The Mysterious Superconductivity of Sr₂RuO₄

The study of unconventional superconductivity is central to modern condensed matter physics research. Unconventional superconductors are materials that exhibit superconductivity through mechanisms that are not explained by the conventional Bardeen-Cooper-Schrieffer (BCS) theory.

Since its discovery in 1994, studies on the superconductivity of strontium ruthenate (Sr₂RuO₄), a layered perovskite, have established it as a strongly correlated multi-band electronic system exhibiting unconventional superconductivity. Initially, a spin-triplet chiral p-wave superconducting state with topological properties was considered as an exciting candidate for its superconducting state. Nevertheless, a coherent understanding of all its properties remained as a puzzle.

However, in 2019, a new experimental development suggested a spin-singlet-like behavior. Furthermore, the dramatic changes in its superconducting properties under uniaxial strains have pointed towards a spin-singlet chiral d-wave superconducting state. Despite this new experimental knowledge and recent research progress, there is no convincing theoretical scenario, and the superconducting state of Sr₂RuO₄ still remains a mystery.

Thanks to the availability of ultra-pure single crystals grown by several groups, the experimental results are consistent as long as the same measurement technique is used. However, some serious controversies become evident among the results of ultrasound, muon spin resonance, and some thermodynamic measurements. To address this knowledge gap, in a recent review published in Journal of the Physical Society of Japan, researchers analyzed recent experiments and theoretical models aimed at identifying its superconducting states.

After thoroughly reviewing numerous recent studies, the researchers summarized the possible superconducting states of Sr₂RuO₄ and discussed approaches for resolving current puzzles. They emphasize the importance of going beyond traditional concepts of superconductivity, especially considering the role of multiple electronic orbitals, and the need to revise the current understanding of subtle and new experimental techniques.

Uncovering these mysteries could deepen our understanding of superconductivity, which is important for applications such as quantum computing and quantum information. Therefore, this review serves as a foundation for modern investigations into unconventional superconductivity in other strongly correlated, multi-band electron systems, potentially leading to the discovery of novel superconductors.