Three-dimensional Pentagon Carbon with a genesis of emergent fermions

TL;DR

This paper introduces a new three-dimensional pentagon-based carbon allotrope with unique electronic properties and emergent topological fermions, revealing novel quasiparticles and phase transitions under strain.

Contribution

It reports the discovery of a metastable pentagon-based carbon structure with unprecedented topological fermions and phase transitions, expanding the understanding of carbon allotropes and topological materials.

Findings

Discovery of a pentagon-only 3D carbon allotrope.

Observation of topological phase transitions under strain.

Identification of novel quasiparticles like isospin-1 triplet fermions.

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 concatenated 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.