Three-dimensional topological critical Dirac semimetal in AMgBi(A= K, Rb, Cs)

We predicted, using first principles calculation, that $A\mathrm{MgBi}$ ($A=$ K, Rb, Cs) are symmetry-protected topological semimetals near the boundary of type-I and type-II Dirac semimetal phases, dubbed topological critical Dirac semimetals. In-plane compressive strain or doping Rb or Cs into KMgBi can drive the transition between the two phases. An effective theory is developed to describe the bands near the Fermi energy, by which we calculate the surface Fermi arcs and the Landau levels throughout the transition. We predict the key features of critical Dirac semimetals that can be observed in photoemission, quantum oscillation, and transport measurements.