Origin of the metal-insulator transition in the parent compounds of Ru-pnictide superconductors*

We study the interplay of the structural phase transition, flat electronic dispersion, and metal-to-insulator transition (MIT) in the parent compounds of the Ru-pnictide superconductors by using first-principles methods. Our electron and phonon calculations reveal that RuP and RuAs undergo MIT accompanied byorthorhombic to monoclinic distortion at low temperature, but RuSb stays orthorhombic and metallic, in agreement with the experimental findings. We find that although monoclinic distortion reduces the van Hove singularity at the Fermi level, a large monoclinic distortion is necessary for a clear MIT. Furthermore, we predict a light-induced two-step insulator-to-metal and structural transitions in the monoclinic phases of RuP and RuAs, which can be tested in future ultrafast pump-probe experiments.