Quantum spin Hall effect in Ta 2 M 3 Te 5 ( M = Pd , Ni )

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 ${\mathrm{Ta}}_{2}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{5}$ can be a QSH insulator based on first-principles calculations. The interlayer binding energy in the layered van der Waals compound ${\mathrm{Ta}}_{2}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{5}$ is $19.6\phantom{\rule{0.16em}{0ex}}\mathrm{meV}/{\AA{}}^{2}$; 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 $\mathrm{Ta}\text{\ensuremath{-}}5d$ and $\mathrm{Pd}\text{\ensuremath{-}}4d$ orbitals without spin-orbit coupling (SOC). The SOC effect opens a global gap and makes the system a QSH insulator. With the $d\text{\ensuremath{-}}d$ band-inverted feature, the nontrivial topology in monolayer ${\mathrm{Ta}}_{2}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{5}$ 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 ${\mathrm{Ta}}_{2}{M}_{3}{\mathrm{Te}}_{5}$ ($M=\mathrm{Pd}$, Ni) can be tuned by external strain. These monolayers and thin films provide feasible platforms for realizing QSH effect as well as related devices.