Hidden Weyl Fermions in Paramagnetic Electride Y2C
Abstract
Recent experimental observations of Weyl fermions in materials opens a new frontier of condensed matter physics. Based on first-principles calculations, we here discover Weyl fermions in a two-dimensional 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 rotation-symmetry protected Weyl nodal lines near E F, which are converted into the multiple pairs of Weyl nodes by including spin-orbit coupling (SOC). However, we reveal that, due to its small SOC effects, Y2C has a topologically nontrivial drumhead-like surface state near E F as well as a very small magnetic anisotropy energy with several μeV per unit cell, consistent with the observed surface state and paramagnetism at low temperatures below 2 K. Our findings propose that the Brillouin zone coordinates of Weyl fermions hidden in paramagnetic electride materials would fluctuate in momentum space with random orientations of the magnetization direction.