Magnetic gating of a 2D topological insulator

Abstract

The compound Bi 14 Rh 3 I 9 has recently been suggested as a weak 3D topological insulator (TI) on the basis of angle-resolved photoemission and scanning-tunneling experiments in combination with density-functional (DF) electronic structure calculations. These methods unanimously support the topological character of the headline compound, but a compelling confirmation could only be obtained by dedicated transport experiments. The latter, however, are biased by an intrinsic n-doping of the material's surface due to its polarity. Electronic reconstruction of the polar surface shifts the topological gap below the Fermi energy, which would also prevent any future device application. Here, we report the results of DF slab calculations for chemically gated and counter-doped surfaces of Bi 14 Rh 3 I 9. We demonstrate that both methods can be used to compensate the surface polarity without closing the electronic gap.tum spin Hall effect In recent years, topological insulators (TIs) have attracted enormous attention due to their intriguing properties. Of particular interest are their massless Dirac-cone-like surface states protected by time-reversal symmetry (TRS). 1-3 In a nutshell, TIs are characterized by these gapless surface states and a bulk energy gap. Three dimensional (3D) TIs are called strong or weak based on four Z 2 invariants (ν 0 ; ν 1 , ν 2 , ν 3). If ν 0 = 1, the material is a strong TI; if ν 0 = 0 and any of the indices (ν 1 , ν 2 , ν 3) is equal to one, it is a weak TI. 4,5 In the former case, including the well-known compounds Bi 2 Se 3 and Bi 2 Te 3 , 1,2 the TRS-protected surface states are present on all facets, while in the latter case, such surface states are present only on certain facets. Their peculiar properties bear the potential to enable novel types of information processing. 6-8 An important step toward so-called topological quantum computing is the recent observation of signatures of Majorana fermion modes. 9 In closer reach could be an application of the recently observed excellent thermoelectric properties of the title compound 10 or of the all-electrical detection of spin polarization by combination of quantum spin Hall and metallic spin Hall transport in a single device. 7 Weak 3D TIs suggested hitherto are usually hosted by layered crystal structures. 11 The strength of the related inter-layer coupling influences the bulk band structure: (i) In Bi 2 TeI with strong inter-layer coupling, this coupling is essential for the formation of the weak 3D TI state; 12 (ii) a weak interlayer coupling, on the other hand, results in a quasi two-dimensional (2D) band structure. This situation is found in KHgSb, 13 ZrTe 5 14 or Bi 14 Rh 3 I 9. 15,16 Weak 3D TIs of the second kind may allow to produce 2D TI structures that are expected to show the quantum spin-Hall (QSH) effect. This can be achieved by cleaving off thin layers from the bulk 3D TI as an alternative way to the fabrication of quantum wells. 17,18 Indeed, 2 a single, charge-compensated layer of Bi 14 Rh 3 I 9 was predicted to be a 2D T...

Conductance of quantum spin Hall edge states from first principles: The critical role of magnetic impurities and inter-edge scattering

“…It is interesting to notice that a similar mechanism has been proposed very recently in the framework of the Kane-Mele-Hubbard model in graphene, where the breakdown of time reversal symmetry at vacancy defects is shown to lead to a corresponding breakdown of conductance quantization 23 . The present ab-initio calculations support this picture and also bear similarities with simple theoretical models in which magnetic scatterers, such as magnetic adatoms or ferromagnetic gates, are introduced in 2D TIs 39,40 .…”

Quasicrystalline Chern Insulators

“…In contrast, they can move dissipationlessly along the edge in Fig. 4 (b) and (c)), preserving the integer conductance.Similar breakdown is also observed in timereversal symmetry protected topological insulators with magnetic [54,55] or nonmagnetic [56] impurities due to strong backscattering or antiresonance, but is never reported in the crystalline CI systems. We believe that the breakdown of conductance quantization should be also found in the CI lattices with singularities [24].…”

Failure of Conductance Quantization in Two-Dimensional Topological Insulators due to Nonmagnetic Impurities

“…Our numerical data in the weak-coupling U/t 1 regime can be explained by utilizing a simple model proposed in Ref. 52, which describes a single edge of a 2DTI in which a pair of helical edge modes is coupled to a δ-like magnetic impurity. According to Ref.…”

“…Since the sample displays a finite spin polarization only around the impurity and the edge-mode wavefunctions decay exponentially away from the edge, the detrimental effects of a vacancy on G rapidly vanish as this is moved towards the center of the sample (see Appendix B). The behavior of T for U/t 1 can be understood by solving the problem of a magnetic δ-like impurity 52,53 at a single edge. In this regime, the dips in T (E) can be parametrized by a Breit-Wigner dependence on E, as shown in Appendix C. Accordingly, such dips can be explained as anti-resonances resulting from the localization of an electron around the impurity.…”