Topological Carbon: a New Perspective 
Time：20161121 
Department of Physics, Applied Physics, & Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180 时间：Tues,20161129,10:0011:00 地点：Room 616,Physics Building Abstract: Topological physics in solids began with carbon but drifted away due to its exceedingly small spinorbit coupling (SOC), which is thought to be essential for any observable effect. Here, I will discuss a different topological classification within carbon that is above the spin degree of freedom, has no need for the SOC, and hence can be observed at any reasonable temperature. It is based on the orbital symmetry, namely, that of the orbital of a carbon. Both spin and orbit are angular degrees of freedoms and both effects may be viewed as a result of (spherical)symmetry breaking – in the former by SOC whereas in the latter by the formation of bonded carbon network. Firstprinciples calculations reveal similarities in their silent and often exotic physical properties.
The simplest topological carbon is polyacetylene. It is a onedimensional (1D) orbital (semimetal) chain with a Dirac point right at the Fermi level. Starting with the polyacetylene, one can construct a whole family of topological matter. Graphene is a celebrated example, where parallel placement of the chains in 2D gives rise to Dirac cones. In 3D carbon, one may have two or three such chain sets intercepting with each other. Various arrangements of the two chain sets give rise to highlystable Weyl semimetals with symmetryprotected loops [1] and surfaces [2] as their Fermi surfaces, which can be reduced to Weyl points with characteristic Fermi arcs for the surface states. Upon breaking the topological protection, e.g., by a biaxial strain, one may also obtain 3D Kagome lattice as a remarkable directgap, blue optoelectronic semiconductor [3]. The recent experimentallyrealized carbon honeycombs (CHCs) [4] may be the first experimental realization of 3D topological carbon. It belongs to the family of three chain set. Besides its interesting but “trivial” applications such as gas storage that have excited the condensed matter physics community, CHC processes 3D Dirac cone that intercepts with a third flat band [5]. Such a triplepoint topological metal [6] has becoming a forefront in today’s topological physics.
[1] Y.P. Chen, et al., Nano Lett. 15, 6974 (2015). [2] C. Zhong, et al., Nanoscale 8, 7232 (2016). [3] Y.P. Chen Y, et al., Phys. Rev. Lett. 113, 085501 (2014). [4] N. V. Krainyukova and E. N. Zubarev, Phys. Rev. Lett. 116, 055501 (2016). [5] Y. Gao, et al., Nanoscale 8, 12863 (2016). [6] Chang, et al., arXiv:1605.06831v1.
Biography: Shengbai Zhang received his Ph. D. in Physics from the University of California at Berkeley in 1989. He moved to Xerox PARC as a postdoc, before joining the National Renewable Energy Laboratory in 1991. In 2008, he became the Senior Kodosky Constellation Chair Professor and Professor in Physics at Rensselaer Polytechnic Institute. His computational research covers a wide range of materials for bulk properties, defect structures, and surface physics. His recent work involves green photovoltaic materials, ultrafast phase change memory materials, topological carbon, twodimensional layered structures, and excited state dynamics. He has published 57 PRL, 5 JACS, 4 Nature Communications, 3 PNAS, and 11 Nano Letters with a total nonselfcitation = 15,176 and Hindex = 62 (by the count of Web of Science).
