The research team led by SPST Professor Yulin Chen is one of teams in China that discover the Weyl semimetals. As a result, this discovery has recently been selected among the “Top 10 Science Breakthroughs of 2015” by Chinese Ministry of Science and Technology. Chen’s team shares the honor with Zhong Fang and Hongming Weng, Genfu Chen, and Hong Ding and Tian Qian from the Chinese Academy of Sciences, whose research teams independently found evidence of this exotic quantum material. The Shanghaitech team includes SPST Assistant Professor Zhongkai Liu, Yanfeng Guo and Associate Professor Binghai Yan.
Back in 2014, Chen’s team first discovered a new topological quantum material –Topological Dirac Semimetals experimentally (Science, 343, 864(2014)), leading to the extensive research in the field. In 2015, the research team independently discovered a new type of Fermion: Weyl fermion in crystalline samples, identified as topological Weyl Semimetals (Nature Physics, 11, 728 (2015)). Weyl fermions are massless chiral particles with linear dispersions. It is a concept originally proposed in high energy physics, yet its existence as fundamental particle has never been proved. The discovery of Weyl fermions in solid state materials make us see the generality, the profoundness, and the beauty of science.
This Topological Weyl Semimetals (TWSs) is a new quantum phase, which can be viewed as the hybrid of “3D graphene” and topological insulators: On one hand, a TWS possesses bulk Weyl fermions which are intriguing chiral particles embracing linear band dispersion along all three momentum directions through the Weyl points (which can be viewed as magnetic monopoles in the momentum space). On the other hand, a TWS also possesses non-trivial topological surface states that form exotic “Fermi-arcs”– unusual Fermi-surfaces consisting of unclosed curves (unlike the closed Fermi surface pockets in conventional materials) that start from and end at Weyl points of different chirality. Through a systematic study of a series of TWS candidates (Nb/Ta)(As/P) (Nature Materials, 15, 27 (2016)), Chen’s team not only adds two new members to the TWS family, but also illustrates the Fermiology evolution with the spin-orbit coupling (SOC) strength within this family of compounds.
The unusual bulk and surface electronic structures of TWSs can give rise to many exotic phenomena, such as chiral magnetic effects, negative magnetoresistance, quantum anomalous Hall effect, novel quantum oscillations in magneto-transport and quantum interference in tunnelling spectroscopy. In addition, in the collaboration research with the Max-Planck Institute, Germany, appealing transport properties have also been discovered in the TWS (Nature Physics, 11, 645 (2015)), such as the ultra-high carrier mobility (e.g. 5×106 cm2V?1s?1 in NbP) and extremely large magnetoresistance (e.g. 850,000% in NbP), making TWSs not only ideal for fundamental research, but also promising materials for novel applications.
Chinese Top 10 Science Breakthroughs of the Year selection, initiated in 2005, is held by fundamental research administration center of the Ministry of Science and Technology. Researches are recommended by CAS Bulletin and other four science journals. 30 research progresses passed the first round among 256 recommended works. The final round was organized in the form of a vote among academicians of Chinese Academy of Sciences, directors of state key laboratory and other scholars. Other breakthroughs this year include: simultaneous quantum teleportation of two inherent properties of the photon, revelation of the evolution and diversity of the Ebola virus, the first measurement of the interaction between two antiprotons, discovery of an ultra-luminous quasar with a most massive black hole of the early universe, discovery of East Asia’s earliest ancient human fossil, revelation of human primordial germ cell’s gene expression and epigenetic regulation, analysis of the key molecular mechanism of cell’s inflammatory necrosis, development of carbon-based high-efficiency photolysis aqueous-phase catalyst and single protein molecule detection by magnetic resonance.
Fig. 1. The family photo of Weyl semimetals: a. Art illustration of Weyl points, Weyl fermions and Fermi arc. b. The calculated and measured Weyl points and Fermi arcs in three transition metal pnictides. c. Extracted Weyl point splitting plotted against the spin-orbit coupling strength.
Fig. 2. DCPS of SPST researchers involved in the research. From left to right: Yulin Chen, Zhongkai Liu, Yanfeng Guo and Binghai Yan.