Two-dimensional Dirac fermions and quantum magnetoresistance in CaMnBi2

We report two-dimensional Dirac fermions and quantum magnetoresistance in single crystals of CaMnBi2. The nonzero Berry's phase, small cyclotron resonant mass, and first-principles band structure suggest the existence of the Dirac fermions in Bi square nets. The in-plane transverse magnetoresistance exhibits a crossover at a critical field B* from semiclassical weak-field B-2 dependence to the high-field unsaturated linear magnetoresistance (similar to 120% in 9 T at 2 K) due to the quantum limit of the Dirac fermions. The temperature dependence of B* satisfies quadratic behavior, which is attributed to the splitting of linear energy dispersion in high field. Our results demonstrate the existence of two-dimensional Dirac fermions in CaMnBi2 with Bi square nets.