From node-line semimetals to large-gap quantum spin Hall states in a family of pentagonal group-IVA chalcogenide

Two-dimensional (2D) topological insulators (TIs) have attracted tremendous\nresearch interest from both theoretical and experimental fields in recent\nyears. However, it is much less investigated in realizing node line (NL)\nsemimetals in 2D materials.Combining first-principles calculations and $k \\cdot\np$ model, we find that NL phases emerge in p-CS$_2$ and p-SiS$_2$, as well as\nother pentagonal IVX$_2$ films, i.e. p-IVX$_2$ (IV= C, Si, Ge, Sn, Pb; X=S, Se,\nTe) in the absence of spin-orbital coupling (SOC). The NLs in p-IVX$_2$ form\nsymbolic Fermi loops centered around the $\\Gamma$ point and are protected by\nmirror reflection symmetry. As the atomic number is downward shifted, the NL\nsemimetals are driven into 2D TIs with the large bulk gap up to 0.715 eV\ninduced by the remarkable SOC effect.The nontrivial bulk gap can be tunable\nunder external biaxial and uniaxial strain. Moreover, we also propose a quantum\nwell by sandwiching p-PbTe$_2$ crystal between two NaI sheets, in which\np-PbTe$_2$ still keeps its nontrivial topology with a sizable band gap ($\\sim$\n0.5 eV). These findings provide a new 2D materials family for future design and\nfabrication of NL semimetals and TIs.\n