报告时间：2018年1月19日14:30-15:30
报告地点：近代物理系210会议室
报告人：于伟强教授（中国人民大学）
报告题目：Protonation in iron-based superconductors to explore high-Tc phases and supply high-sensitive NMR studies
报告摘要：In condensed matter physics, nuclear magnetic resonance (NMR) is a powerful probe for exploring quantum states and their low-energy fluctuations. In this talk, I will first briefly introduce our recent efforts to construct NMR systems with combination of leading pressure, field, and temperature conditions. These facilities are important for us to reveal novel magnetic structures [1], the interplay of nematicity, magnetism and superconductivity [2], and the quantum spin liquid behavior [3] in various strongly correlated electron systems.
 Next I will focus on our most recent experimental development to insert protons into iron-based superconductors, assisted by ionic liquids and electric fields [4]. This methodology not only produces novel high-Tc phases due to its carrier doping effect, but also supplies proton as a high-sensitive NMR isotope for studies of superconductivity. For example, protonation induces bulk superconductivity with Tc at 20 K in undoped BaFe2As2, 42.5 K in FeSe0.93S0.07, and 18 K in FeS. Unconventional superconductivity is indicated in these protonated materials, by the absence of the coherence peak in the NMR spin-lattice relaxation rate at Tc.
This ion-liquid assisted protonation technique may sever as an convenient and nonvolatile carrier doping method, could be immediately applicable to a wide range of materials for tuning insulating, metallic, or superconducting phases, and allows for rich bulk spectroscopic studies in the emergent phases.
References:
[1] P. S. Wang et al., PRL 117, 237001 (2016);
[2] P. S. Wang et al., PRB 96, 094528 (2017);
[3] J. Zheng et al., PRL 119, 227208 (2017);
[4] Y. Cui, arXiv: 1712.01191. Science Bulletin (2018, cover article).
报告人简介：于伟强，中国人民大学物理系教授。1992-1999 在北京师范大学获得学士和硕士学位。2004年获得UCLA博士学位，2004-2008年先后在马里兰大学、马克马斯特大学做博士后，2008年起任现职。
回国后发展极低温和高压核磁共振技术，专注于高温超导材料和量子磁性的物性研究，在铁基高温超导材料的磁性、向列序和超导电性，自旋阻挫和自旋液体材料的量子相变和低能元激发等方面做出前沿研究成果。2011年获得国家基金委首届优青资助。目前共发表60多篇学术论文（含11篇PRL）。