Superconductors are materials that can conduct electricity without resistance, usually only at extremely low temperatures. Although most follow well-understood physical laws, strontium ruthenate (Sr2RuO4) has remained elusive since its superconducting behavior was first identified in 1994. It is one of the most precisely studied unconventional superconductors, but researchers still disagree on how its electrons pair up and what symmetries govern the process.
One way scientists study superconductors is by observing how the superconducting transition temperature, known as Tc, responds to strain. When a crystal is stretched, compressed, or twisted, the different superconducting states react in different ways. Previous studies, particularly those using ultrasound, suggested that Sr2RuO4 can host binary superconducting states. This more complex geometry can produce unusual effects such as internal magnetic fields and the simultaneous presence of multiple superconducting regions. However, such a state is expected to exhibit a strong response to shear strain.
Surprise revealed by precision shear strain experiment
To investigate this further, a research team from Kyoto University designed an experiment focused on applying controlled strains to Sr2RuO4. They developed a method to introduce three different types of shear strain into very thin crystals of the material. Shear strain involves moving parts of the crystal sideways, similar to sliding the top of a deck of cards relative to the bottom. Using high-resolution optical imaging, they precisely measured strain at temperatures as low as 30 degrees K (-243 degrees Celsius).
The results were unexpected. The superconducting transition temperature hardly changed. All variations in Tc are less than 10 millikelvin per percent strain, effectively too small to be detected with confidence.
Findings Question Major Theories
These observations indicate that shear strain has little effect on when Sr2RuO4 becomes superconducting. This result rules out several existing theories and imposes strong limits on the types of superconducting states that can survive. Rather than supporting a two-component state, the findings suggest a one-component superconducting state, or perhaps a more unconventional state that has not yet been fully explored.
“Our study represents a major step toward solving one of the most long-standing mysteries in condensed matter physics,” said lead author Giordano Mattoni of Toyota Physical and Chemical Research Institute.
A new puzzle appears
While the results narrow the possibilities, they also bring new challenges. Although previous ultrasound experiments clearly showed a strong response to shear strain, these direct strain measurements showed little. Explaining this contradiction is currently an important open question for researchers.
Broad impact beyond Sr2RuO4
The strain control approach developed in this study may be useful for studying other superconductors that may exhibit multicomponent behavior, including materials such as UPt₃. It could also help scientists better understand systems with complex phase transitions.
