  Dirac points in two-dimensional (2D) materials have been a fascinating
subject of research, with graphene as the most prominent example. However, the
Dirac points in existing 2D materials, including graphene, are vulnerable
against spin-orbit coupling (SOC). Here, based on first-principles calculations
and theoretical analysis, we propose a new family of stable 2D materials, the
HfGeTe-family monolayers, which represent the first example to host so-called
spin-orbit Dirac points (SDPs) close to the Fermi level. These Dirac points are
special in that they are formed only under significant SOC, hence they are
intrinsically robust against SOC. We show that the existence of a pair of SDPs
are dictated by the nonsymmorphic space group symmetry of the system, which are
very robust under various types of lattice strains. The energy, the dispersion,
and the valley occupation around the Dirac points can be effectively tuned by
strain. We construct a low-energy effective model to characterize the Dirac
fermions around the SDPs. Furthermore, we find that the material is
simultaneously a 2D $\mathbb{Z}_2$ topological metal, which possesses
nontrivial $\mathbb{Z}_2$ invariant in the bulk and spin-helical edge states on
the boundary. From the calculated exfoliation energies and mechanical
properties, we show that these materials can be readily obtained in experiment
from the existing bulk materials. Our result reveals HfGeTe-family monolayers
as a promising platform for exploring spin-orbit Dirac fermions and novel
topological phases in two-dimensions.
