Abstract : A metal, such as aluminium, feeling no electrical resistance below a critical temperature Tc, is called a superconductor. In most superconductors including high-Tc copper-oxides, electrons form pairs with anti-parallel spins. Such superconducting state is called spin-singlet state. Two electrons can also form a pair with parallel spins to enter a spin-triplet state. Spin-triplet superconductivity is a rather rare quantum phenomenon, but is fascinating particularly from topological perspectives, as such novel superconductors can host Majorana bound states that can be used in fault-tolerant quantum computing. Topology is a branch of mathematics but has been widely used to classify the wave functions of states of mater in recent years. A spin-triplet state with odd-parity wave function is topologically nontrivial.

Although spin-triplet state has long been sought, concrete evidence had been lacking until 2016 when CuxBi2Se3 with Tc~3.5K was unambiguously proved to be in such state. Recently, A2Cr3As3 (A = Na, K, Rb, Cs) superconductors with Tc as high as 8 K have emerged as a new class of materials with ferromagnetic electron correlations. In this seminar, I will demonstrate that the spin susceptibility measured by 75As NMR Knight shift unambiguously indicates that K2Cr3As3 is a spin-triplet superconductor described by a vector order parameter d parallel to the c axis. We further show that the d -vector direction can be manipulated by magnetic fields. I will discuss the multiple-phases feature of the superconducting state, and show that K2Cr3As3 is a new platform for the study of topological superconductivity and its applications at the highest temperature ever.

主讲人简介：郑国庆，1985年日本神户大学(Kobe Univ.)物理系毕业，1990年大阪大学(Osaka Univ.)博士。1990年至2004年，大阪大学助教，副教授。2004年起担任日本国立冈山大学(Okayama Univ.)物理系教授。其间的学术兼职包括：美国加利福尼亚大学(UCLA)访问学者，大阪大学客座教授，东京大学客座教授，冈山大学物理系主任/自然科学研究院副院长，日本物理学会理事，中国科学院物理研究所杰出研究员，北京凝聚态物理国家实验室首席科学家。基金委杰出青年基金B, 基金委重点项目，科技部重大研发项目等资金获得者。