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A family photo of Topological Weyl Semimetal using an "Electron Camera"
Author:            Date:2015-11-18            Browse:433

Different branches of science typically deal with very different concepts and research subjects that seldom overlap. Though once in a while, a common idea can emerge, propagate across different fields, and lead to new discoveries, which are rare and appreciated by scientists in all fields. They make us see both the generality, the profoundness, and the beauty of science.

Recent discovery of massless Dirac fermions in graphene, topological insulators and topological Dirac semimetals; and the proposal of Majorana fermions in topological superconductors are such examples. Although the concepts of both Dirac and Majorana fermions were originated from high energy physics, their existence in solid materials has ignited tremendous interest and generated many exciting results in condensed matter physics.

More recently, a new state of quantum matter – the 3D topological Weyl semimetal (TWS) was proposed to exist in solid. This type of materials 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.

These unusual bulk and surface electronic structures of 3D 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, appealing transport properties have also been discovered in some 3D TWS candidates, such as the ultra-high carrier mobility (e.g. 5×106cm2V?1s?1in NbP) and extremely large magnetoresistance (e.g. 850,000% in NbP), making 3D TWSs not only ideal for fundamental research, but also promising materials for novel applications.

To search for TWSs, the most direct way is to look for their unique electronic structures that contains the characteristic “Fermi-arcs” – which is topologically distinct from the Fermi-surfaces of conventional materials whose Fermi-surfaces have to be closed pockets. Working with other researchers, scientists from Division of Condensed matter Physics and Photon Science (DCPS) in School of Physical Science and Technology (SPST) at ShanghaiTech University have made significant contributions in the discovery of TWS.

In a work just published online in Nature Materials on Nov. 3rd, SPST researchers reported the electronic structures of most (three) of the TWS candidates (NbP, TaP and TaAs) in a transition metal monopnictide family. The researchers carried out comprehensive study on these exotic materials using both angle resolved photoemission spectroscopy (ARPES) and ab initiocalculations, and clearly identified the characteristic surface “Fermi-arcs” in all three compounds.This work not only adds two new members to the TWS family, it also illustrates the Fermiology evolution with the spin-orbit coupling (SOC) strength within this family of compounds. "This finding proves SOC as the underlying mechanism to realize and fine-tune the electronic structures of a TWS." said Prof. Zhongkai Liu from SPST, the lead author of the work,"It also gives us a ‘knob’ to control the properties of TWS materials."

The discovery provides a rich material base for exploring both unusual physical phenomena (e.g. chiral magnetic effects, negative magnetoresistance, and quantum anomalous Hall effect) and novel future applications (due to their appealing physical properties).

Research conducted by: Z. K. Liu, Y.F. Guo, B.H. Yan and Y.L. Chen (Division of Condensed Matter Physics and Photon Science, SPST, ShanghaiTech University) in collaboration with researchers from Oxford University, Tsinghua University, Max-Planck Institute at Dresden (Germany).

The full credit of this research: Z. K. Liu, L. X. Yang, Y. Sun, T. Zhang, H. Peng, H. F. Yang, C. Chen. Y. Zhang, Y. F. Guo, D. Prabhakaran, M. Schmidt, Z. Hussain, S.-K. Mo, C. Felser, B. Yan and Y. L. Chen, “Evolution of the Fermi surface of Weyl semimetals in the transition metal pnictide family” Nature Materials, advance online publication,http://dx.doi.org/10.1038/nmat4457

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.