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Epitaxial Growth of Ultraflat Stanene with Topological Band Inversion

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Epitaxial Growth of Ultraflat Stanene with Topological Band Inversion ARTICLES https://doi.org/10.1038/s41563-018-0203-5 Epitaxial growth of ultraflat stanene with topological band inversion Jialiang Deng 1,7, Bingyu Xia2,3,7, Xiaochuan Ma1,7, Haoqi Chen1, Huan Shan1, Xiaofang Zhai1, Bin Li1, Aidi Zhao 1*, Yong Xu 2,3,4*, Wenhui Duan2,3,5, Shou-Cheng Zhang6, Bing Wang 1* and J. G. Hou1 Two-dimensional (2D) topological materials, including quantum spin/anomalous Hall insulators, have attracted intense research efforts owing to their promise for applications ranging from low-power electronics and high-performance thermo- electrics to fault-tolerant quantum computation. One key challenge is to fabricate topological materials with a large energy gap for room-temperature use. Stanene—the tin counterpart of graphene—is a promising material candidate distinguished by its tunable topological states and sizeable bandgap. Recent experiments have successfully fabricated stanene, but none of them have yet observed topological states. Here we demonstrate the growth of high-quality stanene on Cu(111) by low-temperature molecular beam epitaxy. Importantly, we discovered an unusually ultraflat stanene showing an in-plane s–p band inversion together with a spin–orbit-coupling-induced topological gap (~0.3 eV) at the Γ point, which represents a foremost group-IV ultraflat graphene-like material displaying topological features in experiment. The finding of ultraflat stanene opens opportuni- ties for exploring two-dimensional topological physics and device applications. wo-dimensional (2D) topological materials, including quan- control over substrate/interface structures at the atomic level and tum spin Hall (QSH) and quantum anomalous Hall (QAH) still remains elusive. Tinsulators, are intriguing emergent states of quantum mat- In this work, we grew Sn films on Cu(111) by molecular beam ter characterized by a nontrivial band topology and topologically epitaxy (MBE) and measured the atomic and electronic structures protected metallic edge states, which are useful for realizing dis- by scanning tunnelling microscopy (STM) and angle-resolved sipationless conduction, magnetic monopoles, Majorana fermi- photoemission spectroscopy (ARPES) for comparison with first- ons, and so on1–7. Various 2D topological materials have recently principles calculations. Unexpectedly, we obtained high-quality ML been theoretically proposed and investigated8–22, but very few have stanene samples crystallized in an unusually ultraflat honeycomb been experimentally fabricated and confirmed to be topologically lattice, although structural buckling is well known to be a key ingre- nontrivial. Among them, one promising material candidate is sta- dient to lower the free energy and a planar stanene is believed to be nene10, which is theoretically predicted to be the latest cousin of unstable. Further analysis shows that the substrate not only stabi- graphene, composed of an atomic layer of tin (Sn) in a low-buckled lizes the ultraflat structure, but also causes significant lattice stretch- honeycomb lattice. Benefitting from the heavy element and struc- ing and orbital filtering effects in stanene. These substrate effects tural buckling, stanene has strong spin–orbit coupling (SOC) and synergistically generate an ultraflat stanene with an s–p band inver- shows extraordinarily large QSH and QAH gaps (~0.3 eV), feasible sion together with a sizeable SOC-induced gap (~0.3 eV), giving for room-temperature applications10,23. Moreover, the topological topologically derived edge states within the energy gap. Our finding states of stanene are highly tunable and can be varied from π to σ of ultraflat stanene sheds light on future research and applications of orbitals by chemical functionalization. Thus, the two classical planar graphene-like materials. models (that is, the Kane–Mele8 and Bernevig–Hughes–Zhang3 The schematic sample preparation set-up is shown in Fig. 1a. Sn models) can both be studied in stanene, offering an ideal platform atoms were evaporated from an effusion cell onto a Cu(111) sur- to explore 2D topological physics12,24,25. The intriguing proposals face held at ~200 K in ultrahigh vacuum. The samples were then have stimulated intensive experimental efforts. In 2015, mono- cooled to 5 K for STM and scanning tunneling spectroscopy (STS) layer (ML) stanene was successfully fabricated on Bi2Te 3(111), measurements without annealing (see Methods). Figure 1b shows confirming the existence of the theoretically predicted structure17. a representative STM topographic image of a 0.9 ML sample. Most Further experiments on different substrates obtained ML stanene of substrate is covered by uniform continuous Sn film except a with distinct material properties26–30. However, none of the experi- few small regions of bare substrate, which clearly reflects a typi- ments reported the finding of topological states in stanene, due cal 2D growth mode. The apparent height of the Sn film is 0.18 nm to unfavourable substrate effects. Very recently, superconductiv- (Fig. 1b), indicating it is a single atomic layer in thickness. The ity was discovered in few-layer stanene, offering opportunities detailed atomic network is visualized in Fig. 1c, mimicking a well- to explore Majorana fermions31. A critical issue is to grow sta- known graphene structure but with a much larger lattice constant nene with topologically nontrivial states, which requires accurate of 0.51 nm, which is exactly twice that of the underlying Cu(111) 1Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, China. 2State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing, China. 3Collaborative Innovation Center of Quantum Matter, Beijing, China. 4RIKEN Center for Emergent Matter Science (CEMS), Wako, Japan. 5Institute for Advanced Study, Tsinghua University, Beijing, China. 6Stanford Center for Topological Quantum Physics, Stanford University, Stanford, CA, USA. 7These authors contribute equally: Jialiang Deng, Bingyu Xia, Xiaochuan Ma. *e-mail: [email protected]; [email protected]; [email protected] NATURE MATERIALS | VOL 17 | DECEMBER 2018 | 1081–1086 | www.nature.com/naturematerials 1081 ARTICLES NATURE MATERIALS a b cd A B Atomic Sn source 0.18 nm Stanene e 1.5 Sn Cu 2.94 Å 1.0 AB 0.5 Sn 0.51 nm Height (nm) 50 nm 1 nm 0.0 Cu(111) 0123 Distance (nm) f g h 0.0 0.0 Cu(111) Sn Γ2 −0.5 −0.5 M' M Γ 2 M K −1.0 −1.0 0.3 eV Binding energy (eV) Binding energy (eV) −1.5 −1.5 −2.0 −2.0 M Γ K M2 M Γ M' Γ2 Fig. 1 | Atomic and electronic structures of ultraflat stanene on Cu(111). a, Schematic sample preparation set-up. b, STM image of Sn deposited on Cu(111) with 0.9 ML coverage (sample bias Vs = − 1 V, tunnelling current I = 0.1 nA). c, High-resolution STM image of the stanene film (Vs = 1 V, I = 0.4 nA). d, Schematic atomic model of the honeycomb stanene in c. e, Profile along the line in c showing that the adjacent Sn atoms are identical in apparent height. f, 2D BZs of ultraflat stanene on Cu(111). The second BZ of stanene is also shown. g,h, ARPES spectra of 0.9 ML stanene on Cu(111) along the M–Γ –K–M2 (red line in f) (g) and M–Γ –M′ –Γ2 (green line in f) (h) directions. The orange lines outline the energy bands which are contributed mainly by orbitals of Sn. surface (2.55 Å). Considering the relative orientation between the found that such a band structure as well the noticeable gap open- Sn film and the substrate, the honeycomb structure can be viewed ing at the Γ point present significant similarities to the pxy orbit- as a (2 ×​ 2) superstructure. The most intriguing fact is that this hon- als and large SOC gap predicted for fluorinated stanene despite the eycomb lattice is completely flat, as illustrated in Fig. 1d,e. There are energy difference10, implying a possible similar mechanism for the two nonequivalent Sn atoms in a unit cell (Fig. 1d). In the height ultraflat stanene. profile along the blue line (Fig. 1e), the two adjacent Sn atoms are 2D Xenes (X =​ Si, Ge, Sn) all have buckled honeycomb struc- clearly resolved and identical in height. The Sn–Sn bond length tures due to their weak π bonds, thus π–σ interactions are allowed in this honeycomb structure is 2.94 Å, close to the bond length of by buckling, leading to a more stabilized structure than the planar 2.81 Å in bulk α -Sn and 2.83 Å in freestanding stanene10. Our STM one. This is well established both theoretically and experimentally12. results clearly show that an atomic layer of Sn in a planar honey- Previous experiments on stanene mostly obtained low-buckled comb lattice with zero structural buckling, namely ultraflat stanene, structures26–29. Nearly planar stanene with low height variations is formed on Cu(111). was obtained only on Ag(111) with a Ag–Sn surface alloy30. The Such an unexpected ultraflat stanene inspires us to look further growth of ultraflat stanene with zero height variation in the pres- into its electronic structure. The high uniformity of the ultraflat ent experiment seems contradictory to common sense. Something stanene grown on Cu(111) enables high-resolution ARPES mea- unusual must happen to explain our experimental finding. We surements. The stanene/Cu(111) surface Brillouin zones (BZs) are performed first-principles calculations to reveal the underlying shown in Fig. 1f. The ARPES spectra of 0.9 ML stanene along M–Γ mechanism (Fig. 2). Generally one would expect a smaller buckling –K–M2 are presented in Fig. 1g. Here M2 is the M point of the sec- (δ) in a stretched lattice. For a freestanding stanene, δ decreases ond BZ. The ARPES spectra show additional fine features in com- from 0.85 Å to 0.72 Å by varying the lattice constant (a) from 4.68 Å parison with those of clean Cu(111), which are marked by orange (a0) to 5.1 Å.
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