For thirty years, the strontium ruthenate puzzle has stood as one of the most stubborn open challenges in condensed matter physics. The material, Sr₂RuO₄, discovered in 1994, is a layered perovskite superconductor that doesn’t contain copper, setting it apart from the high-temperature cuprate families. Its transition temperature is a meager 1.5 Kelvin—roughly minus 272 degrees Celsius—so the intense, decades-long interest has never been about operating at practical temperatures. The fixation has always been about symmetry: how electrons pair up to generate superconductivity in this material has remained a question without a definitive answer, despite continuous scrutiny from the world’s top experimental and theoretical groups.
The Two-Component vs. One-Component Debate
At the heart of the dispute is the order parameter—a physical quantity that describes how electron pairs organize themselves in the superconducting state. For ordinary superconductors, this parameter is simple: a single component suffices to describe the pairing. For Sr₂RuO₄, the situation has been maddeningly ambiguous. Ultrasound experiments have long pointed toward a two-component order parameter, implying that two distinct pairing states intertwine. Meanwhile, experiments that uniaxially strain the crystal—essentially squeezing it along a single axis—have consistently supported a one-component scenario. The same material was yielding contradictory conclusions depending on the experimental probe, creating one of the most vexing unsolved problems in the field.
Shear Strain as a Decisive Probe
A research group at Kyoto University’s Hakubi Center for Advanced Research and the Tomonaga-RIKEN-Kyoto University Collaborative Laboratory (TRiKUC) has now tackled this conflict head-on. The team, led by Giordano Mattoni, a specific assistant professor, along with doctoral researcher Thomas Johnson and collaborative laboratory professor Yuehui Qian, developed a new approach that no one had tried before: applying shear strain to the crystals.
Shear strain is a deformation that distorts a crystal laterally, akin to sliding the top of a deck of cards sideways relative to the bottom. The Kyoto group thinned single crystals of Sr₂RuO₄ down to approximately 30 micrometers, glued them directly onto piezoelectric actuators, and imprinted three distinct types of shear strain. They then developed a custom optical imaging technique capable of measuring the amount of strain directly at temperatures down to 30 Kelvin (minus 243 degrees Celsius).
The difficulty of the task was not lost on the researchers. Mattoni described the process as requiring extremely fragile crystals to be subjected to precise deformation, which involved a long series of trial and error attempts to get the technique to work.
Why the Superconducting Transition Temperature Stayed Put
The experimental result was unambiguous. For all three shear strain directions, the superconducting transition temperature, Tc, changed by less than 10 millikelvin per 1% strain. In practical terms, the change was below the detection limit of the experiment.
This null result carries devastating weight for the two-component theory. Under a two-component order parameter, shear strain should produce a distinct V-shaped suppression of Tc. Using the Ehrenfest relation—a thermodynamic principle governing phase transitions—predictions from prior ultrasound data suggested that a 1% shear strain would shift Tc by tens to hundreds of millikelvins. The actual measurement was orders of magnitude below those predictions.
In plain terms, shear strain and superconductivity barely interact in Sr₂RuO₄. That lack of coupling imposes a severe constraint against the two-component order parameter hypothesis.
The Puzzle Deepened Rather Than Solved
Disproving the two-component model, however, does not automatically validate the one-component scenario. Sr₂RuO₄ continues to display phenomena that are difficult to square with a simple one-component description. Experiments below Tc have repeatedly detected an internal magnetic field, known as time-reversal symmetry breaking, through muon spin rotation and the Kerr effect. Tunneling junction experiments have signaled the existence of multiple superconducting domains. Evidence also points to horizontal line nodes—lines where the superconducting gap goes to zero—which are difficult to generate within a conventional one-component framework.
A New Tool for Quantum Materials Research
The technique developed for this study—applying and measuring shear strain in micron-scale crystals—extends beyond Sr₂RuO₄. It opens a new experimental channel for investigating other quantum materials where the nature of the order parameter remains debated. Heavy-fermion superconductors like UPt₃, where a two-component order parameter has been proposed, are a natural next target. The addition of shear strain to the condensed matter toolkit gives researchers a direct, clean way to probe the symmetry of superconducting pairing.
The dissonance between the ultrasound experiments, which saw a clear response to shear, and the direct strain measurement, which saw none, remains an open problem. Resolving that inconsistency is itself a new research question that could reveal deeper physics about how strain couples to electronic states in correlated materials. The findings were published in Nature Communications on December 16, 2025, under the title “Direct evidence for the absence of coupling between shear strain and superconductivity in Sr₂RuO₄.”
Thirty years of mystery have not produced a final answer for Sr₂RuO₄. But one major hypothesis has been removed from the board, and a sharper experimental method is now available to test the alternatives. The path to understanding this enigmatic superconductor has narrowed—and that, in science, counts as real progress.
