  Nonsymmorphic space group symmetries can generate exotic band-crossings in
topological metals and semimetals. Here, based on symmetry analysis and
first-principles calculations, we reveal rich band-crossing features in the
existing layered compounds Ta$_3$SiTe$_6$ and Nb$_3$SiTe$_6$, enabled by
nonsymmorphic symmetries. We show that in the absence of spin-orbit coupling
(SOC), these three-dimensional (3D) bulk materials possess accidental Dirac
loops and essential fourfold nodal lines. In the presence of SOC, there emerges
an hourglass Dirac loop---a fourfold degenerate nodal loop, on which each point
is a neck-point of an hourglass-type dispersion. We show that this interesting
type of band-crossing is protected and dictated by the nonsymmorphic space
group symmetries, and it gives rise to drumhead-like surface states.
Furthermore, we also investigate these materials in the monolayer form. We show
that these two-dimensional (2D) monolayers host nodal lines in the absence of
SOC, and the nodal lines transform to essential spin-orbit Dirac points when
SOC is included. Our work suggests a realistic material platform for exploring
the fascinating physics associated with nonsymmorphic band-crossings in both 3D
and 2D systems.
