Abstract
We predict a new class of large band gap quantum spin Hall insulators, the fluorinated PbX (X = C, Si, Ge and Sn) compounds, that are mechanically stable two-dimensional materials. Based on first principles calculations we find that, while the PbX systems are not topological insulators, all fluorinated PbX (PbXF2) compounds are 2D topological insulators. The quantum spin Hall insulating phase was confirmed by the explicitly calculation of the Z2 invariant. In addition we performed a thorough investigation of the role played by the (i) fluorine saturation, (ii) crystal field, and (iii) spin-orbital coupling in PbXF2. By considering nanoribbon structures, we verify the appearance of a pair of topologically protected Dirac-like edge states connecting the conduction and valence bands. The insulating phase which is a result of the spin orbit interaction, reveals that this new class of two dimensional materials present exceptional nontrivial band gaps, reaching values up to 0.99 eV at the Γ point, and an indirect band gap of 0.77 eV. The topological phase is arisen without any external field, making this system promising for nanoscale applications, using topological properties.
Highlights
One year after its successful synthesis[2], graphene was the first theoretical proposal of a 2D TI3
In this letter we present a new class of large band gap quantum spin Hall insulators (QSHIs) composed by a fully fluorinated binary compounds, PbXF2 (X =C, Si, Ge, and Sn)
Based on first principles calculations, we find that fluorinated binary compounds of 2D PbX, PbXF2 for X =C, Si, Ge, and Sn present topological nontrivial phase
Summary
One year after its successful synthesis[2], graphene was the first theoretical proposal of a 2D TI3. By making a suitable combination of elements with large SOC, e.g. like TlBi23, the 2D TI phase may appear at the (fully relaxed) equilibrium geometry without any external agent like strain or electric fields. In this case, we can infer that 2D systems composed by lead is quite interesting. Our first principles results shows that all PBXF2 are 2D TIs based on the explicitly calculation of Z2 invariant This group-IV compounds present quite large band gaps, reaching up to 0.77 eV, more than two times the largest band gap for a two dimensional material that has been experimentally obtained, the 2D TI stanene[17,18]. By considering nanoribbon structures, we verify the formation of spin-locked 1D Dirac-like edge states, providing further support the nontrivial phase of PbXF2
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