二维有机拓扑绝缘体的研究进展

新材料的发现促进了科学与技术的进步.拓扑绝缘体是近期材料领域新的研究热点,相关研究的进一步深入,不仅加深了人们对材料物理性质的理解,也为其在自旋电子学和量子计算机等领域的潜在应用提供了有价值的参考.近年来,理论工作预测了一系列由金属和有机物构筑的二维有机拓扑绝缘体,本文主要介绍六角对称的金属有机晶格与Kagome金属有机晶格两类典型的二维有机拓扑绝缘体的研究进展,其中重点介绍了理论预测的氰基配位二维本征有机拓扑绝缘体.除了理论计算方面的工作,还简要介绍了关于二维有机拓扑绝缘体材料合成方面的实验工作.二维有机拓扑绝缘体的理论与实验研究不仅拓展了拓扑绝缘体的研究体系,还为寻找新的拓扑绝缘体材料提供了思路.     

Two-dimensional topological insulators with large bulk energy gap

Two-dimensional(2D) topological insulators(TTs,or quantum spin Hall insulators) are special insulators that possess bulk 2D electronic energy gap and time-reversal symmetry protected one-dimensional(1D) edge state.Carriers in the edge state have the property of spin-momentum locking,enabling dissipation-free conduction along the 1D edge.The existence of 2D TIs was confirmed by experiments in semiconductor quantum wells.However,the 2D bulk gaps in those quantum wells are extremely small,greatly limiting potential application in future electronics and spintronics.Despite this limitation,2D TIs with a large bulk gap attracted plenty of interest.In this paper,recent progress in searching for TIs with a large bulk gap is reviewed briefly.We start by introducing some theoretical predictions of these new materials and then discuss some recent important achievements in crystal growth and characterization.     

Dephasing effects in topological insulators

Topological insulators, a class of typical topological materials in both two dimensions and three dimensions,are insulating in bulk and metallic at surface. The spin-momentum locked surface states and peculiar transport properties exhibit promising potential applications on quantum devices, which generate extensive interest in the last decade. Dephasing is the process of the loss of phase coherence, which inevitably exists in a realistic sample. In this review, we focus on recent progress in dephasing effects on the topological insulators. In general, there are two types of dephasing processes: normal dephasing and spin dephasing. In two-dimensional topological insulators, the phenomenologically numerical investigation shows that the longitudinal resistance plateaus is robust against normal dephasing but fragile with spin dephasing. Several microscopic mechanisms of spin dephasing are then discussed. In three-dimensional topological insulators, the helical surface states exhibit a helical spin texture due to the spin-momentum locking mechanism. Thus, normal dephasing has close connection to spin dephasing in this case, and gives rise to anomalous “gap-like” feature. Dephasing effects on properties of helical surface states are investigated.     

Magnetic quantum ring in two-dimensional topological insulators

The quantum properties of topological insulator magnetic quantum rings formed by inhomogeneous magnetic fields are investigated using a series expansion method for the modified Dirac equation. Cycloid-like and snake-like magnetic edge states are respectively found in the bulk gap for the normal and inverted magnetic field profiles. The energy spectra, current densities and classical trajectories of the magnetic edge states are discussed in detail. The bulk band inversion is found to manifest itself through the angular momentum transition in the ground state for the cycloid-like states and the resonance tunneling effect for the snake-like states.     

Strain engineering of topological properties in lead‐salt semiconductors

Rock‐salt chalcogenide SnTe represents the simplest realization of a topological insulator where a crystal symmetry allows for the appearance of surface metallic states. Here, we theoretically predict that strain, as realized in thin films grown on (001) substrates, may induce a transition to a topological crystalline insulating phase in related lead‐salt chalcogenides. Furthermore, relevant topological properties of the surface states, such as the location of the Dirac cones on the surface Brillouin zone or the decay length of edge states, appear to be tunable with strain, with potential implications for technological devices benefiting from those additional degrees of freedom. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photonic Floquet topological insulators in atomic ensembles

The feasibility of realizing a photonic Floquet topological insulator (PFTI) in an atomic ensemble is demonstrated. The interference of three coupling fields will split energy levels periodically, to form a periodic refractive index structure with honeycomb profile that can be adjusted by different frequency detunings and intensities of the coupling fields. This in turn will affect the appearance of Dirac cones in momentum space. When the honeycomb lattice sites are helically ordered along the propagation direction, gaps open at Dirac points, and one obtains a PFTI in an atomic vapor. An obliquely incident beam will be able to move along the zigzag edge of the lattice without scattering energy into the PFTI, due to the confinement of edge states. The appearance of Dirac cones and the formation of a photonic Floquet topological insulator can be shut down by the third‐order nonlinear susceptibility and opened up by the fifth‐order one.

     

拓扑绝缘体表面态的STM研究

文章主要介绍了利用扫描隧道显微镜对拓扑绝缘体表面态进行的一系列研究工作,包括拓扑绝缘体表面态的电子驻波以及拓扑表面态的朗道量子化现象.这些工作对于拓扑绝缘体基本性质的确立以及深入理解具有十分重要的意义.     

拓扑绝缘体表面态的STM研究

文章主要介绍了利用扫描隧道显微镜对拓扑绝缘体表面态进行的一系列研究工作,包括拓扑绝缘体表面态的电子驻波以及拓扑表面态的朗道量子化现象.这些工作对于拓扑绝缘体基本性质的确立以及深入理解具有十分重要的意义.     

拓扑绝缘体的普适电导涨落

拓扑绝缘体因其无能量耗散的拓扑表面输运而备受关注, 揭示拓扑表面态因其 的贝利相位而产生的拓扑输运现象, 将有助于拓扑绝缘体相关器件的应用开发. 本文回顾了普适电导涨落(UCF) 揭示拓扑绝缘体奇异输运性质的研究进展. 通过调控温度、角度、门电压、垂直磁场和平行磁场等外部参量, 实现了对拓扑绝缘体的UCF 效应的系统研究, 证实了拓扑绝缘体中二维UCF 的输运现象, 并通过尺寸标度规律获得了UCF 的拓扑起源的实验证据, 讨论了拓扑表面态的UCF 的统计对称规律. 从而实现了对拓扑绝缘体UCF 效应的较为完整的理解.     

Efficiency of electrical manipulation in two-dimensional topological insulators

We investigate the efficiency of electrical manipulation in a two-dimensional topological insulator by inspecting the electronic states of a lateral electrical potential superlattice in the system. The spatial distribution of the electron density in the system can be tuned by changing the strength of the externally applied lateral electrical superlattice potential. This provides us the information about how efficiently one can manipulate the electron motion inside a two-dimensional topo- logical insulator. Such information is important in designing electronic devices, e.g., an electric field effect transistor made of the topological insulator. The electronic states under various conditions are examined carefully. It is found that the dispersion of the mini-band and the electron distribution in the potential well region both display an oscillatory behavior as the potential strength of the lateral superlattice increases. The probability of finding an electron in the potential well region can be larger or smaller than the average as the potential strength varies. These features can be attributed to the coupled multiple-band nature of the topological insulator. In addition, it is also found that these behaviors are not sensitive to the gap parameter of the two-dimensional topological insulator model. Our study suggests that the electron density manipulation via electrical gating in a two-dimensional topological insulator is less effective and more delicate than that in a traditional single-band semiconductor.     

三聚化非厄密晶格中具有趋肤效应的拓扑边缘态

近年来,探索新的拓扑量子结构、深入分析各种多聚化拓扑晶格中的新奇物理性质已经成为热点.并且,多聚化拓扑模型在量子光学等领域的研究也愈发深入,拥有广阔的发展前景.本文聚焦于研究三聚化非厄密晶格中的新奇拓扑特性.首先,若晶胞内最近邻正反向耦合不相等,三聚化模型中的体态和边缘态出现趋肤效应.其中,随着最近邻耦合正反系数差的增大,拓扑保护的边缘态的宽度和简并度均可被调制,边缘态数量也会减少.其次,当在考虑次近邻耦合的影响时,随着次近邻耦合系数在适当范围内变化,系统本征能谱的上下能隙及其中具有趋肤效应的边缘态也会发生不对称的变化.此外,当适当改变两种耦合系数,三聚化非厄密模型的体态和边缘态的局域程度也会随之发生变化.     

Topological nature of in‐gap bound states in disordered large‐gap monolayer transition metal dichalcogenides

We propose a physical model based on disordered (a hole punched inside a material) monolayer transition metal dichalcogenides (TMDs) to demonstrate a large‐gap quantum valley Hall insulator. We find an emergence of bound states lying inside the bulk gap of the TMDs. They are strongly affected by spin–valley coupling, rest‐ and kinetic‐mass terms and the hole size. In addition, in the whole range of the hole size, at least two in‐gap bound states with opposite angular momentum, circulating around the edge of the hole, exist.Their topological insulator (TI) feature is analyzed by the Chern number, characterized by spacial distribution of their probabilities and confirmed by energy dispersion curves (energy vs. angular momentum). It not only sheds light on overcoming low‐temperature operating limitation of existing narrow‐gap TIs, but also opens an opportunity to realize valley‐ and spin‐qubits. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

First principle study of edge topological defect-modulated electronic and magnetic properties in zigzag graphene nanoribbons

Zigzag graphene nanoribbon(ZGNR) is a promising candidate for next-generation spintronic devices. Development of the field requires potential systems with variable and adjustable electromagnetic properties. Here we show a detailed investigation of ZGNR decorated with edge topological defects(ED-ZGNR) synthesized in laboratory by Ruffieux in 2015[Pascal Ruffieux, Shiyong Wang, Bo Yang, et al. 2015 Nature 531 489]. The pristine ED-ZGNR in the ground state is an antiferromagnetic semiconductor, and the acquired band structure is significantly changed compared with that of perfect ZGNR. After doping heteroatoms on the edge, the breaking of degeneration of band structure makes the doped ribbon a half-semi-metal, and nonzero magnetic moments are induced. Our results indicate the tunable electronic and magnetic properties of ZGNR by deriving unique edge state from topological defect, which opens a new route to practical nano devices based on ZGNR.     

磁性拓扑绝缘体中的量子化反常霍尔效应

文章从平常霍尔效应出发,介绍了反常霍尔效应及其内秉物理机制,并在此基础上介绍了其量子化版本——量子化反常霍尔效应.然后从拓扑有序态的角度,重点讨论了量子化反常霍尔效应与量子霍尔效应、量子自旋霍尔效应、拓扑绝缘体等之间的区别与内在联系.最后介绍了通过在拓扑绝缘体（Bi2Se3, Bi2Te3 和 Sb2Te3）薄膜中掺杂过渡金属元素（Cr 或 Fe）实现量子化反常霍尔效应的方法.     

On the photon-drag effect of photocurrent of surface states of topological insulators

The photocurrent of surface states of topological insulator due to photon-drag effect is computed, being based on pure Dirac model of surface states. The scattering by disorder is taken into account to provide a relaxation mechanism for the photocurrent. The Keldysh–Schwinger formalism has been employed for the systematic calculation of photocurrent. The helicity dependent photocurrent of sizable magnitude transverse to the in-plane photon momentum is found, which is consistent with experimental data. Other helicity independent photocurrents with various polarization states are also calculated.     

A new topological insulator built from quasi one‐dimensional atomic ribbons

A novel topological insulator with orthorhombic crystal structure is demonstrated. It is characterized by quasi one‐dimensional, conducting atomic chains instead of the layered, two‐dimensional sheets known from the established Bi₂(Se,Te)₃ system. The Sb‐doped Bi₂Se₃ nanowires are grown in a TiO₂‐catalyzed process by chemical vapor deposition. The binary Bi₂Se₃ is transformed from rhombohedral to orthorhombic by substituting Sb on ～38% of the Bi sites. Pure Sb₂Se₃ is a topologically trivial band insulator with an orthorhombic crystal structure at ambient conditions, and it is known to transform into a topological insulator at high pressure. Angle‐resolved photoemission spectroscopy shows a topological surface state, while Sb doping also tunes the Fermi level to reside in the bandgap. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Electron on a cylinder with topological defects in a homogeneous magnetic field

In this work we study the effects of the geometry and topology of a cylinder on the energy levels of an electron moving in a homogeneous magnetic field. We consider the existence of topological defects as a screw dislocation and a disclination. When we take the region of movement as the full cylindrical surface, we find that, by increasing the strength of the screw dislocation, the dispersion on the electronic energy levels is affected and monotonically increasing. For an electron moving in an almost flat region we show that the dispersion on the Landau levels decrease monotonically as we increase the strength of the screw dislocation. The lowest Landau level can reach a zero value, leaving the energy of the system solely given by the geometry of the cylinder, which does not depend on the magnetic field. In both situations, as we change the deficit angle of the disclination, we observe that the energy levels are shifted and the magnitude of such shift depends on the magnetic field. The Landau levels for a flat sample are recovered in the limit of an infinite cylinder radius.     

Quantum Probing Topological Phase Transitions by Non‐Markovianity

Understanding the physical significance and probing the global invariants characterizing quantum topological phases in extended systems is a main challenge in modern physics with major impact in different areas of science. Here, a quantum‐information‐inspired probing method is proposed where topological phase transitions are revealed by a non‐Markovianity quantifier. The idea is illustrated by considering the decoherence dynamics of an external read‐out qubit that probes a Su–Schrieffer–Heeger (SSH) chain with either pure dephasing or dissipative coupling. Qubit decoherence features and non‐Markovianity measure clearly signal the topological phase transition of the SSH chain.     

Predication of topological states in the allotropes of group-IV elements

Three-dimensional (3D) topological insulators (TIs) have been studied for approximately fifteen years, but those made from group-IV elements, especially Ge and Sn, seem particularly attractive owing to their nontoxicity, sizable intrinsic spin–orbit coupling (SOC) strength and natural compatibility with the current semiconductor industry. However, group-IV elemental TIs have rarely been reported, except for the low temperature phase of α-Sn under strain. Here, based on first-principles calculations, we propose new allotropes of Ge and Sn, named T5-Ge/Sn, as desirable TIs. These new allotropes are also highly anisotropic Dirac semimetals if the SOC is turned off. To the best of our knowledge, T5-Ge/Sn are the first 3D allotropes of Ge/Sn that possess topological states in their equilibrium states at room temperature. Additionally, their isostructures of C and Si are metastable indirect and direct semiconductors. Our work not only reveals two promising TIs, but more profoundly, we justify the advantages of group-IV elements as topological quantum materials (TQMs) for fundamental research and potential practical applications, and thus reveal a new direction in the search for desirable TQMs.