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.
