# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1706.03293