Layered compounds AMnBi 2 (A = Ca, Sr, Ba, or rare earth element) have been established as Dirac materials. Dirac electrons generated by the two-dimensional (2D) Bi square net in these materials are normally massive due to the presence of a spin-orbital coupling (SOC) induced gap at Dirac nodes. Here we report that the Sb square net in an isostructural compound BaMnSb 2 can host nearly massless Dirac fermions. We observed strong Shubnikov-de Haas (SdH) oscillations in this material. From the analyses of the SdH oscillations, we find key signatures of Dirac fermions, including light effective mass (∼0.052m 0; m 0, mass of free electron), high quantum mobility (1280 cm 2 V â 1 S â 1) and a I € Berry phase accumulated along cyclotron orbit. Compared with AMnBi 2, BaMnSb 2 also exhibits much more significant quasi two-dimensional (2D) electronic structure, with the out-of-plane transport showing nonmetallic conduction below 120 K and the ratio of the out-of-plane and in-plane resistivity reaching ∼670. Additionally, BaMnSb 2 also exhibits a G-Type antiferromagnetic order below 283 K. The combination of nearly massless Dirac fermions on quasi-2D planes with a magnetic order makes BaMnSb 2 an intriguing platform for seeking novel exotic phenomena of massless Dirac electrons.
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