Beyond Dirac and Weyl fermions: Unconventional quasiparticles in conventional crystals

TLDR

Quantum field theory classifies fermions as Majorana, Weyl, or Dirac. The authors systematically classify linear and quadratic 3‑, 6‑, and 8‑band crossings protected by space‑group symmetries in spin‑orbit coupled, time‑reversal invariant crystals. They discover several new symmetry‑protected fermionic quasiparticles beyond the Dirac, Weyl, and Majorana types, including distinct degeneracies and phenomena such as Fermi arcs and Dirac lines, and identify candidate materials via ab initio calculations.

Abstract

In quantum field theory, we learn that fermions come in three varieties: Majorana, Weyl, and Dirac. Here we show that in solid state systems this classification is incomplete and find several additional types of crystal symmetry-protected free fermionic excitations . We exhaustively classify linear and quadratic 3-, 6- and 8- band crossings stabilized by space group symmetries in solid state systems with spin-orbit coupling and time-reversal symmetry. Several distinct types of fermions arise, differentiated by their degeneracies at and along high symmetry points, lines, and surfaces. Some notable consequences of these fermions are the presence of Fermi arcs in non-Weyl systems and the existence of Dirac lines. Ab-initio calculations identify a number of materials that realize these exotic fermions close to the Fermi level.