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 research interest from both the theoretical and the 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 symmetry analysis, we find that NL phases emerge in p-CS2 and p-SiS2, as well as other pentagonal IVX2 films, i.e., p-IVX2 (IV=C, Si, Ge, Sn, Pb; X=S, Se, Te) in the absence of spin-orbit coupling (SOC). The NLs in p-IVX2 consist of 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 strain and uniaxial strain. Moreover, we also propose a quantum well by sandwiching a p-PbTe2 crystal between two NaI sheets in which p-PbTe2 still keeps its nontrivial topology with a sizable band gap (similar to 0.5 eV). These findings provide a new 2D material platform for exploring fascinating physics in both NL semimetals and TIs.