|Topological Carbon: a New Perspective|
Department of Physics, Applied Physics, & Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180
地点：Room 616,Physics Building
Topological physics in solids began with carbon but drifted away due to its exceedingly small spin-orbit 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. First-principles calculations reveal similarities in their silent and often exotic physical properties.
The simplest topological carbon is polyacetylene. It is a one-dimensional (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 highly-stable Weyl semimetals with symmetry-protected loops  and surfaces  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 direct-gap, blue optoelectronic semiconductor . The recent experimentally-realized carbon honeycombs (CHCs)  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 . Such a triple-point topological metal  has becoming a forefront in today’s topological physics.
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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, two-dimensional 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 non-self-citation = 15,176 and H-index = 62 (by the count of Web of Science).