Eightfold Degenerate Fermions in Two Dimensions.

Area of Science:

• Condensed matter physics
• Materials science
• Quantum mechanics

Background:

• Multifold degenerate fermions are of significant research interest.
• Previous studies have lacked two-dimensional (2D) realizations.
• Exploring novel fermionic states in lower dimensions is crucial.

Purpose of the Study:

• To demonstrate the realization of eightfold degenerate fermions in 2D systems.
• To identify the symmetry protection mechanisms for these fermions.
• To predict a material candidate for hosting these exotic fermions.

Main Methods:

• Symmetry analysis and lattice model construction.
• Investigation of magnetic space groups and spin rotation symmetry.
• First-principles electronic structure calculations.

Main Results:

• Eightfold degenerate fermions can exist in 2D systems.
• Protection by gray magnetic space groups and SU(2) spin rotation symmetry in nonmagnetic materials.
• Protection by spin space groups in antiferromagnetic materials.
• Prediction of monolayer LaB$_{8}$ as a 2D eightfold degenerate Dirac node-line semimetal.

Conclusions:

• The study establishes a pathway for realizing 2D eightfold degenerate fermions.
• Monolayer LaB$_{8}$ is a promising platform for experimental investigation.
• This work advances the understanding of topological fermions in condensed matter systems.