Unconventional superconductivity in Sr₂RuO₄

Sr₂RuO₄, featuring a layered perovskite crystalline and quasi-two-dimensional electronic structure, was first synthesized in 1959. Unconventional, p-wave pairing was predicted for Sr₂RuO₄ by Rice and Sigrist and Baskaran shortly after superconductivity in this material was discovered in 1994. Experimental evidence for unconventional superconductivity in Sr₂RuO₄ has been accumulating in the past two decades and reviewed previously. In this article, we will first discuss constraints on the pairing symmetry of superconductivity in Sr₂RuO₄ and summarize experimental evidence supporting the unconventional pairing symmetry in this material. We will then present several aspects of the experimental determination of the unconventional superconductivity in Sr₂RuO₄ in some detail. In particular, we will discuss the phase-sensitive measurements that have played an important role in the determination of the pairing symmetry in Sr₂RuO₄. The responses of superconductivity to the mechanical perturbations and their implications on the mechanism of superconductivity will be discussed. A brief survey of various non-bulk Sr₂RuO₄ will also be included to illustrate the many unusual features resulted from the unconventional nature of superconductivity in this material system. Finally, we will discuss some outstanding unresolved issues on Sr₂RuO₄ and provide an outlook of the future work on Sr₂RuO₄.