Beyond Dirac and Weyl fermions: Unconventional quasiparticles in conventional crystals

TL;DR

This paper uncovers new types of fermionic quasiparticles in crystals beyond the traditional Dirac and Weyl types, classifying their symmetries and identifying real materials hosting these exotic states.

Contribution

It provides a comprehensive classification of novel fermionic excitations in crystals, expanding the understanding of fermion types beyond standard quantum field theory.

Findings

Identification of new fermionic quasiparticles in crystals

Discovery of materials hosting these exotic fermions

Prediction of Fermi arcs and Dirac lines in non-Weyl systems

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.