This powerful material had to go through a torture test to help solve long standing puzzle

Kyoto researchers narrowed theories about strontium ruthenate superconductivity, but major quantum mysteries and contradictions remain unresolved.

Scientists at Kyoto University took a fresh look at Strontium Ruthenate (Sr₂RuO₄), a material that has puzzled researchers since it was found to be superconducting in 1994. Superconductors are materials that can carry electricity with zero resistance, usually at very low temperatures. Most follow well-understood rules based on conventional BCS theory, but Sr₂RuO₄ has remained an exception. It belongs to a class known as unconventional superconductors, where electrons pair through mechanisms that are still not fully understood. Despite being one of the cleanest and best-studied unconventional superconductors, the exact way its electrons pair up to create superconductivity is still debated.

For years, experiments have given conflicting results. Ultrasound studies suggested that Sr₂RuO₄ might have a two-component superconducting state. The superconducting state is described using something called an order parameter — a mathematical framework that explains how electrons organize themselves inside the material. In a two-component state, multiple interacting quantum states can coexist, making the superconductivity more complex and capable of supporting unusual effects like internal magnetic fields or multiple superconducting domains. On the other hand, some uniaxial pressure experiments pointed toward a simpler one-component state. This disagreement has been at the center of the debate.

To help resolve it, the Kyoto team tried a new approach. They applied three different kinds of shear strain to very thin crystals of Sr₂RuO₄. Shear strain is a sideways distortion of the crystal, similar to sliding the top of a deck of cards relative to the bottom. It is applied to uncover the quantum mechanisms behind superconductivity.

The strain was carefully measured using optical imaging, and the superconducting transition temperature (Tc) — the temperature at which the material enters the superconducting state — was tracked using low-frequency magnetic susceptibility, which measures how a material responds to magnetic fields, down to 30 Kelvin (−243 °C).

The outcome was striking: Tc barely changed. Any variation was smaller than 10 millikelvin per percent strain, which is effectively undetectable. This shows that shear strain has little to no effect on superconductivity in Sr₂RuO₄.

These results line up with a one-component order parameter model, but the story is not that simple. A one-component model cannot explain other experimental findings, such as time-reversal symmetry breaking, superconducting domains, and horizontal line nodes. Time-reversal symmetry breaking refers to a situation where the superconducting state behaves differently if time is mathematically reversed, implying the existence of spontaneous internal magnetic fields. Horizontal line nodes are regions in momentum space where the superconducting energy gap drops to zero, offering clues about how electron pairing changes throughout the crystal. This means that while the new data rules out several theories, it also points to the need for alternative explanations that go beyond conventional models.

“Our study represents a major step toward solving one of the longest-standing mysteries in condensed-matter physics,” said Giordano Mattoni, first author and researcher at the Toyota Riken–Kyoto University Research Center (TRiKUC).

The findings also highlight a puzzle. Earlier ultrasound experiments showed a strong shear effect, while the new direct strain measurements do not. Understanding why these two methods disagree is now a major open question.

Beyond Sr₂RuO₄, the strain-control technique developed in this study could be useful for other superconductors that may have multi-component states, such as UPt₃, and for materials with complex phase transitions.

In short, the Kyoto team’s work narrows the possibilities for what kind of superconducting state Sr₂RuO₄ can host. It strengthens the case against a two-component state but leaves unexplained features that continue to fuel debate. The mystery of how superconductivity works in this compound is not solved yet, but the path forward is clearer.

Source: Kyoto University, Nature

This article was generated with some help from AI and reviewed by an editor. Under Section 107 of the Copyright Act 1976, this material is used for the purpose of news reporting. Fair use is a use permitted by copyright statute that might otherwise be infringing.

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Posted Thursday 28 May 2026 at 7:53 am AEST (my time).

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