Semimetallic carbon allotrope with a topological nodal line in mixed sp2-sp3 bonding networks

Graphene is known as a two-dimensional Dirac semimetal, in which electron states are described by the Dirac equation of relativistic quantum mechanics. Three-dimensional analogs of graphene are characterized by Dirac points or lines in momentum space, which are protected by symmetry. Here, we report a novel 3D carbon allotrope belonging to a class of topological nodal line semimetals, discovered using an evolutionary structure search method. The new carbon phase in the monoclinic C2/m space group, termed m-C8, consists of five-membered rings with sp3 bonding interconnected by sp2-bonded carbon networks. Enthalpy calculations reveal that m-C8 is more favorable than recently reported topological semimetallic carbon allotropes, and the dynamic stability of m-C8 is verified by phonon spectra and molecular dynamics simulations. Simulated X-ray diffraction patterns indicate that m-C8 could be one of the unidentified carbon phases observed in detonation shoot. The analysis of electronic properties indicates that m-C8 exhibits a nodal line protected by both inversion and time-reversal symmetries in the absence of spin-orbit coupling and the surface band connecting the projected nodal points. Our results may help design new carbon allotropes with exotic electronic properties. A 3D form of graphene made from interlinked ribbons may hold surface states where charge carriers act as massless, high-speed particles. Topological semimetals are materials whose valence and conduction bands intersect and create localized zones with special magnetic and electric properties. While pristine graphene molecules do not show topological behavior, Kee Joo Chang from Korea's KAIST and colleagues predict that coupling bands of five-membered carbon rings into a 3D framework can generate these intriguing states. The team used global optimization algorithms to search for 3D graphene with plentiful connections between single- and double-bonded carbons, and found a new, thermodynamically stable phase with a band structure that crosses along curved paths. X-ray simulations revealed that this allotrope may naturally occur in soot produced by high-pressure explosions, bolstering prospects for experimental synthesis. We report a new carbon allotrope belonging to a class of topological nodal line semimetals, based on global optimization and first-principles density functional calculations. The new carbon phase in monoclinic C2/m space group, termed m-C8, consists of five-membered rings with sp3 hybridized bonds and sp2-bonded carbon networks. The band structure exhibits linear dispersions around the Fermi level where the valence and conduction bands touch each other. Based on the analysis of X-ray diffraction spectra and enthalpy-pressure curves, we propose that m-C8 can be present in detonation soot and a phase transition from graphite to m-C8 can occur under pressure.