Theoretical prediction of Weyl fermions in the paramagnetic electride Y2 C

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 ${\mathrm{Y}}_{2}\mathrm{C}$. We find that the Y $4d$ orbitals and the anionic $s$-like orbital confined in the interstitial spaces between ${[{\mathrm{Y}}_{2}\mathrm{C}]}^{2+}$ cationic layers are hybridized to give rise to van Have singularities near the Fermi energy ${E}_{\mathrm{F}}$, 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 ${E}_{\mathrm{F}}$, which are converted into the multiple pairs of Weyl nodes by including spin-orbit coupling. Furthermore, we find that ${\mathrm{Y}}_{2}\mathrm{C}$ has a topologically nontrivial surface state near ${E}_{\mathrm{F}}$ as well as a tiny magnetic anisotropy energy, consistent with the observed surface state and paramagnetism at low temperatures below $\ensuremath{\sim}2\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. 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.