  Layered compounds AMnBi2 (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 BaMnSb2 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
(~0.052m0; m0, mass of free electron), high quantum mobility (1280 cm2V-1S-1)
and a Pi Berry phase accumulated along cyclotron orbit. Compared with AMnBi2,
BaMnSb2 also exhibits much more significant quasi two-dimensional (2D)
electronic structure, with the out-of-plane transport showing nonmetallic
conduction below 120K and the ratio of the out-of-plane and in-plane
resistivity reaching ~670. Additionally, BaMnSb2 also exhibits an
antiferromagnetic order with a weak ferromagnetic component. The combination of
nearly massless Dirac fermions on quasi-2D planes with a magnetic order makes
BaMnSb2 an intriguing platform for seeking novel exotic phenomena of massless
Dirac electrons.
