Sixfold, fourfold, and threefold excitations in the rare-earth metal carbide <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>R</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>
Lei Jin, Ying Liu, Xiaoming Zhang, Xuefang Dai, Guodong Liu
Abstract
Unconventional fermions, such as threefold, fourfold, sixfold, and eightfold fermions have attracted intense attention in recent years. However, the concrete materials hosting unconventional fermions are still in urgent scarcity. In this work, based on first-principle calculations and symmetry analysis, we reveal rich unconventional fermions in existing compound ${R}_{2}{\mathrm{C}}_{3}$ ($R=\mathrm{Y}$, La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm, Yb, Lu). We show that these compounds host quadratic dispersive threefold, linear dispersive fourfold, and sixfold points (SPs) near the Fermi level in their electronic band structures when spin-orbital coupling (SOC) is not included. We also find that a uniaxial strain can transform the unconventional fermions into other types of fermions, depending on the directions of strain. When SOC is considered, a SP transforms to an eightfold degenerate point and a fourfold degenerate point. Overall, our work provides a family of realistic materials to study the unconventional fermions.