单层二维量子自旋霍尔绝缘体1<i>T'</i>-WTe<sub>2</sub>研究进展

贾亮广; 刘猛; 陈瑶瑶; 张钰; 王业亮

doi:10.7498/aps.71.20220100

摘要

量子自旋霍尔效应通常存在于二维拓扑绝缘体中, 其具有受拓扑保护的无耗散螺旋边界态. 2014年, 理论预言单层1T' 相过渡金属硫族化合物是一类新型的二维量子自旋霍尔绝缘体. 其中, 以单层1T'-WTe₂为代表的材料体系具有原子结构稳定、体带隙显著、拓扑性质易于调控等许多独特的优势, 对低功耗自旋电子器件的发展具有重要的意义. 本文总结了单层1T'-WTe₂在实验上的最新进展, 包括基于分子束外延生长的材料制备, 螺旋边界态的探测及其对磁场的响应, 掺杂、应力等手段在单层1T'-WTe₂中诱导出的新奇量子物态等. 也对单层1T'-WTe₂未来可能的应用前景进行了展望.

关键词:

Abstract

Quantum spin Hall effect, usually existing in two-dimensional (2D) topological insulators, has topologically protected helical edge states. In the year 2014, there was raised a theoretical prediction that monolayer transition metal dichalcogenides (TMDs) with 1T' phase are expected to be a new class of 2D quantum spin Hall insulators. The monolayer 1T'-WTe₂ has attracted much attention, because it has various excellent characteristics such as stable atomic structures, an obvious bandgap opening in the bulk of monolayer 1T'-WTe₂, and tunable topological properties, which paves the way for realizing a new generation of spintronic devices. In this review, we mainly summarize the recent experimental progress of the 2D quantum spin Hall insulators in monolayer 1T'-WTe₂, including the sample preparation via a molecular beam epitaxy technique, the detection of helical edge states and their response on external magnetic fields, as well as the modulation of more rich and novel quantum states under electron doping or strain. Finally, we also prospect the future applications based on monolayer 1T'-WTe₂.

Keywords:

作者及机构信息

北京理工大学集成电路与电子学院, 低维量子结构与器件工信部重点实验室, 北京　100081

通信作者: 张钰, yzhang@bit.edu.cn ; 王业亮, yeliang.wang@bit.edu.cn

基金项目: 国家自然科学基金(批准号: 92163206, 61725107)、国家重点研发计划(批准号: 2020YFA0308800, 2021YFA1400100)、北京市自然科学基金(批准号: Z190006)和中国博士后科学基金(批准号: 2021M700407)资助的课题.

Authors and contacts

School of Integrated Circuits and Electronics, MIIT Key Laboratory for Low-Dimensional Quantum Structure and Devices, Beijing Institute of Technology, Beijing 100081, China

Corresponding author: Zhang Yu, yzhang@bit.edu.cn ; Wang Ye-Liang, yeliang.wang@bit.edu.cn

Funds: Project supported by the National Natural Science Foundation of China (Grant Nos. 92163206, 61725107), the National Key Research and Development Program of China (Grant Nos. 2020YFA0308800, 2021YFA1400100), the Natural Science Foundation of Beijing, China (Grant No. Z190006), and the China Postdoctoral Science Foundation (Grant No. 2021M700407).

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参考文献

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施引文献

图 1 量子霍尔效应和量子自旋霍尔效应的边界态和能带结构示意图

Fig. 1. Schematic of edge states and band structures in quantum Hall effect and quantum spin Hall effect.

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图 2 单层1T'-WTe₂中量子自旋霍尔绝缘体的探测及调控

Fig. 2. Detection and modulation of quantum spin Hall effect in monolayer 1T'-WTe₂

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图 3 单层过渡金属硫族化合物MX₂的结构示意图^[15]　(a) 1H-MX₂^[15]; (b) 1T-MX₂^[15]; (c) 1T'-MX₂^[15]

Fig. 3. Atomic structures of monolayer TMD MX₂^[15]: (a) 1H-MX₂^[15]; (b) 1T-MX₂^[15]; (c) 1T'-MX₂^[15].

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图 4 单层1T'-TMD的能带结构　(a) 能带从拓扑平庸相转变为拓扑非平庸相的示意图. 能带的反转导致其从拓扑平庸相转变为拓扑非平庸相, 自旋-轨道耦合效应可以进一步使轨道交叠处的简并度解除^[35]. (b)单层1T'-TMD的能带结构^[15]

Fig. 4. Band structures of monolayer 1T'-TMD: (a) Schematic of band evolution from a topologically trivial phase to a nontrivial phase. Band inversion causes the band changing from topologically trivial to topologically nontrivial^[35]; (b) band structure of monolayer 1T'-TMD^[15].

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图 5 单层1T'-WTe₂的制备与表征　(a)利用MBE技术生长单层1T'-WTe₂的示意图^[30]; (b)在石墨烯表面生长单层1T'-WTe₂后的RHEED图^[30], 其中蓝色箭头表示来自BLG/SiC(0001)基底的条纹, 而红色箭头表示来自单层1T'-WTe₂的3个等能畴方向的条纹^[30]; (c) 单层1T'-WTe₂的原子分辨STM图^[54]; (d) 单层1T'-WTe₂的布里渊区图^[54]

Fig. 5. Preparation and characterization of the monolayer 1T'-WTe₂: (a) Schematic of sample preparation of monolayer 1T'-WTe₂ via an MBE method^[30]. (b) RHEED pattern of monolayer 1T'-WTe₂^[30]. The blue arrow marks the streaks from the BLG/SiC(0001) substrate, while the red arrows represent the ones from WTe₂ domains of three equivalent orientations^[30]. (c) Atomic-resolution STM image of monolayer 1T'-WTe₂^[54]. (d) Brillouin zone of monolayer 1T'-WTe₂^[54].

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图 6 单层1T'-WTe₂的体能带结构　(a) 利用ARPES探测单层1T'-WTe₂沿着Γ-X方向的体能带结构^[55]; (b) 利用第一性原理计算单层1T'-WTe₂的能带结构^[55]

Fig. 6. Band structure of monolayer 1T'-WTe₂: (a) Band structure of monolayer 1T'-WTe₂ acquired by ARPES along the Γ-X direction^[55]; (b) calculated band structure of monolayer 1T'-WTe₂ along the Γ-X direction by first-principles^[55]

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图 7 利用STM探测1T'-WTe₂单层台阶处的一维边界态^[29]　(a) 1T'-WTe₂单层台阶的STM图; (b) 1T'-WTe₂内部(黑线)和单层台阶边缘处(红线)的STS谱; (c)空间分辨的STS谱, 横坐标表示探测点到1T'-WTe₂单层台阶的距离, x = 0 nm为1T'-WTe₂的单层台阶

Fig. 7. STM detection of edge states in 1T'-WTe₂ monolayer step: (a) STM image of 1T'-WTe₂ monolayer step^[29]; (b) typical STS spectra recorded at the step edge (red curve) and at a location at the inner terrace (black curve) of 1T'-WTe₂^[29]; (c) spatial-resolved STS spectra recorded perpendicular to the 1T'-WTe₂ monolayer step. The position x = 0 nm is at the monolayer step^[29].

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图 8 利用MIM探测单层1T'-WTe₂的边界态^[32]　(a)样品的光学显微镜图, 单层1T'-WTe₂被转移到SiO₂/Si衬底上, 并覆盖10 nm厚的hBN; (b)—(d) 图(a)中不同区域对应的零磁场下MIM-Im图; (e)穿过单层1T'-WTe₂边界的零磁场下MIM-Im信号随栅极电压的变化情况, 其中E_Gate = –15 V为电中性点的位置; (f)穿过单层1T'-WTe₂边界的B = 9 T下MIM-Im信号随栅极电压的变化情况

Fig. 8. Detection of edge states in monolayer 1T'-WTe₂ via an MIM technique: (a) Optical image of monolayer 1T'-WTe₂ exfoliated onto SiO₂/Si substrate and covered with a 10-nm-thick hBN^[32]. (b)–(d) MIM-Im images of the regions marked in panel (a)^[32]. (e) MIM-Im images obtained across the edge of monolayer 1T'-WTe₂ as a function of gate voltage E_Gate under B = 0 T^[32]. The charge neutral point is located at E_Gate = –15 V^[32]. (f) MIM-Im images obtained across the edge of monolayer 1T'-WTe₂ as a function of gate voltage E_Gate under B = 9 T^[32].

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图 9 利用电荷输运测量单层1T'-WTe₂的量子自旋霍尔效应^[31]　(a)基于单层1T'-WTe₂器件的示意图. 器件由单层1T'-WTe₂、用于封装的hBN、石墨顶栅、8个接触电极, 以及长度不一的一系列局域背栅组成. (b)电阻差值ΔR随局域栅极电压V_c的变化曲线, 其中局域背栅的宽度分别为100, 70 和60 nm. (c)长度依赖的电阻. 在短通道极限下, ΔR_s接近于电阻最小值h/(2e²), 这意味着每个边界的电导为e²/h, 满足量子自旋霍尔效应. (d)在垂直磁场下, 边界电导G_S随V_c的变化曲线, 背栅宽度为100 nm. (e)在特定的V_c下, G_S随磁场的变化曲线. 电导是否出现饱和取决于费米面的位置. (f) –ln(G_S/G₀)随μ_BB/(k_BT)的变化. 黑线为线性拟合的结果. 插图: 对于电导不饱和的情况, 在不同温度下测量G_S随磁场的变化; (g)边界电导随温度的变化情况. 插图: 不同温度下ΔR随V_c的变化曲线^[31]

Fig. 9. Observation of quantum spin Hall effect up to 100 K in monolayer 1T'-WTe₂ via transport measurements^[31]: (a) Schematic of monolayer 1T'-WTe₂ encapsulated with hBN. Graphite is applied for the top gate, eight contact electrodes are applied to minimize the effect of contact resistance, and a series of in-channel local bottom gates are applied to study the length-dependent feature. (b) ΔR versus V_c for the gate with the width of 60, 70, and 100 nm. (c) Length dependence of ΔR_s. In the short-channel limit, the ΔR_s values approach a minimum of h/(2e²), in agreement with quantum spin Hall effect. (d) G_S versus V_c under perpendicular magnetic fields for a 100-nm-width gate. (e) G_S versus B at specific V_c. The saturation or not of G_S depends on the Fermi energy. (f) –ln(G_S/G₀) versus μ_BB/(k_BT). The black line is a linear fit. Inset: Temperature dependence of G_S versus B for the non-saturating curves. (g) Temperature dependence of the edge conductance. Inset: gate dependence of ΔR at various temperatures.

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图 10 利用STS探测单层1T'-WTe₂中体绝缘态的物理起源^[54]　(a)在单层1T'-WTe₂体内不同位置的低能STS谱, 红色箭头表示库仑能隙, 蓝色箭头表示局域态密度降低的能量位置; (b)不同能量STS图的傅里叶变换结果; (c)沿着动量Y-Γ-Y方向的能带结构示意图, 其中q₁为导带的带内散射, q₂为两个导带间的带间散射, q₃为导带和价带间的带间散射, q₄为价带的带内散射; (d)通过不同能量STS图的傅里叶变换得到的单层1T'-WTe₂能量-动量色散关系, 其中黑线为q₁, q₂, q₃带色散; (e)不同浓度K掺杂单层1T'-WTe₂得到的STS谱, 费米面处存在库仑能隙

Fig. 10. STS evidence of the physical origin of the bulk insulator in monolayer 1T'-WTe₂^[54]: (a) Spatially resolved low-energy STS spectra recorded in the bulk of monolayer 1T'-WTe₂. The Coulomb gap and the minimum of local density of states are marked by the red and blue arrows, respectively. (b) The fast Fourier transform (FFT) image of the STS maps at different energies. (c) Band structures of monolayer 1T'-WTe₂ along the direction of Y-Γ-Y in the reciprocal space. The corresponding scattering channels of are the intra-band scattering of the conduction band (q₁), the inter-conduction band scattering (q₂), the inter-band scattering between the valence and conduction bands (q₃), and the intra-band scattering of the valence band (q₄). (d) Energy-momentum dispersion along the Y-Γ-Y direction. The black lines schematically illustrate the band dispersion of q₂, q₃, and q₄. (e) STS spectra taken on the 1T'-WTe₂ surface with different potassium coverage. The Coulomb gap is located at the Fermi energy.

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图 11 在单层1T'-WTe₂中实现二维拓扑绝缘体-超导相变^[36]　(a) 基于单层1T'-WTe₂器件的光学显微镜图及模型图, 其中单层1T'-WTe₂用hBN介电层封装, 并通过石墨引入电极; (b)在不同温度下, R_xx随电子掺杂浓度n_e的变化关系; (c)在单层1T'-WTe₂中发生二维拓扑绝缘体-超导转变的相图; (d)在不同n_e下, R_xx随温度的变化曲线; (e)在高n_e时, R_xx随垂直磁场强度的变化关系, 其中插图为T_1/2随温度的变化以及B_1/2随垂直磁场的变化; (f)在高n_e时, R_xx随平行磁场强度的变化关系, 其中插图为T_1/2随平行磁场的变化, 虚线为g因子为2时的泡利极限值B_P; (g)在不同温度和垂直磁场下, 电导随掺杂浓度的变化曲线, 其中示意图为不同位置单层1T'-WTe₂体和边界的到点情况, 其中体态的颜色与相图一致, 边界态用红色表示

Fig. 11. Realization of phase transition between two-dimensional topological insulating states and superconductivity in monolayer 1T'-WTe₂^[36]: (a) Optical image and schematic device structure of monolayer 1T'-WTe₂ with encapsulated hBN dielectric layers and two graphite gates. (b) R_xx as a function of electrostatic doping n_e under different temperatures. (c) experimental phase diagram of phase transition between two-dimensional topological insulating states and superconductivity in monolayer 1T'-WTe₂. (d) R_xx as a function of temperature under different n_e. (e) R_xx as a function of perpendicular magnetic field at the highest n_e value. Inset: T_1/2 as a function of temperature, as well as B_1/2 as a function of perpendicular magnetic field. (f) R_xx as a function of parallel magnetic field at the highest n_e value. Inset: T_1/2 as a function of parallel magnetic field. The Pauli limit B_P, assuming g factor of 2, is indicated by the dashed line. (g) Conductance as a function of n_e under different temperatures and perpendicular magnetic field. Schematics indicate the state of edge and bulk conduction of monolayer 1T'-WTe₂ at different points. The bulk is colored to match the phase diagram, and red indicates a conducting edge state.

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图 12 利用应力在单层1T'-WTe₂中实现绝缘体-半金属相变^[38]　(a)—(c)在应力作用下, 单层1T'-WTe₂的原子分辨STM图及其STS谱; (d), (e)单层1T'-WTe₂的体带隙随其晶格常数a和b的变化; (f)单层1T'-WTe₂随晶格常数a和b变化的相图, 其中实验上测得的结果用黑色圆圈标注; (g)理论计算单层1T'-WTe₂的A边界能带结构, 计算的晶格常数a = 6.33 Å, b = 3.54 Å; (h)垂直于A边界的空间分辨STS谱, 可以看到明显的一维边界态

Fig. 12. Strain tunable phase transition between topological insulator and semimetal insulator in monolayer 1T'-WTe₂^[38]: (a)− (c) Atomically resolved STM images and corresponding STS spectra in monolayer 1T'-WTe₂ under strain. (d), (e) Energy gap as a function of strains along the a or b directions in monolayer 1T'-WTe₂. (f) Phase diagram of monolayer 1T'-WTe₂ as a function of lattice constants a and b. Strain conditions acquired from the experimental data are marked by black circles. (g) Calculated edge states along the A edge with the lattice constants a = 6.33 Å, b = 3.54 Å. (h) Spatially resolved STS spectra recorded across the A edge. One-dimensional edge states can be clearly identified.

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单层二维量子自旋霍尔绝缘体1T'-WTe₂研究进展