Despite extensive research, Sr₂RuO₄ has remained an enigmatic unconventional superconductor for nearly three decades. Initially categorized as a prime example of a chiral spin-triplet superconductor, doubts regarding this classification only emerged four years ago when evidence for spin-singlet pairing appeared. This unexpected shift has introduced a significant conundrum as the newly proposed spin-singlet nature presents considerable new obstacles in establishing a pairing symmetry that aligns with the wealth of existing experimental data.
This colloquium aims to provide an overview of the current challenges in harmonizing theoretical perspectives with present state of experimental reality.