Ying Liu
Pennsylvania State University and Shanghai Jiao Tong University
地点：B501
时间：2019-06-05 15:30
Motivated originally by theoretical predictions that Sr2RuO4 is a p-wave, spin-triplet superconductor, much of the work on this material has focused on its pairing symmetry since the discovery of its superconductivity 25 years ago. The earliest and perhaps most frequently cited evidence for a spin-triplet Sr2RuO4 is the NMR Knight shift measurements showing that the spin susceptibility of Sr2RuO4 was a constant across Tc. In the spin-triplet, “equal spin pairing” picture, spin susceptibilities in the superconducting and the normal states are the same given that individual parallel pairs of spin of Cooper pairs are still randomly oriented even though they have condensed into a plane perpendicular to the d-vector. The spin-triplet picture for Sr2RuO4 is currently being challenged because new NMR Knight shift measurements carried out very recently have shown that the original NMR Knight shift data obtained more than 20 years ago did not reflect the intrinsic properties of Sr2RuO4 - the spin susceptibility of Sr2RuO4 is in fact substantially lower than that of the normal state. The simplest interpretation of the new data is that Sr2RuO4 is a spin-singlet. 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 crystalline symmetry, this also means that Sr2RuO4 must be spin-triplet. Within the spin-triplet scenario, half-quantum vortices (HQVs) will form, as predicted theoretically. Evidence for HQVs in Sr2RuO4 was indeed found in the cantilever magnetometry measurements. Our work aimed at detecting HQVs through magnetoresistance oscillations (MRO) measurements also yielded evidence supporting HQVs. Similar to the phase-sensitive experiment, this result cannot be explained in a spin-singlet picture. In this informal talk, I will present some details of our earlier phase-sensitive and more recent MRO measurements on Sr2RuO4. I will discuss the challenge imposed by the new development and possible experiments that will help resolved the dilemma.
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. He earned a MS degree, under the direction of Professor Zhao-Qing Zhang at Institute of Physics, Chinese Academy of Sciences, Beijing in 1984. Prof. Liu did his Ph. D. thesis research under the direction of Professor Allen M. Goldman at University of Minnesota, and received his Ph.D. degree in 1991. After three-year postdoctoral research at University of Colorado, Boulder, Prof. Liu joined the faculty of Department of Physics of the 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, odd-parity, spin-triplet superconductor 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 a “1000-Talent” program support of Chinese central government, an Outstanding Young Investigator (Type B) award from National Science Foundation of China and a Chang Jiang Lecturer Professorship from Chinese Ministry of Education.