The Electronic Properties And Topological Effects In Three-dimensional Carbon Networks

Carbon material is one of the oldest known materials for humankind,always companying the development of human beings' civilization.The knowledge of carbon materials started from the naturally existed graphite and diamond;in recent decades,people consecutively in experiments realized three novel carbon allotropes with receiving tremendous attentions,which are fullerene,carbon nanotube and graphene.Carbon has powerful ability of hybridization,can form three typical hybridized orbitals: sp,sp2 and sp3 hybridization.Carbon allotropes have abundant physical properties,especially of the optoelectronic properties,orbital physics and topological properties.The strength of spin-orbital coupling in carbon is weak enough to make the topological classification of carbon materials have a fundamentally distinct from strong spin-orbital coupling materials.The orbital physics and topological properties in carbon materials can also be generalized to other materials consisting of group-IV elements or light elements.In order to have more deeply understanding and make a generalization of the knowledge of orbital physics and topological properties in carbon materials,we performed few works as below.1.We proposed and studied a carbon-Kagome-lattice family(CKL),and found out that coexistence of Dirac and flat bands in the proximity of Fermi level are observed in this series of three-dimensional carbon structures,implying those structures maybe can serve as superconductors.The flat bands are originated from the orbital frustration of the kagome lattices,while the Dirac bands are related to the carbon zigzag chains.In addition,the flat bands are also found in the thinnest CKL slabs,only consisting of benzene rings,and would be split because of strong correlated effect in the case of hole doping.2.We proposed a new class of Weyl semimetal — Weyl-surface semimetal,based on three types of 3D graphene networks.The slabs and nanowires of those structures remain to be semimetallic with Weyl lines and points as the Fermi surfaces,respectively.Between the Weyl lines,flat surface bands emerge with possible strong magnetism.The robustness of these structures can be traced back to a bulk topological invariant,ensured by the sublattice symmetry,and to the one-dimensional Weyl semimetal behavior of zigzag carbon chain.3.We proposed and studied a new metastable three-dimensional carbon allotrope entirely composed of pentagon rings — Pentagon Carbon.Under lattice strain,Pentagon Carbon exhibits topological phase transitions,generating a series of novel topological fermions beyond the Dirac and Weyl paradigm,from isospin-1 triplet fermions,to triply-degenerate fermions,and further to Hopf-link Weyl-loop fermions.Its Landau level spectrum also exhibits distinct features.4.Based on the understanding of orbital physics in carbon materials,we absorbed the group-VI atoms on graphene and group-IV graphene-like materials to tune the relative magnitude of the internal orbital interactions,and with the aid of tiny external strain additionally,the semi-Dirac semimetal is achieved in silence oxide eventually.