar picture of electrons acting as distinct carriers of charge.

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

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

Topology and the topological semimetal phase

In physics, topology concerns the geometric character of material structures. Certain topological states can preserve particle properties, rather than letting neighbouring particles buffet one another and spoil their behaviour.

Normally, making sense of topological states involves assembling properties into particle-like maps - something researchers did not expect a material to sustain while it is at quantum criticality.

Quantum criticality and topology each offer their own advantages in materials. Bringing them together could point to a new family of materials whose quantum responses are highly sensitive yet remain robust and stable.

Hall effect observations without a magnetic field

After cooling CeRu₄Sn₆ to near absolute zero and applying an electric charge, the team detected the Hall effect in the electrons carrying current through the material. In practice, the current deflected sideways.

The researchers argue that this provided an unambiguous signature of topological behaviour. Typically, the Hall effect needs a magnetic field to push electrons off course, but here no magnetic field was applied; instead, something intrinsic to the material was shaping the current’s trajectory.

"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.

The team also found that the topological signal was strongest precisely where the material’s electron patterns were most unstable; in other words, the quantum-critical fluctuations themselves helped stabilise the newly identified phase.

A great deal remains to be explored. The researchers aim to find out whether the same quantum state appears in other materials, to determine how broadly it applies.

They also plan to examine the topology seen here in more detail, along with the exact set of 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 has been published in Nature Physics.