Direct visualization of a two-dimensional topological insulator in the single-layer $1{T}^{\ensuremath{'}}\ensuremath{-}\mathrm{WT}{\mathrm{e}}_{2}$

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Direct visualization of a two-dimensional topological insulator in the single-layer $1 T^{'} - WT e_{2}$

Zhen-Yu Jia, Ye-Heng Song, Xiang-Bing Li, Kejing Ran, Pengchao Lu, Hui-Jun Zheng, Xin-Yang Zhu, Zhi-Qiang Shi, Jian Sun, Jinsheng Wen, Dingyu Xing, and Shao-Chun Li

Phys. Rev. B 96, 041108(R) – Published 7 July 2017

Abstract

We have grown nearly freestanding single-layer $1 T^{'} - WT e_{2}$ on graphitized $6 H$ -SiC(0001) by using molecular beam epitaxy (MBE), and characterized its electronic structure with scanning tunneling microscopy/spectroscopy (STM/STS). The existence of topological edge states at the periphery of single-layer $WT e_{2}$ islands was confirmed. Surprisingly, a bulk band gap at the Fermi level and insulating behaviors were also found in single-layer $WT e_{2}$ at low temperature, which are likely associated with an incommensurate charge order transition. The realization of two-dimensional topological insulators (2D TIs) in single-layer transition-metal dichalcogenide provides a promising platform for further exploration of the 2D TIs’ physics and related applications.

Received 11 February 2017
Revised 5 April 2017

DOI:https://doi.org/10.1103/PhysRevB.96.041108

Physics Subject Headings (PhySH)

Topological insulators

Monolayer films Transition metal dichalcogenides

Molecular beam epitaxy Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Zhen-Yu Jia¹, Ye-Heng Song¹, Xiang-Bing Li¹, Kejing Ran¹, Pengchao Lu¹, Hui-Jun Zheng¹, Xin-Yang Zhu¹, Zhi-Qiang Shi¹, Jian Sun^1,2, Jinsheng Wen^1,2, Dingyu Xing^1,2, and Shao-Chun Li^1,2,*

¹National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing, Jiangsu 210093, China
²Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

^*scli@nju.edu.cn

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Vol. 96, Iss. 4 — 15 July 2017

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Figure 1
Single-layer $WT e_{2}$ islands grown on BLG/SiC(0001). (a) Top and side view sketches of single-layer $1 T^{'} - WT e_{2}$ on BLG/SiC(0001); (b) RHEED pattern of as-grown single-layer $WT e_{2}$ . The blue arrow marks the streaks from the BLG/SiC(0001) substrate, while the red arrows represent the ones from $WT e_{2}$ domains of three equivalent orientations. (c) Large-scale STM topographic image of single-layer $WT e_{2}$ islands on the BLG/SiC(0001) substrate ( $150 \times 150 n m^{2}, U = + 1.0 V, I_{t} = 100 pA, T ≅ 4.7$ K). The length of the scale bar is 30 nm. Inset: Atomic resolution STM image of the single-layer $WT e_{2}$ surface ( $U = + 50 mV, I_{t} = 500 pA, T ≅ 4.7$ K) showing that single-layer $WT e_{2}$ is in the $1 T^{'}$ phase. The red rectangle represents the surface unit cell. (d) Line scan profile taken across both the first- and the second-layer $WT e_{2}$ . (e) $d I / d V$ spectra taken on surfaces of single-layer $WT e_{2}$ (red), second-layer $WT e_{2}$ (green), and the bulk termination (blue), respectively. The labels C2, C3 and V2, V3 mark the edges of valence bands and conduction bands, according to the calculated band structure [9]. (f) Small energy scale (from $- 300$ to $+ 300 mV$ ) $d I / d V$ spectra. (g) High-resolution $d I / d V$ spectrum (red) taken at $\sim 6.7 K$ indicating the features of the bulk band gap. Both the band gap and the coherence peaks are fitted (black dashed) based on a CDW gap format adopted from the literature [26].
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Figure 2
Edge states of single-layer $1 T^{'} - WT e_{2}$ on BLG/SiC(0001). (a) Large energy scale $d I / d V$ spectra (from $- 1.0$ to $+ 1.0 V$ ) taken along the rainbow-colored dotted line in the inset at $\sim 4.7 K$ , sweeping from the terrace (red end) to the step edge (purple end). The inset shows the STM topographic image ( $U = + 500 mV, I_{t} = 200 pA$ ) where the $d I / d V$ spectra were taken. The colored dots mark the locations where the same colored spectra were taken, respectively. (b) $d I / d V$ spectra (from $- 300$ to $+ 300 mV$ ) taken across the step edge. The corresponding line scan profile of the step edge is plotted in the inset, which is parallel to the $y$ axis. The edge-state feature is marked by the red vertical line, and the bulk band gap the black vertical lines. (c) STM topographic image ( $28 \times 28 n m^{2}, U = + 1.0 V, I_{t} = 100 pA$ ). The length of the scale bar is 5.6 nm. (d)–(h) $d I / d V$ maps measured on the same area of (c) with various bias voltages of $+ 200, + 100, + 60, - 40$ , and $- 80 mV$ , respectively. (i) Typical line cuts extracted from (d)–(g) indicating the penetration length of the edge state is $\sim 5.0 nm$ .
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Figure 3
Transport behavior of single-layer $1 T^{'} - WT e_{2}$ . (a) The measured square resistance $R_{sq}$ as a function of temperature. The black dotted line is $R_{total}$ as measured on the Te-capped $WT e_{2}$ /BLG/SiC, and the red dotted line $R_{BLG}$ as measured on the Te-capped BLG/SiC. The inset shows the schematic illustration of the resistance measurement. (b) The extracted effective resistance of single-layer $WT e_{2}, R_{WT e_{2}}$ , assuming $G_{total} = x G_{BLG} + G_{WT e_{2}}$ , with various values of $x$ . Here, $0 \leq x < x_{m}$ indicates the BLG/SiC substrate effect on $G_{total}$ , with $x = 0$ standing for no substrate effect and $x = x_{m}$ corresponding to the maximal substrate effect.
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Figure 4
Incommensurate charge order in single-layer $1 T^{'} - WT e_{2}$ at low temperature. (a)–(c) STM topographic images (left panel) and the corresponding FFT (right panel) taken at $\sim 4.7 K, L N_{2}$ , and room temperature (RT), respectively. The bias voltages $U$ are marked in the STM images, and $I_{t} = 0.1 nA$ . The red circles in the FFT mark the Bragg vectors, and the blue ones the wave vector for incommensurate charge order.
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Physical Review B

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Direct visualization of a two-dimensional topological insulator in the single-layer $1 T^{'} - WT e_{2}$

Zhen-Yu Jia, Ye-Heng Song, Xiang-Bing Li, Kejing Ran, Pengchao Lu, Hui-Jun Zheng, Xin-Yang Zhu, Zhi-Qiang Shi, Jian Sun, Jinsheng Wen, Dingyu Xing, and Shao-Chun Li

Phys. Rev. B 96, 041108(R) – Published 7 July 2017

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Physical Review B

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Direct visualization of a two-dimensional topological insulator in the single-layer 1T′−WTe2

Zhen-Yu Jia, Ye-Heng Song, Xiang-Bing Li, Kejing Ran, Pengchao Lu, Hui-Jun Zheng, Xin-Yang Zhu, Zhi-Qiang Shi, Jian Sun, Jinsheng Wen, Dingyu Xing, and Shao-Chun Li

Phys. Rev. B 96, 041108(R) – Published 7 July 2017

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Direct visualization of a two-dimensional topological insulator in the single-layer $1 T^{'} - WT e_{2}$