Nontrivial topological electronic structures in a single Bi(111) bilayer on different substrates: A first-principles study

Abstract

Electronic structures, minimum energy configurations, and band topology of strained Bi(111) single bilayers placed on a variety of semiconducting and insulating substrates are investigated using first-principles calculations. A topological phase diagram of a free-standing Bi bilayer is presented to help guide the selection of suitable substrates. The insulating hexagonal-BN is identified as the best candidate substrate material for supporting nontrivial topological insulating phase of Bi bilayer thin films. A planar hexagonal Bi layer is predicted under tensile strain, which we show could be realized on a SiC substrate. The Bi bilayer becomes metallic under the compressive strain induced by Si and Ge substrates.The recent discovery of topological insulators (TIs) which possess a nontrivial Z 2 topological invariant is attracting worldwide attention, making this a fast developing area in materials sciences. 1-4 TIs host spin-polarized surface/edge states, which are not allowed to backscatter due to the constraint of time-reversal symmetry and are thus very desirable for spintronics and other applications. 5,6 The development of topological band theory combined with the predictive power of first-principles calculations, has led to the discovery of many new families of TIs such as Li 2 AgSb, 18 ternary tetradymite, 19 quaternary chalcogenides, and famatinites. 20 While a number of three-dimensional (3D) topological insulators have been realized experimentally, there are only very few materials realizations of the two-dimensional (2D) TIs, also referred to as quantum spin Hall (QSH) insulators. The key feature of a QSH insulator is the presence of protected gapless edge states which carry two spin-polarized currents propagating in opposite directions. Graphene was the first system which was proposed to support a QSH state through spin-orbit coupling effects, but the associated gap in graphene is too small to be accessible experimentally. 4 To date, the only experimental realizations of the QSH state are HgTe/CdTe (Refs. 21-23) and InAs/GaSb/AlSb (Refs. 24 and 25) quantum well systems. No stand-alone thin film or a thin film supported on a suitable substrate has been experimentally demonstrated to harbor a QSH state. The great need for finding new QSH insulator materials is for these reasons clear.Theoretical studies have shown the sensitivity of the Z 2 topological invariant to film thickness in Bi 2 Se 3 and Bi 2 Te 3 ultrathin films, 26,27 suggesting that the 2D QSH phase could be induced through the reduced dimensionality in thin films of 3D TIs. 28-30 Accordingly, the search for 2D QSH phases has focused on Bi and Sb films in view of their strong spin-orbit interaction, 31-35 and the fact that Bi/Sb alloys were the first 3D TIs to be discovered experimentally. 36 In particular, a single Bi(111) bilayer (BL) film has been predicted to be an elemental 2D QSH insulator, 31,32 and ultrathin Bi(111) films are predicted to be topologically nontrivial for a wide range of film thicknesses. 33 In contrast, Sb(111) films w...

Topological phases in two-dimensional materials: a review

“…The influence of several traditional substrates, e.g. hexagonal boron-nitride (h-BN) and silicon-carbide [165], has been intensively studied. It has been shown that h-BN is an ideal substrate for stabilizing the topologically nontrivial phase of a freestanding Bi bilayer.…”

Large-gap quantum spin Hall state in functionalized dumbbell stanene

“…Considering compatibility of lattice constant, we explore strain-driven TI phase transition in DB-SnCH 3 . Figure 3 SiC is an important semiconductor to support 2D materials to realize QSHE, including Bi bilayers [47] . Here, we construct two kinds of HTSs, such as monolayer (ML) SiC and 5-ML SiC layers, to support DB-SnCH 3 .…”

The nontrivial electronic structure of Bi/Sb honeycombs on SiC(0001)

“…Beyond carbon [12], the predicted 2D-TIs include films of silicon, germanium [13], tin [14], and bismuth [15], and InBi, GaBi, and TlBi [16] alloys at their equilibrium honeycomb structures, while arsenic [17], antimony [18], BBi, and AlBi [16] transition to a 2D TI phase under strain. Several of the newly predicted 2D-TIs exhibit band gaps large enough to exceed thermal excitation energy at room temperature [15,16,[18][19][20]. Notably, in analogy with graphene, silicene, germanene, and stanene [21], we may refer to a monolayer film of bismuth and antimony of interest to this study as bismuthene and antimonene.…”