Kyoto Experiment Finds Shear Strain Has No Effect on Strontium Ruthenate Superconductivity, Deepening a 30-Year Mystery
A Kyoto University team applied three types of shear strain to ultra-thin crystals of the unconventional superconductor Sr2RuO4 and found virtually no change in its transition temperature, ruling out several leading theoretical models and opening new questions about the material's pairing symmetry.
Overview
Researchers at Kyoto University have found that shear strain has virtually no effect on the superconducting transition temperature of strontium ruthenate (Sr2RuO4), a result that eliminates several prominent theoretical explanations for the material’s behavior and deepens one of the longest-running puzzles in condensed matter physics. The study, published in Nature Communications, represents one of the most direct experimental tests yet of how this enigmatic superconductor responds to mechanical deformation.
Sr2RuO4 has confounded physicists since its superconducting properties were first identified in 1994. Despite being one of the cleanest and most carefully studied unconventional superconductors, the precise way its electrons pair up to achieve frictionless current flow has remained stubbornly unresolved. The new Kyoto result narrows the field of viable theories but also creates a fresh contradiction with earlier measurements, according to ScienceDaily.
The Experiment
The research team, led by first author Giordano Mattoni of the Toyota Riken — Kyoto University Research Center, developed a precision technique to introduce three distinct types of shear strain to extremely thin crystals of Sr2RuO4. Shear strain works by shifting part of a crystal sideways relative to the rest, analogous to sliding a deck of cards. The experiments were conducted at temperatures as low as 30 Kelvin (minus 243 degrees Celsius), with high-resolution optical imaging used to measure the effects with unprecedented accuracy, as described by Phys.org.
The central finding was striking in its simplicity: the superconducting transition temperature barely changed. Any variation was smaller than 10 millikelvin per percent of applied strain, a shift so small it falls below the threshold of confident detection. In practical terms, twisting the crystal had essentially no measurable impact on its superconductivity, according to ScienceDaily.
Why It Matters
The result carries significant theoretical weight because it directly tests a core prediction of several proposed models for Sr2RuO4. A number of theories have suggested that the material harbors a two-component superconducting state, in which electrons pair in a complex pattern involving two intertwined order parameters. Such two-component states would be expected to respond strongly to shear strain, since the deformation would break the symmetry that holds the two components together.
The absence of any detectable response to shear effectively rules out these two-component models, at least in their standard formulations. The findings instead point toward either a simpler one-component superconducting state or an entirely unconventional pairing mechanism that has not yet been fully theorized, as reported by Phys.org.
A New Contradiction
The Kyoto result also creates a notable discrepancy with earlier experimental work. Previous ultrasound measurements on Sr2RuO4 had shown a strong response linked to shear, suggesting that the superconducting state did couple to this type of deformation. The new direct strain measurements, however, show virtually no such coupling.
Understanding why these two experimental methods yield conflicting answers is now a major open question. One possibility is that ultrasound probes a different aspect of the material’s behavior than static strain does, but resolving this disagreement will require further investigation, according to ScienceDaily.
Broader Implications
Mattoni stated that the study “represents a major step toward solving one of the longest-standing mysteries in condensed-matter physics,” according to Phys.org. The shear strain technique developed for this work is not limited to Sr2RuO4. The researchers noted it can now be applied to other multi-component superconductors such as UPt3, potentially helping to resolve similar symmetry questions in other exotic materials.
The study was conducted with collaborators including Thomas Johnson, Atsutoshi Ikeda, Shubhankar Paul, Jake Bobowski, Manfred Sigrist, and senior author Yoshiteru Maeno. While the 30-year mystery of Sr2RuO4 remains unsolved, the Kyoto team has significantly narrowed the search space, bringing physicists closer to identifying the true nature of this material’s unconventional superconducting state.