  Quantum spin Hall (QSH) effect with great promise for the potential
application in spintronics and quantum computing has attracted extensive
research interest from both theoretical and experimental researchers. Here, we
predict monolayer Ta$_2$Pd$_3$Te$_5$ can be a QSH insulator based on
first-principles calculations. The interlayer binding energy in the layered van
der Waals compound Ta$_2$Pd$_3$Te$_5$ is 19.6 meV/A$^2$; thus, its
monolayer/thin-film structures could be readily obtained by exfoliation. The
band inversion near the Fermi level ($E_F$) is an intrinsic characteristic,
which happens between Ta-$5d$ and Pd-$4d$ orbitals without spin-orbit coupling
(SOC). The SOC effect opens a global gap and makes the system a QSH insulator.
With the $d$-$d$ band-inverted feature, the nontrivial topology in monolayer
Ta$_2$Pd$_3$Te$_5$ is characterized by the time-reversal topological invariant
$\mathbb Z_2=1$, which is computed by the one-dimensional (1D) Wilson loop
method as implemented in our first-principles calculations. The helical edge
modes are also obtained using surface Green's function method. Our calculations
show that the QSH state in Ta$_2M_3$Te$_5$ ($M=$ Pd, Ni) can be tuned by
external strain. These monolayers and thin films provide feasible platforms for
realizing QSH effect as well as related devices.
