The propagation of electrons and photons can respectively have the spin-momentum locking effect which correlates the spin with the linear momentum. For the surface electrons in three-dimensional topological insulators (TIs), their spin is locked to the transport direction. For photons in optical fibers and photonic waveguides, they carry transverse spin angular momentum (SAM) which is also locked to the propagation direction. A direct connection between the electronic and the optical spins occurs in TIs with lifted spin degeneracy, which leads to spin-dependent selection rules of optical transitions and results in phenomena such as circular photogalvanic effect (CPGE). Here, we demonstrate an optoelectronic device that integrates a TI with a photonic waveguide. Interaction between the photons in the transverse-magnetic (TM) mode of the waveguide, which carries transverse SAM, and the surface electrons in a Bi2Se3 layer generates a directional, spin-polarized photocurrent. Because of optical spin-momentum locking, the device works in a non-reciprocal way such that changing the light propagation direction reverses the photon spin and thus the direction of the photocurrent in the TI. This novel device provides a directional interface that directly converts the photon propagation path to the direction and spin polarization of the photo-excited surface current in the TI. It represents a new way of implementing coupled spin-orbit interaction between electrons and photons and may lead to significant applications in opto-spintronics and quantum information processing.
Spin-momentum locking in protected surface states enables efficient electrical detection of magnon decay at a magnetic-insulator/topological-insulator heterojunction. Here we demonstrate this property using the spin Seebeck effect, i.e. measuring the transverse thermoelectric response to a temperature gradient across a thin film of yttrium iron garnet, an insulating ferrimagnet, and forming a heterojunction with (BixSb1-x)2Te3, a topological insulator. The non-equilibrium magnon population established at the interface can decay in part by interactions of magnons with electrons near the Fermi e…
We numerically study silicon waveguides on silica showing that it is possible to simultaneously guide optical and acoustic waves in the technologically important silicon on insulator (SOI) material system. Thin waveguides, or fins, exhibit geometrically softened mechanical modes at gigahertz frequencies with phase velocities below the Rayleigh velocity in glass, eliminating acoustic radiation losses. We propose slot waveguides on glass with telecom optical frequencies and strong radiation pressure forces resulting in Brillouin gains on the order of 500 and 50,000 1/(Wm) for backward and forwar…
Quantum spin Hall (QSH) insulator with large gap is highly desirable for potential spintronics application. Here we realize electrically tunable QSH insulator with large gap in van der Waals heterobilayer of monolayer transition metal dichalcogenide (TMD) and hexagonal BAs. When the type II band alignment gets inverted in an electric field, the hybridization by interlayer hopping between the spin-valley locked valence band edges in TMD and the BAs conduction band edges leads to a stacking-configuration dependent topological band inversion. In the non-interacting limit, the double spin degenera…
Pseudo-spin and valley degrees of freedom (DOFs) engineered in photonic analogues of topological insulators (TI) provide potential approaches to optical encoding and robust signal transport. Here we observe a ballistic edge state whose spin-valley indices are locked to the direction of propagation along the interface between a valley photonic crystal and a metacrystal emulating the quantum spin Hall effect. We demonstrate the inhibition of inter-valley scattering at a Y-junction formed at the interfaces between photonic TIs carrying different spin-valley Chern numbers. These results open up th…
Klein tunneling, the perfect transmission of normally incident Dirac electrons across a potential barrier, has been widely studied in graphene and explored to design switches, albeit indirectly. We show that Klein tunneling maybe easier to detect for spin-momentum locked electrons crossing a PN junction along a three-dimensional topological insulator surface. In these topological insulator PN junctions (TIPNJs), the spin texture and momentum distribution of transmitted electrons can be measured electrically using a ferromagnetic probe for varying gate voltages and angles of current injection. …
The spin-momentum locked nature of the robust surface states of three dimensional topological insulators (3D TI) make them promising candidates for spintronics applications. Surface potentials which respect time reversal symmetry can exist at the surface between a 3D TI and the trivial vacuum. These potentials can distort the spin texture of the surface states while retaining their gapless nature. In this work, the effect of all such surface potentials on the spin textures is studied. Since, a tunnel magnetoresistance signal carries the information of the spin texture, it is proposed that spin…
Electrical injection and detection of spin-polarized currents in topological insulator Bi2Te2Se
拓扑绝缘体（TI）是一种具有非平凡自旋-动量锁定拓扑表面态（TSS）的量子物质。直到最近，三维TI中TSS的自旋动量锁定的电检测一直缺乏。许多结果是在具有显著体传导的样品上测量的，例如金属Bi 2Se 3，其中分离表面和体对测量的自旋信号的贡献可能具有挑战性。在这里，我们报告的自旋电位测量薄片剥落的大块绝缘3D TI Bi 2 Te 2Se（BTS 221）晶体，使用两个外部的金（Au）接触驱动的DC自旋螺旋电流和中间的铁磁（FM）-Al 2 O3隧道接触检测自旋极化。当沿FM触点的易磁化轴在相反方向之间扫描平面内磁场以切换其磁化时，由FM电极测量的电压呈现滞后阶梯状变化。重要的是，该阶跃状电压变化的方向可以通过反转DC电流的方向来反转，并且如通过相反FM磁化之间的检测器电压的差所测量的变化的幅度随着偏置电流的增加而线性增加，这与自旋动量锁定TSS的电流诱导的自旋极化一致。我们的工作直接证明了电注入和检测自旋极化TI，并可能使利用自旋螺旋TSS在纳米电子学和自旋电子学的未来应用。
Spin-momentum locked (SML) topological surface state (TSS) provides exotic properties for spintronics applications. The spin-polarized current, which emerges owing to the SML, can be directly detected by performing spin potentiometric measurement. We observed spin-polarized current using a bulk insulating topological insulator (TI), Bi1.5Sb0.5Te1.7Se1.3, and Co as the ferromagnetic spin probe. The spin voltage was probed with varying the bias current, temperature, and gate voltage. Moreover, we observed non-local spin-polarized current, which is regarded as a distinguishing property of TIs. Th…
Optical control of electrons in a Floquet topological insulator
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)（埃尔朗根-纽伦堡弗里德里希·亚历山大大学）；University of Central Florida（中佛罗里达大学）；Technion - Israel Institute of Technology（以色列理工学院）；Max Planck Institute for the Structure and Dynamics of Matter（马普物质结构与动力学研究所）；Ludwig-Maximilians-Universität, München（慕尼黑路德维希-马克西米利安大学）
Robustness of helical hinge states of three-dimensional weak second-order topological i sulators (WSOTIs) against disorders is studied. The pure WSOTI is obtained from a weak $\mathbb{Z}_2$ first-order topological insulator through a surface band inversion. Both bulk states and surface states in the WSOTI are gapped, and in-gap valley-momentum locked helical hinge states are topologically protected by the surface valley-Chern number. In the presence of weak disorders, helical hinge states are robust against disorders while the quantized conductance of the states is fragile due to the inter-val…
Surface conduction and pi-bonds in graphene and topological insulator Bi2Se3
提出了拓扑绝缘体Bi 2 Se 3的杂化轨道模型，并与石墨烯的杂化轨道模型进行了比较。在Se 1层的范德华间隙和表面上的π键三聚体的存在被认为是负责在Bi 2 Se 3中发现的不寻常的表面导电机制作为拓扑绝缘体。这三个π键被锁定为范德华能隙中Se 1层之间的永久电偶极，作为带隙半导体对体块的吸引力。相反，表面上的π键三聚体被解锁，并表现出两个简并量子态，在跨桥模型中产生局部电荷转移机制，该机制负责表面导电。Bi 2 Se 3表面上的π键的作用进行了比较，在石墨烯中的一个类似的二维能带结构包含狄拉克锥。
Topological insulators are promising for spintronics and related technologies due to their spin-momentum-locked edge states, which are protected by time-reversal symmetry. In addition to the unique fundamental physics that arises in these systems, the potential technological applications of these protected states has also been driving TI research over the past decade. However, most known topological insulator materials naturally contain spinful nuclei, and their hyperfine coupling to helical edge states intrinsically breaks time-reversal symmetry, removing the topological protection and enabli…
Three-dimensional topological insulators are a class of Dirac materials, wherein strong spin-orbit coupling leads to two-dimensional surface states. The latter feature spin-momentum locking, i.e., each momentum vector is associated with a spin locked perpendicularly to it in the surface plane. While the principal spin generation capability of topological insulators is well established, comparatively little is known about the interaction of the spins with external stimuli like polarized light. We observe a helical, bias-dependent photoconductance at the lateral edges of topological Bi2Te2Se pla…
The binary chalcogenides Bi2Te3 and Bi2Se3 are the most widely studied topological insulators. Although the quantum anomalous Hall effect has recently been observed in magnetically doped Sb2Te3 this compound has been studied to a much lesser extend. Here, by using energy resolved quasiparticle interference mapping, we investigate the scattering events of pristine Sb2Te3 surfaces. We find that, in addition to the Dirac fermions, another strongly spin polarized surface resonance emerges at higher energies in the unoccupied electronic states. Although the two surface states are of different origi…
Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi2Se3. Using transient second harmonic generation, a technique capable of independently mon…
The surface state of a 3D topological insulator (3DTI) is a spin-momentum locked conductive state, whose large spin hall angle can be used for the energy-efficient spin orbit torque based switching of an overlying ferromagnet (FM). Conversely, the gated switching of the magnetization of a separate FM in or out of the TI surface plane, can turn on and off the TI surface current. The gate tunability of the TI Dirac cone gap helps reduce its sub-threshold swing. By exploiting this reciprocal behaviour, we can use two FM/3DTI heterostructures to design a 1-Transistor 1-magnetic tunnel junction ran…
Introducing, observing, and manipulating individual impurities coupled to a host material offers the opportunity to create new device concepts based on single spin and charge states. Because of potential applications in spintronics and magneto-electrics, such an approach would be particularly useful for topological insulators (TI), a recently discovered material class hosting spin-momentum- locked surface states. To make them useful for new technologies, a robust control of their interaction with external perturbations is required. However, traditional approaches such as metal electrodes or do…
Incorporating relativistic physics into quantum tunneling can lead to exotic behavior such as perfect transmission via Klein tunneling. Here, we probe the tunneling properties of spin-momentum locked relativistic fermions by designing and implementing a tunneling geometry that utilizes nanowires of the topological Kondo insulator candidate, SmB6. The nanowires are attached to the end of scanning tunneling microscope tips, and used to image the bicollinear stripe spin-order in the antiferromagnet Fe1.03Te with a Neel temperature of ~50 K. The antiferromagnetic stripes become invisible above 10 …
The search for novel magnetic quantum phases, phenomena and functional materials has been guided by relativistic magnetic-symmetry groups in coupled spin and real space from the dawn of the field in 1950s to the modern era of topological matter. However, the magnetic groups cannot disentangle non-relativistic phases and effects, such as the recently reported unconventional spin physics in collinear antiferromagnets from the typically weak relativistic spin-orbit coupling phenomena. Here we discover that more general spin symmetries in decoupled spin and crystal space categorize non-relativisti…
We investigate a spin-electricity conversion effect in a topological insulator/ferromagnet heterostructure. In the spin-momentum-locked surface state, an electric current generates nonequilibrium spin accumulation, which causes a spin-orbit torque that acts on the ferromagnet. When spins in the ferromagnet are completely parallel to the accumulated spin, this spin-orbit torque is zero. In the presence of spin excitations, however, a coupling between magnons and electrons enables us to obtain a nonvanishing torque. In this paper, we consider a model of the heterostructure in which a three-dimen…
Ambipolar Field Effect in Topological Insulator Nanoplates of (BixSb1-x)2Te3
拓扑绝缘体代表了量子物质的一种新状态，对基础物理和技术应用（如自旋电子学和量子信息处理）都有吸引力。在拓扑绝缘体中，体能隙被自旋-动量锁定的表面态穿过，形成具有独特电子性质的奇数个表面带。然而，输运测量通常由来自晶体缺陷或环境掺杂的残留体载流子主导，其掩盖了拓扑表面贡献。在这里，我们证明（BixSb 1-x）2 Te 3作为一个可调的拓扑绝缘体系统，通过改变Bi/Sb的组成比来操纵体电导率。通过角分辨光电子能谱（ARPES）测量和从头计算，证实了（BixSb 1-x）2 Te 3在整个组成范围内都是拓扑绝缘体。此外，我们观察到一个明确的双极门控效应类似于在石墨烯中观察到的（BixSb 1-x）2 Te 3的场效应晶体管（FET）器件中使用纳米片。本研究中展示的拓扑绝缘体纳米结构中载流子类型和浓度的操纵为拓扑绝缘体在纳米电子学和自旋电子学中的实现铺平了道路。
Sign control of photocurrents by spin-group-symmetry breaking in altermagnetic insulators
Institute for Theoretical Solid State Physics, IFW Dresden（德累斯顿亥姆霍兹中心IFW理论固体物理研究所）；State Key Laboratory of Ultra-intense laser Science and Technology, Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS)（中国科学院上海光学精密机械研究所超强激光科学技术国家重点实验室）；Würzburg-Dresden Cluster of Excellence ctd.qmat（维尔茨堡-德累斯顿卓越集群ctd.qmat）；Collective Dynamics and Quantum Transport Unit, Okinawa Institute of Science and Technology Graduate University（冲绳科学技术大学院大学集体动力学与量子输运单元）
Topological insulators (TIs) with spin momentum locked topological surface states (TSS) are expected to exhibit a giant spin-orbit torque (SOT) in the TI/ferromagnet systems. To date, the TI SOT driven magnetization switching is solely reported in a Cr doped TI at 1.9 K. Here, we directly show giant SOT driven magnetization switching in a Bi2Se3/NiFe heterostructure at room temperature captured using a magneto-optic Kerr effect microscope. We identify a large charge to spin conversion efficiency of ~1-1.75 in the thin TI films, where the TSS is dominant. In addition, we find the current densit…
Dislocations are ubiquitous in three-dimensional solid-state materials. The interplay of such real space topology with the emergent band topology defined in reciprocal space gives rise to gapless helical modes bound to the line defects. This is known as bulk-dislocation correspondence, in contrast to the conventional bulk-boundary correspondence featuring topological states at boundaries. However, to date rare compelling experimental evidences are presented for this intriguing topological observable, owing to the presence of various challenges in solid-state systems. Here, using a three-dimens…
Electrical transport in three dimensional topological insulators(TIs) occurs through spin-momentum locked topological surface states that enclose an insulating bulk. In the presence of a magnetic field, surface states get quantized into Landau levels giving rise to chiral edge states that are naturally spin-polarized due to spin momentum locking. It has been proposed that p-n junctions of TIs in the quantum Hall regime can manifest unique spin dependent effects, apart from forming basic building blocks for highly functional spintronic devices. Here, for the first time we study electrostaticall…
A topological insulator surface under strong Coulomb, magnetic and disorder perturbations
三维拓扑绝缘体体现了一种新发现的物质状态，其特征在于通过自旋分辨的ARPES测量证明了跨越体带隙的传导自旋动量锁定表面状态。这种极不寻常的地表环境为发现新的物理现象提供了丰富的基础。在这里，我们提出了第一个控制下的拓扑绝缘体表面的库仑，磁和无序扰动的研究。我们使用具有大库仑态和显着磁矩的铁的相互作用作为模型拓扑绝缘体Bi $_2$ Se $_3$的拓扑表面态的\textit{systematically test the robustness}探针。我们观察到，强扰动导致狄拉克费米子的奇数倍的创建和磁相互作用打破时间反演对称性的存在下，带杂交。我们还提出了一个理论模型来解释改变表面的Bi $_2$ Se $_3$。总的来说，这些结果是一个重要的指导，在操纵拓扑表面探测基础物理或开发设备的应用。
There has been considerable interest in exploiting the spin degrees of freedom of electrons for potential information storage and computing technologies. Topological insulators (TI), a class of quantum materials, have special gapless edge/surface states, where the spin polarization of the Dirac fermions is locked to the momentum direction. This spin-momentum locking property gives rise to very interesting spin-dependent physical phenomena such as the Edelstein and inverse Edelstein effects. However, the spin injection in pure surface states of TI is very challenging because of the coexistence …
Three-dimensional topological insulator (3D TI) nanowires display various interesting magnetotransport properties that can be attributed to their spin-momentum-locked surface states such as quasiballistic transport and Aharonov-Bohm oscillations. Here, we focus on the transport properties of a 3D TI nanowire with a gated section that forms an electronic Fabry-Pérot (FP) interferometer that can be tuned to act as a surface-state filter or energy barrier. By tuning the carrier density and length of the gated section of the wire, the interference pattern can be controlled and the nanowire can bec…
We investigate symmetry-protected topological water waves within a strategically engineered square lattice system. Thus far, symmetry-protected topological modes in hexagonal systems have primarily been studied in electromagnetism and acoustics, i.e. dispersionless media. Herein, we show experimentally how crucial geometrical properties of square structures allow for topological transport that is ordinarily forbidden within conventional hexagonal structures. We perform numerical simulations that take into account the inherent dispersion within water waves and devise a topological insulator tha…
Quantum spin Hall insulator is characterized by the helical edge states, with the spin polarization of electron being locked to its direction of motion. Although the edge-state conduction has been observed, unambiguous evidence of the helical spin texture is still lacking. Here, we investigate the coherent edge-state transport in an interference loop pinched by two point contacts. Due to the helical character, the forward inter-edge scattering enforces a $π$ spin rotation. Two successive processes can only produce a nontrivial $2π$ or trivial $0$ spin rotation, which can be controlled by the R…
Gate-tuned Differentiation of Surface-conducting States in Bi1.5Sb0.5Te1.7Se1.3 Topological-insulator Thin Crystals
使用场角，温度和背栅电压依赖性的弱反本地化（WAL）和普遍的电导波动薄Bi 1.5 Sb 0.5 Te 1.7 Se 1.3拓扑绝缘体单晶，结合栅极调谐霍尔电阻率测量，我们可靠地分离的拓扑性质的表面导电体导电和拓扑平凡的表面导电。我们最大限度地减少了晶体中的体传导和背栅调谐费米能级的拓扑底表面带，同时保持顶面不敏感的背栅与最佳晶体厚度？100 nm。我们认为，在相对较低的磁场的载流子的相干扩散运动发生的WAL效应是更重要的比其他运输工具，如Shubnikov-de Hass振荡确认导电的拓扑保护的表面状态。我们的方法提供了一个高度连贯的图片的表面输运性质的TI和一个可靠的手段，调查的基本拓扑性质的表面导电和可能的量子器件的应用相关的动量锁定自旋极化的表面状态。
