LAUSR

Two-dimensional (2D) topological insulators (TIs) have attracted tremendous research interest from both theoretical and experimental fields in recent years. However, it is much less investigated in realizing node line (NL) semimetals in 2D materials.Combining first-principles calculations and $k \cdot p$ model, we find that NL phases emerge in p-CS$_2$ and p-SiS$_2$, as well as other pentagonal IVX$_2$ films, i.e. p-IVX$_2$ (IV= C, Si, Ge, Sn, Pb; X=S, Se, Te) in the absence of spin-orbital coupling (SOC). The NLs in p-IVX$_2$ form symbolic Fermi loops centered around the $\Gamma$ point and are protected by mirror reflection symmetry. As the atomic number is downward shifted, the NL semimetals are driven into 2D TIs with the large bulk gap up to 0.715 eV induced by the remarkable SOC effect.The nontrivial bulk gap can be tunable under external biaxial and uniaxial strain. Moreover, we also propose a quantum well by sandwiching p-PbTe$_2$ crystal between two NaI sheets, in which p-PbTe$_2$ still keeps its nontrivial topology with a sizable band gap ($\sim$ 0.5 eV). These findings provide a new 2D materials family for future design and fabrication of NL semimetals and TIs.