Sr2RuO4

Sr2RuO4 is a layered ruthenate oxide that belongs to the Ruddlesden–Popper series Srn+1RunO3n+1 (n = 1). It crystallizes in a tetragonal K2NiF4–type structure with RuO2 planes separated by SrO layers. In the crystal, ruthenium sits in an octahedral environment, and the formal oxidation state is Ru4+ (4d4). The electronic structure is highly anisotropic, yielding a quasi-two-dimensional Fermi surface with three sheets (α, β, γ) derived from Ru 4d t2g states, and substantial spin–orbit coupling.

Superconductivity in Sr2RuO4 was discovered in 1994, with a transition temperature Tc ≈ 1.5 K. For many years it was hailed as a leading candidate for unconventional, possibly spin-triplet, superconductivity with a chiral p-wave order parameter and broken time-reversal symmetry. Experimental probes such as muon spin relaxation and polar Kerr effect provided support for TRS breaking, while phase-sensitive measurements remained challenging.

In the years since, results have grown more nuanced. Several experiments have questioned a simple spin-triplet, chiral p-wave state and have pointed toward either a singlet or a more complex order parameter, potentially involving multiple components. Complementary evidence from strain-tuning experiments shows that Tc can be enhanced by uniaxial strain, with reports of a pronounced Tc increase near certain strain values, in connection with changes to the electronic structure near a van Hove singularity.

Normal-state properties are metallic and largely consistent with a Fermi-liquid description at low temperatures in high-quality crystals, with a highly anisotropic, quasi-two-dimensional Fermi surface. Sr2RuO4 remains an active platform for exploring unconventional superconductivity and potential topological phenomena, and its exact pairing symmetry is a topic of ongoing research.