Motivated originally by theoretical predictions that Sr₂RuO₄ is a p-wave, spin-triplet superconductor, much of the work on this material has focused on the issue of pairing symmetry since the discovery of its superconductivity 25 years ago. A constant spin susceptibility across and well below T_c in Sr₂RuO₄ obtained in NMR Knight shift measurements has frequently been cited as the direct evidence for a spin-triplet pairing. This result can be understood easily in the “equal spin pairing” picture in which the spin susceptibilities in the superconducting and the normal states are the same given that electron spins paired in parallel in the superconducting state are still randomly oriented as in the normal state even though they have condensed into the plane perpendicular to the d-vector. The spin-triplet picture for Sr₂RuO₄ is currently being challenged as new NMR Knight shift measurements carried out very recently showed that the original data obtained more than 20 years ago did not reflect the intrinsic properties of Sr₂RuO₄ - the spin susceptibility in fact decreases continuously substantially as the temperature lowers below the T_c. A possible interpretation of the new data is that Sr₂RuO₄ could be spin-singlet rather than spin-triplet. On the other hand, the phase-sensitive measurement carried out at Penn State aiming at detecting the symmetry of the orbital part of the order parameter showed that an odd-parity, most likely chiral p-wave, is the most reasonable interpretation of the data, a conclusion remains true today. Given that Sr₂RuO₄ possesses an inversion center, this also means that Sr₂RuO₄ must be spin-triplet. Furthermore, half-quantum vortices (HQVs) was obsered in the cantilever magnetometry measurements, as predicted theoretically. Our work aimed at detecting HQVs through magnetoresistance oscillations (MRO) measurements also yielded evidence supporting HQVs. Similar to the phase-sensitive experiment, the existence of HQVs in Sr₂RuO₄ cannot be explained in a spin-singlet picture. In this talk, I will present some details of our phase-sensitive and more recent MRO measurements on Sr₂RuO₄ and discuss the challenge imposed and opportunities offered by this new development.

Ying Liu, professor of physics at Pennsylvania State University and Hongwen Professor of Physics at Shanghai Jiao Tong University (part-time), received his BS degree from Peking University in 1982 and a MS degree, under the direction of Professor Zhao-Qing Zhang at Institute of Physics, Chinese Academy of Sciences in 1984. Prof. Liu did his Ph. D. thesis research under the direction of Professor Allen M. Goldman at University of Minnesota, Twin Cities and received his Ph.D. degree in 1991. After performing postdoctoral research at University of Colorado, Boulder, Prof. Liu joined the faculty of Pennsylvania State University in 1994, becoming tenured full professor in 2005. His research has focused on the study of low-dimensional and unconventional superconductors, in particular, Sr₂RuO₄. He also worked on graphene, topological insulators, 2D crystals of transition metal oxides and chalcogenides, as well as strongly correlated electronic systems. Professor Liu received an NSF Career Award in 1997 and was selected as a fellow of the American Physical Society in 2006. He was awarded an Outstanding Young Investigator (Type B) and a Chang Jiang Lecturer Professorship in China in 2005 and 2006, respectively.