Kyoto, Japan -- Superconductors are materials that can conduct electricity with zero resistance, usually only at very low temperatures. Most superconductors behave according to well-established rules, but strontium ruthenate, Sr₂RuO₄, has defied clear understanding since its superconducting properties were discovered in 1994. It is considered one of the cleanest and best-studied unconventional superconductors, yet scientists still debate the precise structure and symmetry of the electron pairing that gives rise to its remarkable properties.
One powerful way to identify the underlying superconducting state is to measure how the superconducting transition temperature, or Tc, changes under strain, since different superconducting states respond differently when a crystal is stretched, compressed, or twisted. Many earlier experiments, especially ultrasound studies, suggested that Sr₂RuO₄ might host a two-component superconducting state, a more complex form of superconductivity that can support exotic behaviors such as internal magnetic fields or multiple coexisting superconducting domains. But a genuine two-component state is expected to respond strongly to shear strain.
This inspired a team of researchers from Kyoto University to use strain to understand the true nature of the superconducting state of Sr₂RuO₄. The researchers developed a technique that allowed them to apply three distinct kinds of shear strain to extremely thin Sr₂RuO₄ crystals. Shear strain is a type of distortion that shifts part of the crystal sideways, similar to sliding the top of a deck of cards relative to the bottom. The strain levels were carefully measured using high-resolution optical imaging down to 30 degrees K (−243 degrees C). The key discovery: the superconducting temperature hardly changed at all. Any shift in Tc was smaller than 10 millikelvin per percent strain, effectively below the detection limit.
These results show that shear strain has virtually no effect on the temperature at which Sr₂RuO₄ becomes superconducting, ruling out several proposed theories and setting strict limits on what kinds of superconducting states are still possible. The findings instead point toward a one-component superconducting state, or perhaps even more unusual, still-unexplored superconducting states that behave differently from conventional theoretical expectations.
"Our study represents a major step toward solving one of the longest-standing mysteries in condensed-matter physics," says first author Giordano Mattoni, Toyota Riken - Kyoto University Research Center.
This study tightens the search for the correct explanation of how superconductivity occurs in this compound. Yet a puzzle remains: earlier ultrasound measurements clearly showed a strong effect linked to shear, 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 can be applied to other superconductors that exhibit multi-component behavior, such as UPt₃, as well as other materials with intricate phase transitions.
