Two dimensional Dirac fermions and quantum magnetoresistance in CaMnBi${}_2$

We report two dimensional Dirac fermions and quantum magnetoresistance in single crystals of CaMnBi${}_2$. The non-zero Berry’s phase, small cyclotron resonant mass and first-principle band structure suggest the existence of the Dirac fermions in the Bi square nets. The in-plane transverse magnetoresistance exhibits a crossover at a critical field $B^{\ast }$ from semiclassical weak-field $B^2$ dependence to the high-field unsaturated linear magnetoresistance ($\sim 120%$ in 9 T at 2 K) due to the quantum limit of the Dirac fermions. The temperature dependence of $B^{\ast }$ 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 CaMnBi${}_2$ with Bi square nets.