Even odder after twenty-three years: the superconducting order parameter puzzle of Sr2RuO4

In this short review, we aim to provide a topical update on the status of efforts to understand the superconductivity of Sr 2 RuO 4 . We concentrate on efforts to identify a superconducting order parameter symmetry that is compatible with all the major pieces of experimental knowledge of the material, and highlight some major discrepancies that have become even clearer in recent years. As the pun in the title suggests, we have tried to start the discussion from scratch, making no assumptions even about fundamental issues such as the parity of the superconducting state. We conclude that no consensus is currently achievable in Sr 2 RuO 4 , and that the reasons for this go to the heart of how well some of the key probes of unconventional superconductivity are really understood. This is, therefore, a puzzle that merits continued in-depth study.