Twenty-five years of research on Sr2RuO4: a model correlated electron metal and a confounding superconductor

In this colloquium, I will describe the history of research on Sr₂RuO₄ since its superconductivity was first discovered in 1994.  In a flurry of early work, it was shown to have an unconventional order parameter, providing strong evidence for the superconductivity being driven by electron-electron interactions.  The resulting superconducting state is extremely fragile, being destroyed by minute amounts of disorder.  This has motivated the growth of extremely pure single crystals, which has in turn enabled experiments that allow us to understand, in exquisite detail, the metallic state from which the superconductivity condenses.  Indeed, some regard Sr₂RuO₄ as the ‘drosophila material’ for refining many-body theories of metals.  However, in spite of the wonderful purity and the understanding of the metallic state, the path to understanding the superconductivity is not proving to be nearly as straightforward.  Indeed, the field has recently been turned upside down by new work overturning a famous result that had been a cornerstone of the spin-triplet interpretation of the superconductivity.  I will discuss the physics and the sociology of the problem and the lessons that experiences on Sr₂RuO₄ can give us for research in other fields.