Interfacing 2D and 3D Topological Insulators: Bi(111) Bilayer on Bi 2 Te 3 Toru Hirahara, 1, * Gustav Bihlmayer, 2 Yusuke Sakamoto, 1 Manabu Yamada, 1 Hidetoshi Miyazaki, 3 Shin-ichi Kimura, 3 Stefan Blu ¨gel, 2 and Shuji Hasegawa 1 1 Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 2 Peter Gru ¨nberg Institut and Institute for Advanced Simulation, Forschungszentrum Ju ¨lich and JARA, 52425 Ju ¨lich, Germany 3 UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan (Received 8 May 2011; published 10 October 2011) We report the formation of a bilayer Bi(111) ultrathin film, which is theoretically predicted to be in a two-dimensional quantum spin Hall state, on a Bi 2 Te 3 substrate. From angle-resolved photoemission spectroscopy measurements and ab initio calculations, the electronic structure of the system can be understood as an overlap of the band dispersions of bilayer Bi and Bi 2 Te 3 . Our results show that the Dirac cone is actually robust against nonmagnetic perturbations and imply a unique situation where the topologically protected one- and two-dimensional edge states are coexisting at the surface. DOI: 10.1103/PhysRevLett.107.166801 PACS numbers: 73.25.+i, 73.20.r, 73.61.Ng Topological insulators, realized in materials with strong spin-orbit interaction, are gaining increasing attention in condensed matter physics. Mathematically characterized by the Z 2 topological number, they are band insulators but have metallic edge (surface) modes that are protected by time-reversal symmetry. It is predicted that they may be used in application for spintronics or quantum comput- ing [1]. The presence of such systems has been well established experimentally by measuring the helical Dirac-like surface-state band dispersion for various Bi-based materi- als with (spin- and) angle-resolved photoemission spec- troscopy (ARPES) [2–4] for three-dimensional (3D) materials. Theoretically, the topological surface states are described as ‘‘domain-wall’’ states at the interface of two systems that have different topological properties (a sign change of the Dirac mass) [1]. Their existence is always guaranteed at the topological-trivial interface. To investi- gate how the Dirac cone is affected by perturbations, theorists have calculated the electronic structure for sys- tems where the whole surface is terminated with dielectrics [5] or surfaces that have different terminations [6]. In terms of experiment, an apparent ‘‘gap opening’’ of the Dirac cones was found by nonmagnetic impurity adsorption pos- sibly due to band bending effects [7,8]. For two-dimensional quantum spin Hall states (2D QSH), there has been only one case where elaborated work has been performed: HgTe=CdTe quantum wells [9]. Although the topological protection is more complete in the one-dimensional (1D) edge states compared to the 2D surface states of 3D topological insulators, only a few works have been done experimentally [10]. An interesting system predicted to be in a 2D QSH phase is a single- bilayer Bi(111) ultrathin film, which is said to have much shorter edge-state penetration length scales compared to HgTe=CdTe quantum wells [11,12]. But since it is the thinnest limit for a 2D system similar to graphene, it has not been realized experimentally. One reason is that Bi prefers the black-phosphorous structure instead of the rhombohedral structure for atomic-layer thicknesses [13]. In the present Letter, we show that we succeeded in fabricating a single bilayer of Bi(111) on Bi 2 Te 3 ð111Þ since they both form in layers with a hexagonal lattice [Figs. 1(a) and 1(b)] and investigated how the Dirac cone is affected by this Bi termination. From ARPES and first principles calculations, we show for the first time that the Dirac cone of Bi 2 Te 3 is actually robust against nonmag- netic perturbations. The present system is ideal to study the 1D edge states of a 2D QSH Bi bilayer as well as to explore the novel spin transport phenomena due to the interplay of 1D edge and 2D surface states. The ARPES experiments were performed at BL-5U of UVSOR using the MBS-Toyama A-1 analyzer at 10 K. The energy and the angular resolutions were 20 meV and 0.2  , respectively. The calculations have been performed by the full-potential linearized augmented plane wave method in film geometry as implemented in the FLEUR program [14], and the generalized gradient approximation [15] has been used for the description of the exchange-correlation potential. All the film fabrication and measurements were done in situ. First, a clean Sið111Þ-ð7  7Þ surface was prepared on an n-type substrate (P-doped, 1–10   cm at room temperature) by a cycle of resistive heat treatments. Then Bi was deposited on the 7  7 structure at 400 K under Te-rich conditions. Such a procedure is reported to result in a quintuple-layer-by-quintuple-layer [(QL), 1 QL ¼ 10:2  A, Fig. 1(f)] epitaxial film formation [16]. This can be seen in the clear reflection high-energy electron diffrac- tion (RHEED) pattern in Fig. 1(e), indicating the 1  1 periodicity of the Bi 2 Te 3 ð111Þ surface as well as the Kikuchi pattern. It is also known that high-quality epitaxial Bi(111) films can be formed on Sið111Þ-ð7  7Þ in bilayers [Fig. 1(c)] [(BL), 1 BL ¼ 3:9  A][13]. Theory predicts that PRL 107, 166801 (2011) PHYSICAL REVIEW LETTERS week ending 14 OCTOBER 2011 0031-9007= 11=107(16)=166801(5) 166801-1 Ó 2011 American Physical Society