Black-hole horizon in the Dirac semimetal Zn2In2 S5

Recently, realizing new fermions, such as type-I and type-II Dirac/Weyl fermions in condensed matter systems, has attracted considerable attention. Here, we show that the transition state from type-I to type-II Dirac fermions can be viewed as a "type-III" Dirac fermion, which exhibits unique characteristics, including a Dirac-line Fermi surface with a nontrivial topological invariant and critical chiral anomaly effect, distinct from previously known Dirac semimetals. Most importantly, we discover Zn2In2S5 is a type-III Dirac semimetal, characterized by a pair of Dirac points in the bulk and Fermi arcs on the surface. We further propose a solid-state realization of the black-hole-horizon analog in inhomogeneous Zn2In2In5 to simulate black-hole evaporation with a high Hawking temperature. We envision that our findings will stimulate researchers to study the physics of type-III Dirac fermions, as well as astronomical problems in a condensed matter analog.