Coexistence of Type-\uppercase\expandafter{\romannumeral2} and Type-\uppercase\expandafter{\romannumeral4} Dirac Fermions in SrAgBi.

Until now scientists have found tree types of Dirac Fermions in crystals and these three types of Dirac Fermions can be described by one model. Here, we find a type of Dirac fermions that escapes this model. We find this type of Dirac fermions can exist in SrAgBi and is dubbed type-\uppercase\expandafter{\romannumeral4} Dirac Fermions. The band near the type-\uppercase\expandafter{\romannumeral4} Dirac points is nonlinear and the electron pocket and the hole pocket are from the same band. It is worth pointing out that there is a type-\uppercase\expandafter{\romannumeral2} Dirac Fermion near this new Dirac Fermion. So we used two models to describe the coexistence of these two Dirac fermions. Topological surface states of these two Dirac points are also calculated. We envision that our findings will stimulate researchers to study novel physics of type-\uppercase\expandafter{\romannumeral4} Dirac fermions, as well as the interplay of type-\uppercase\expandafter{\romannumeral2} and type-\uppercase\expandafter{\romannumeral4} Dirac fermions.