  The Dirac materials, such as graphene and three-dimensional topological
insulators, have attracted much attention because they exhibit novel quantum
phenomena with their low energy electrons governed by the relativistic Dirac
equations. One particular interest is to generate Dirac cone anisotropy so that
the electrons can propagate differently from one direction to the other,
creating an additional tunability for new properties and applications. While
various theoretical approaches have been proposed to make the isotropic Dirac
cones of graphene into anisotropic ones, it has not yet been met with success.
There are also some theoretical predictions and/or experimental indications of
anisotropic Dirac cone in novel topological insulators and AMnBi2 (A=Sr and Ca)
but more experimental investigations are needed. Here we report systematic high
resolution angle-resolved photoemission measurements that have provided direct
evidence on the existence of strongly anisotropic Dirac cones in SrMnBi2 and
CaMnBi2. Distinct behaviors of the Dirac cones between SrMnBi2 and CaMnBi2 are
also observed. These results have provided important information on the strong
anisotropy of the Dirac cones in AMnBi2 system that can be governed by the
spin-orbital coupling and the local environment surrounding the Bi square net.
