Nearly massless Dirac fermions hosted by Sb square net in BaMnSb2

<jats:title>Abstract</jats:title><jats:p>Layered compounds AMnBi<jats:sub>2</jats:sub> (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<jats:sub>2</jats:sub> 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 (~0.052<jats:italic>m</jats:italic><jats:sub>0</jats:sub>; <jats:italic>m</jats:italic><jats:sub>0</jats:sub>, mass of free electron), high quantum mobility (1280 cm<jats:sup>2</jats:sup>V<jats:sup>−1</jats:sup>S<jats:sup>−1</jats:sup>) and a π Berry phase accumulated along cyclotron orbit. Compared with AMnBi<jats:sub>2</jats:sub>, BaMnSb<jats:sub>2</jats:sub> 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<jats:sub>2</jats:sub> 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<jats:sub>2</jats:sub> an intriguing platform for seeking novel exotic phenomena of massless Dirac electrons.</jats:p>