Abstract
Carbon, the basic building block of our universe, enjoys a vast number of allotropic structures. Owing to its bonding characteristic, most carbon allotropes possess the motif of hexagonal rings. Here, with first-principles calculations, we discover a new metastable three-dimensional carbon allotrope entirely composed of pentagon rings. The unique structure of this Pentagon Carbon leads to extraordinary electronic properties, making it a cornucopia of emergent topological fermions. Under lattice strain, Pentagon Carbon exhibits topological phase transitions, generating a series of novel quasiparticles, 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, including a huge number of almost degenerate chiral Landau bands, implying pronounced magneto-transport signals. Our work not only discovers a remarkable carbon allotrope with highly rare structural motifs, it also reveals a fascinating hierarchical particle genesis with novel topological fermions beyond the Dirac and Weyl paradigm.
Highlights
Carbon, the basic building block of our universe, enjoys a vast number of allotropic structures
Our work discovers a new metastable 3D carbon allotrope—the Pentagon Carbon, which exhibits a beautiful lattice structure entirely composed of pentagonal rings—a highly rare lattice motif for 3D crystals
The triply degenerate points here are distinct from those previous examples in the following aspects: first, due to the negligible SOC, Pentagon Carbon is essentially a spinless system, and, as we mentioned at the beginning, spinful and spinless systems belong to different symmetry/topological classes
Summary
The basic building block of our universe, enjoys a vast number of allotropic structures. Pentagon Carbon exhibits topological phase transitions, generating a series of novel quasiparticles, from isospin-1 triplet fermions to triply degenerate fermions and further to Hopf-link Weyl-loop fermions. The research on topological metals and semimetals (TMs) has been attracting tremendous interest[7,8,9,10,11,12,13,14] In these materials, topologically protected quasiparticles emerge at protected band-crossing points around Fermi Level, giving rise to condensed matter realizations of many peculiar fermions with a variety of distinct electronic properties. The breaking of Lorentz symmetry at lattice scale makes it possible to have TMs that can host new types of fermions with different degeneracies beyond the Dirac and Weyl fermions[21,22,23] These new fermions are actively searched for, and if realized, they are believed to host a wealth of exotic physical properties unprecedented in high-energy physics. We discuss a possible reaction pathway for the experimental synthesis of Pentagon Carbon
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