Large Fermi arc and robust Weyl semimetal phase in Ag2 S

Three-dimensional Dirac and Weyl semimetals have attracted widespread interest in condensed matter physics and material science. Here, based on the first-principle calculations and symmetry analysis, we report that Ag2S with P212121 symmetry is a topological Dirac semimetal in the absence of spin-orbit coupling (SOC). Every Dirac point is composed of two Weyl points with the same chirality overlapping in the momentum space. After taking the SOC into consideration, each Dirac point is split into two Weyl points. The Weyl points with the opposite chirality are far from each other in the momentum space. Therefore, the Weyl pairs are not easy to annihilate and are robust in the Ag2S compound, which also leads to long Fermi arcs on material surfaces. The robustness of the Weyl points against the strain is discussed.