Theoretical prediction of Weyl fermions in the paramagnetic electride Y2C

Abstract

Recent experimental observations of Weyl fermions in materials open a new frontier of condensed-matter physics. Based on first-principles calculations, we here discover the Weyl fermions in a two-dimensional (2D) layered electride material Y2C. We find that the Y 4d orbitals and the anionic s-like orbital confined in the interstitial spaces between [Y2C](2+) cationic layers are hybridized to give rise to van Have singularities near the Fermi energy E-F, which induce a ferromagnetic (FM) order via the Stoner-type instability. This FM phase with broken time-reversal symmetry hosts the Weyl nodal lines near E-F, which are converted into the multiple pairs of Weyl nodes by including spin-orbit coupling. Furthermore, we find that Y2C has a topologically nontrivial surface state near E-F as well as a tiny magnetic anisotropy energy, consistent with the observed surface state and paramagnetism at low temperatures below similar to 2 K. Our findings demonstrate the existence of Weyl fermions in a 2D electride material thereby providing a platform to study the interesting interplay of Weyl fermion physics and electride materials.