Quasi-two-dimensional massless Dirac fermions in CaMnSb2

We report a study of the magnetic and magnetotransport properties of layered transition-metal pnictide CaMnSb2. CaMnSb2 is a quasi-two-dimensional system with an orthorhombic crystal structure, containing two distinct layers of a distorted Sb square sheet and an edge-sharing MnSb4 tetrahedron. It was found that CaMnSb2 orders antiferromagnetically below T-N = 302 K and exhibits the expected highly anisotropic magnetic and electrical transport properties. At low temperatures and high fields, we observed quantum oscillations in both magnetization and resistivity, which reveal the multiband Fermi surfaces with very small cross-sectional areas. Nonzero Berry phase, very small cyclotron mass, and high carrier mobility indicate the existence of nearly massless Dirac fermions in CaMnSb2.