  The recent discovery of strong spin Hall effects (SHE) in 2D layered
topological semimetals has attracted intensive attention due to its exotic
electronic properties and potential applications in spintronic devices. In this
paper, we systematically study the topological properties and intrinsic SHE of
layered transition metal carbides $M_{2}\rm{C}$ ($M=\rm{V}, \rm{Nb}, \rm{Ta}$).
The results show that both bulk and monolayer $M_{2}\rm{C}$ have
symmetry-protected nodal points (NPs) and lines (NLs) originating from the $d$
band crossing near the Fermi level ($E_F$). The inclusion of SOC breaks the
degeneracy of NLs and NPs, contributing to large spin Hall conductivity (SHC)
up to $\sim$1100 and $\sim$200 $(\hbar / e)(\Omega \mathrm{cm})^{-1}$ for bulk
and monolayer Ta$_{2}$C, respectively. Remarkably, we find that magnitude of
SHC exhibits a significant enhancement by increasing the layer number. For
eight-layer Ta$_{2}$C, the maximum value of SHC can reach up to $\sim$600
$(\hbar / e)(\Omega \mathrm{cm})^{-1}$, comparable to many reported 3D
topological materials. Analysis of spin Berry curvature reveals that the large
SHC originates from layer-number-dependent nodal line structure near the $E_F$,
in which the repeated crossover between valence and conduction bands creates
large amounts of NPs along the $\Gamma\rm{-K}$ route. Our findings not only
provide a new platform for experimental research of low-dimensional SHE, but
also suggest an effective way of realizing giant SHE by controlling layer
thickness.
