Observation of unconventional six-fold, four-fold and three-fold excitations in rare-earth-metal carbide Re2C3

TL;DR

This study identifies Re2C3 compounds as hosts for various unconventional fermions, revealing their electronic structures and how strain and spin-orbit coupling influence their fermionic states, thus offering new materials for fermion research.

Contribution

The paper provides first-principle calculations and symmetry analysis showing Re2C3 compounds host multiple types of unconventional fermions near the Fermi level, including strain and SOC effects.

Findings

Re2C3 compounds host three-fold, four-fold, six-fold, and eight-fold fermions.

Yb2C3 has a clean band structure with fermions close to the Fermi level.

Strain and SOC can transform fermion types in Re2C3.

Abstract

Unconventional fermions, such as three-fold, four-fold, six-fold, and eight-fold fermions have attracted intense attention in recent years. However, the concrete materials hosting unconventional fermions are still in urgent scarcity. In this work, based first-principle calculations and symmetry analysis, we reveal rich unconventional fermions in existing compound Re2C3 (Re = Y, La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm, Yb, Lu). We show that these compounds host quadratic dispersive three-fold (TP), linear dispersive four-fold (FP) and six-fold points (SP) near the Fermi level in their electric band structures when spin-orbital coupling (SOC) is not included. Notably, the FP is charge-2 Dirac-like point. More importantly, among compound Re2C3, the compound Yb2C3 has very clean band structure, and its unconventional fermions are closed to the Fermi level. We also find that a uniaxial strain can transform the unconventional fermions into other types fermions, depending on the directions of strain. When SOC is considered, a SP transform to an eightfold degenerate point and a fourfold degenerate point. Overall, our work provides a family of realistic materials to study the unconventional fermions.