Unexpected Giant-Gap Quantum Spin Hall Insulator in Chemically Decorated Plumbene Monolayer

Abstract

Quantum spin Hall (QSH) effect of two-dimensional (2D) materials features edge states that are topologically protected from backscattering by time-reversal symmetry. However, the major obstacles to the application for QSH effect are the lack of suitable QSH insulators with a large bulk gap. Here, we predict a novel class of 2D QSH insulators in X-decorated plumbene monolayers (PbX; X = H, F, Cl, Br, I) with extraordinarily giant bulk gaps from 1.03 eV to a record value of 1.34 eV. The topological characteristic of PbX mainly originates from s-px,y band inversion related to the lattice symmetry, while the effect of spin-orbital coupling (SOC) is only to open up a giant gap. Their QSH states are identified by nontrivial topological invariant Z2 = 1, as well as a single pair of topologically protected helical edge states locating inside the bulk gap. Noticeably, the QSH gaps of PbX are tunable and robust via external strain. We also propose high-dielectric-constant BN as an ideal substrate for the experimental realization of PbX, maintaining its nontrivial topology. These novel QSH insulators with giant gaps are a promising platform to enrich topological phenomena and expand potential applications at high temperature.

Two-Dimensional Large Gap Topological Insulators with Tunable Rashba Spin-Orbit Coupling in Group-IV films

“…Afterwards, several candidate materials are proposed to overcome the issue in graphene for the observation of QSH effect. These samples include silicene10, germanene11, stanene12, plumbene13, bismuth14, and arsenene15, in which they exhibit the topology properties alike to grapheme, but their nontrivial bulk gaps are still small. Motivated by the experimental synthesis of hydrogenated germanene (GeH)16, the chemical functionalization on honeycomb structure, e.g., Bi/Sb/As17181920 and Ge/Sn/Pb films21222324252627, has been proposed to host QSH effect with enhanced band gaps.…”

Semimetal behavior of bilayer stanene

“…The overlap between valence band maximum (VBM) and conduction band minimum (CBM) at the Γ point is decreased and finally a very small electronic indirect electronic gap is opened at the Γ point. To investigate the topological phase of FBSn we calculate the Z 2 topological invariant by crossing the parities of occupied bands [4,5,35,36]. Table 1 indicates the occupied bands paritiy at four TRIM points for FBSn unders 2 percent of biaxial tensile strain.…”

“…Due to the relatively large atomic mass of tin atoms the intrinsic spin-orbit interaction opens an energy gap in the electronic structure of stanene which is around 70 meV [24,28]. Theoretical calculations predict a larger gap for halogenated monolayer [24] and ethynyle-derivative [4] stanene, as well as chemically decorated plumbene [5], that suggest these structures for application in nanodevices based on topological insulators. Indeed, it was shown that oxygen adsorption may cause topological behavior to trivial phase transition in stanene nanoribbons [25].…”

“…Recently theoretical investigation of materials based on group IV elements attract considerable attention [1,2,3,4,5]. Among them, Stanene the two-dimensional (2D) tin as a topological insulators has been studied thoroughly because of being ideal candidates to quantum spin Hall (QSH) effect [6,7,8,9,10,11].…”

“…In the pursuit of large-gap QSH insulators, many novel structures have been predicted [11,17,18,19,24,4,5] including slightly buckled honeycomb lattice of tin atoms [26], also known as stanene, which recently has been fabricated by molecular beam epitaxy in laboratory [27]. Due to the relatively large atomic mass of tin atoms the intrinsic spin-orbit interaction opens an energy gap in the electronic structure of stanene which is around 70 meV [24,28].…”