Spin-polarized band structure of the three-dimensional quantum spin Hall insulator $\rm Bi_{1-x}Sb_{x}$ (x=0.12-0.13) was fully elucidated by spin-polarized angle-resolved photoemission spectroscopy using a high-yield spin polarimeter equipped with a high-resolution electron spectrometer. Between the two time-reversal-invariant points, $\bar{\varGamma}$ and $\bar{M}$, of the (111) surface Brillouin zone, a spin-up band ($Σ_3$ band) was found to cross the Fermi energy only once, providing unambiguous evidence for the strong topological insulator phase. The observed spin-polarized band dispersions determine the "mirror chirality" to be -1, which agrees with the theoretical prediction based on first-principles calculations.
We report a proximity-driven large anomalous Hall effect in all-telluride heterostructures consisting of ferromagnetic insulator Cr2Ge2Te6 and topological insulator (Bi,Sb)2Te3. Despite small magnetization in the (Bi,Sb)2Te3 layer, the anomalous Hall conductivity reaches a large value of 0.2e2/h in accord with a ferromagnetic response of the Cr2Ge2Te6. The results show that the exchange coupling between the surface state of the topological insulator and the proximitized Cr2Ge2Te6 layer is effective and strong enough to open the sizable exchange gap in the surface state.
The chirality associated with broken time reversal symmetry in magnetically doped topological insulators has important implications to the quantum transport phenomena. Here we report the anomalous Hall effect studies in Mn- and Cr-doped Bi$_2$Te$_3$ topological insulators with varied thickness and doping content. By tracing the chirality of the Hall loops, we find that the Mn-type anomalous Hall effect with clockwise chirality is strengthened by the reduction of film thickness, which is opposite to that of the Cr-type anomalous Hall effect with counterclockwise chirality. We provide a phenomen…
Quantized Anomalous Hall Effect in Magnetic Topological Insulators
霍尔效应、反常霍尔效应和自旋霍尔效应分别是由洛伦兹力和自旋轨道耦合引起的固体中的基本输运过程。近年来，人们发现了量子形式的霍尔效应和自旋霍尔效应。然而，量子化反常霍尔（QAH）效应尚未在实验上实现。在QAH绝缘体中，自发磁矩和自旋-轨道耦合结合产生拓扑非平凡的电子结构，导致量子化的霍尔效应而无需任何外部磁场。在这项工作中，基于最先进的第一性原理计算，我们预测的辉碲铋矿半导体Bi $_2$ Te $_3$，Bi $_2$ Se $_3$，和Sb $_2$ Te $_3$形成磁有序的绝缘体时，掺杂过渡金属元素（Cr或Fe），在鲜明对比，传统的稀磁半导体中，自由载流子是必要的调解磁耦合。二维薄膜中的磁序产生了以有限的陈数为特征的拓扑电子结构，具有量子化的霍尔电导$e^{2}/h$。长期追求的QAH绝缘体状态的实验实现可以在室温下实现稳健的无耗散电荷传输。
A mechanism for phonon Hall effect (PHE) in non-magnetic insulators under an external magnetic field is theoretically studied. PHE is known in (para)magnetic compounds, where the magnetic moments and spin-orbit interaction play an essential role. In sharp contrast, we here show that a non-zero Berry curvature of acoustic phonons is induced by an external magnetic field due to the correction to the adiabatic Born-Oppenheimer approximation. This results in the finite thermal Hall conductivity $κ_H$ in nonmagnetic band insulators. Our estimate of $κ_H$ for a simple model gives $κ_H \sim 1.0\times…
The spin Hall effect (SHE) is normally discussed in terms of a spin current, which is ill-defined in strongly spin-orbit-coupled systems because of spin non-conservation. In this work we propose an alternative view of SHE phenomena by relating them to a spin analog of charge polarization induced by an electric field. The spin density polarization is most conveniently defined in insulators, which can have a SHE if they break time-reversal symmetry, i.e. if they are magnetic. The reciprocal of this SHE is a counterpart of the inverse SHE (ISHE), and is manifested in magnetic insulators as a char…
A key feature of the topological surface state under a magnetic field is the presence of the zeroth Landau level at the zero energy. Nonetheless, it has been challenging to probe the zeroth Landau level due to large electron-hole puddles smearing its energy landscape. Here, by developing ultra-low-carrier density topological insulator Sb$_2$Te$_3$ films, we were able to reach an extreme quantum limit of the topological surface state and uncover a hidden phase at the zeroth Landau level. First, we discovered an unexpected quantum-Hall-to-insulator-transition near the zeroth Landau level. Then, …
The recently introduced classification of two-dimensional insulators in terms of topological crystalline invariants has been applied so far to "obstructed" atomic insulators characterized by a mismatch between the centers of the electronic Wannier functions and the ionic positions. We extend this notion to quantum spin Hall insulators in which the ground state cannot be described in terms of time-reversal symmetric localized Wannier functions. A system equivalent to graphene in all its relevant electronic and topological properties except for a real-space obstruction is identified and studied …
A topological insulator (TI) interfaced with a magnetic insulator (MI) may host an anomalous Hall effect (AHE), a quantum AHE, and a topological Hall effect (THE). Recent studies, however, suggest that coexisting magnetic phases in TI/MI heterostructures may result in an AHE-associated response that resembles a THE but in fact is not. This article reports a genuine THE in a TI/MI structure that has only one magnetic phase. The structure shows a THE in the temperature range of T=2-3 K and an AHE at T=80-300 K. Over T=3-80 K, the two effects coexist but show opposite temperature dependencies. Co…
In insulators, the longitudinal resistivity becomes infinitely large at zero temperature. For classic insulators, the Hall conductivity becomes zero at the same time. However, there are special systems, such as two-dimensional quantum Hall isolators, in which a more complex scenario is observed at high magnetic fields. Here, we report experimental evidence for a quasi-quantized Hall insulator in the quantum limit of the three-dimensional semimetal SrSi2. Our measurements reveal a magnetic field-range, in which the longitudinal resistivity diverges with decreasing temperature, while the Hall co…
In a magnetic topological insulator, nontrivial band topology conspires with magnetic order to produce exotic states of matter that are best exemplified by quantum anomalous Hall (QAH) insulators and axion insulators. Up till now, such magnetic topological insulators are obtained by doping topological insulators with magnetic atoms. The random magnetic dopants, however, inevitably introduce disorders that hinder further exploration of quantum effects in the material. Here, we resolve this dilemma by probing quantum transport in MnBi$_2$Te$_4$ thin flake - a topological insulator with intrinsic…
Electrical detection of topological magnetic textures such as skyrmions is currently limited to conducting materials. While magnetic insulators offer key advantages for skyrmion technologies with high speed and low loss, they have not yet been explored electrically. Here, we report a prominent topological Hall effect in Pt/Tm$_3$Fe$_5$O$_{12}$ bilayers, where the pristine Tm$_3$Fe$_5$O$_{12}$ epitaxial films down to 1.25 unit cell thickness allow for tuning of topological Hall stability over a broad range from 200 to 465 K through atomic-scale thickness control. Although Tm$_3$Fe$_5$O$_{12}$ i…
We use temperature-dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in heavy metal/ferrimagnetic insulator heterostructures with perpendicular magnetic anisotropy. This approach enables detection of both the magnetic proximity effect onset temperature and the magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. Onset of a magnetic proximity effect yields a local extremum in the temperature-dependent anomalous Hall signal, which occurs at h…
Large-gap quantum spin Hall (QSH) insulators were previously predicted in stanene and its derivatives. Beyond stanene that is the thinnest $α$-Sn(111) film, we propose to explore QSH insulators in $α$-Sn films with different crystallographic orientations. Our first-principles calculations reveal that the thickness-dependent band gap of $α$-Sn (100) and (110) films does not show a monotonic decrease as typically expected by quantum confinement, but displays an oscillating change behavior, an indicative of topological quantum phase transition. While these films are normal insulators in the ultra…
Quantum spin Hall insulators in centrosymmetric thin films composed from topologically trivial BiTeI trilayers
量子自旋霍尔绝缘体预测十年前，现在实验观察到的是有助于突破纳米电子学，由于非耗散的自旋极化电子输运通过其边缘。为了在正常条件下持续这种传输，绝缘体应该在稳定的拓扑相中具有足够大的带隙。在这里，我们从理论上表明，量子自旋霍尔绝缘体可以实现在超薄膜构造从一个平凡的带绝缘体与强自旋轨道耦合。最薄的薄膜，具有足够大的反向间隙的实际应用是一个中心对称的六重层建立了两个反向堆叠的非中心对称的BiTeI三层。这个非平凡的六重层原来是人工设计的强三维拓扑绝缘体Bi $_2$ Te $_2$ I $_2$的结构元素。我们揭示了拓扑绝缘体如何由BiTeX家族（X=I，Br，Cl）的结构元素组成的一般原理，这为拓扑相的工程设计开辟了新的视角。
Designing in-plane heterostructures of quantum spin hall insulators from first principles: 1T'-MoS2 with adsorbates
正常和拓扑绝缘体之间的界面被绑定到主机金属状态的时间反演对称性的保护，因此对无序和界面重建鲁棒。二维拓扑绝缘体（量子自旋霍尔绝缘体）提供了通过吸附原子或分子来改变局部拓扑的独特机会，因此构成了用于设计拓扑异质结构的理想平台。在这里，我们应用第一性原理计算表明，量子自旋霍尔绝缘体1 T '-MoS 2在吸附各种原子时表现出向平凡绝缘体的相变。然后，它表明，一维金属边界状态确实出现在带状几何形状的交替区域和不吸附氧，这些边界状态一般构成简单的线性连接之间的价带和导带。这与拓扑边缘状态形成鲜明对比，拓扑边缘状态通常表现出对特定边缘终止敏感的强色散。异质结构也暗示了在1 T '-MoS 2片中的一维金属网络的简单设计。
