Chemical Gating of a Weak Topological Insulator: Bi₁₄Rh₃I₉

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. A confirmation of the weak 3D TI state of Bi 14 Rh 3 I 9 would require to observe the QSH effect on the mentioned 1D edge states. 22 However, related transport experiments make only sense if the observed intrinsic doping is compensated by reasonable means and, thus, the topological gap with the edge states is shifted to the Fermi level.There are several possible ways to compensate the surface polarity: (i) physical gating by preparation of a dielectric gate structure and applying the electric field effect; (ii) chemical gating by deposition of an oxidizing agent; or (iii) counter-doping of the surface layer.Here, we report results of investigations into the two latter possibilities by means of DF 3 calculations. In particular, we study the effects of Iodine deposition as a sparse overlayer and of counter-doping by exchanging surface-layer Bi atoms by Sn. The results are expected to provide suggestions for the preparation of forthcoming transport experiments, which are required to confirm the topological state of Bi 14 Rh 3 I 9 or similar systems. We are not aware of any observation of the QSH effect on another suggested weak 3D TI material. All DF calculations were done with the full-potential local-orbital (FPLO) code 23 using the local density approximation (LDA) in the PW92 parametrization. 24 The self-consistent calculations were carried out with spin-orbit coupling (SOC) included. This effort is necessary because the involved elements have a sizable SOC strength which is responsible for opening the band gap. The following basis states are treated as valence states: Bi: 5s, 5p, 5d, 6s, 7s, 6p, 7p, 6d; Rh: 4s, 4p, 5s, 6s, 4d, 5d, 5p; I: 4s, 4p, 4d, 5s, 6s, 5p, 6p, 5d.In order to simulate the [001] surface of a bulk sample, we considered a series of slabs with different thickness (1.25 to 3.75 nm), varying from one to three structural layers. The considered layer stacks have the same lateral cell dimensions as the experimental bulk structure, 15 and equivalent atomic positions. Thus, the elementary cell of a slab with one, two, and three structural layers contains two, four, and six chemical uni...

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