Termination-dependent topological surface states of the natural superlattice phase Bi4Se3

Abstract

We describe the topological surface states of Bi 4 Se 3 , a compound in the infinitely adaptive Bi 2 -Bi 2 Se 3 natural superlattice phase series, determined by a combination of experimental and theoretical methods. Two observable cleavage surfaces, terminating at Bi or Se, are characterized by angle-resolved photoelectron spectroscopy and scanning tunneling microscopy, and modeled by ab initio density functional theory calculations. Topological surface states are observed on both surfaces, but with markedly different dispersions and Kramers point energies. Three-dimensional topological insulators (3D TIs) are a new class of materials that exhibit topologically protected helical metallic topological surface states (TSS) and a bulk band gap. [1][2][3][4][5][6][7][8][9][10][11][12] The great interest in 3D TIs is partly due to the fact that they necessarily host exotic bound states at their boundaries when interfaced with other nontopological or topological materials. 13 In addition, several theoretical studies have predicted that novel properties may emerge when topological insulators are interlaced with other materials in a regular superlattice. 14,15 In order to pursue these promising avenues, however, various experimental challenges have to be solved. For example, a basic understanding of the surface band structures of more complex topological materials, while highly desirable, is not trivial. Although there have been studies of different materials at the interface of 3D TIs, 16,17 and on different surface terminations of the same material, 18,19 no in-depth study of the TSS and electronic band structure of a complex topological material or a true bulk topological superlattice material has yet been reported.Here, we investigate the properties of Bi 4 Se 3 , the simplest topological superlattice material, consisting of single Bi 2 layers interleaved with single Bi 2 Se 3 layers in a 1:1 ratio 20 [ Fig. 1(a)]. While bulk Bi 2 Se 3 is a model 3D TI, an isolated Bi 2 layer is predicted to be a 2D TI; 21 combining these two building blocks into a 3D superlattice offers a unique possibility for studying the effects of interlayer interactions. We investigate the electronic structure of this material experimentally via angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) and theoretically via ab initio density functional theory (DFT) calculations. We observe two types of surfaces after cleaving the crystal, corresponding to Bi 2 -and Bi 2 Se 3 -terminated terraces. We find that both terminations exhibit TSS, but with substantially different Kramers point energies and dispersions. We show that many features of the surface band structure can be derived from the idealized case of weakly coupled Bi 2 and Bi 2 Se 3 layers where the interaction between these building blocks is responsible for the different TSSs. Bi 4 Se 3 and related (Bi 2 ) m (Bi 2 Se 3 ) n (Ref. 22) superlattice phases provide a unique opportunity for studying the coexistence of multiple types of topological surface st...

Electronic band structure for occupied and unoccupied states of the natural topological superlattice phase Sb2Te

“…If one compares our data with the corresponding ones for Sb 2 Te 3 [15,16], it is clear that the surface electronic structure of 'Sb 2 Te 3 -terminated' SbTe 2 is markedly different from Sb 2 Te 3 , in particular for the TSSs. This confirms that, as in the case of Bi 4 Se 3 [18], it is the bulk band structure that determines the TSSs, not the structure of the first quintuple layer of the host. Furthermore, the dynamics presented in Fig.…”

“…High-quality single crystals of Sb 2 Te [18] were cleaved in situ with a top-post under UHV conditions (base pressure better than 2.5 × 10 −10 mbar), and were measured with µ-XPS, µ-ARPES and trARPES.…”

Gapped Surface States in a Strong-Topological-Insulator Material

“…Previously, Dirac cone-like TSS were found in the gap between the first fully occupied bulk valence band (BVB) and the hole-like bulk conduction band (HBCB) on two different surface terminations of Bi 4 Se 3 (111) [10]. It was determined that the TSS result from a parity inversion at the Γ-point of the bulk Brillouin zone (BBZ), a characteristic shared with the Bi 2 Se 3 parent compound [15].…”

“…The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin-helicity. Topological insulator materials (TIM) [1], which include insulators [2][3][4] and semimetals [5][6][7][8][9][10], possess an inverted bulk band gap that hosts unusually robust, spin-helical topological surface states (TSS) at the material's boundaries. The presence of TSS is guaranteed by the topology of the bulk bands and the states can only be removed from the Fermi energy if protecting symmetries are broken.…”

“…Single crystals of Bi 4 Se 3 were synthesized following a previously reported procedure [10]. ARPES was per-formed using a Scienta SES-100 electron spectrometer at beamline 12.0.1 of the Advanced Light Source with a combined instrumental energy resolution of ∼12 meV and an angular resolution better than ± 0.07º.…”