  This review describes recent significant research developments made on the
layered perovskite Sr2RuO4 and discusses current issues from both experimental
and theoretical perspectives. Since the discovery of superconductivity in
Sr2RuO4 in 1994, studies using high-quality single crystals quickly revealed it
to be an archetypal unconventional superconductor among strongly correlated
electron systems. In particular, it was thought that the spin-triplet chiral
p-wave superconducting state, which breaks time-reversal symmetry, was a
prominent possibility. In 2019, however, a new development overturned the past
experimental results, and spin-singlet-like behavior became conclusive.
Furthermore, innovation in uniaxial strain devices has stimulated researchers
to explore changes in the superconducting state by controlling the symmetry and
dimensionality of the Fermi surfaces and enhancing the superconducting
transition temperature Tc from 1.5 K to 3.5 K. A spin-singlet chiral d-wave
superconducting state is consistent with most of these recent experimental
results. Nevertheless, there are still unnatural aspects that remain to be
explained. The focus of this review is on unraveling this mystery. Unlike many
other unconventional superconductors, the normal state of Sr2RuO4 exhibits
typical Fermi liquid behavior. Nevertheless, to elucidate its superconducting
state, it may be essential to go beyond the traditional framework of
unconventional superconductivity and recast the theory explicitly considering
the multi-orbital aspects of its electronic states. In this review, we describe
the frontiers of superconductivity research in Sr2RuO4 and discuss how the
remaining issues may be resolved.
