We analyze the four-dimensional Hamiltonian proposed to describe the band structure of the single-Dirac-cone family of topological insulators in the presence of a uniform perpendicular magnetic field. Surface Landau level(LL) states appear, decoupled from the bulk levels and following the quantized energy dispersion of a purely two-dimensional surface Dirac Hamiltonian. A small hybridization gap splits the degeneracy of the central n=0 LL with dependence on the film thickness and the field strength that can be obtained analytically. Explicit calculation of the spin and charge densities show that surface LL states are localized within approximately one quintuple layer from the surface termination. Some new surface-bound LLs are shown to exist at a higher Landau level index.
We study spin conductivity of the surface states in a thin film of a topological insulator with Kubo formulas. Hybridization between the different sides of the film opens a gap at the Dirac point. We found that in the gapped region spin conductivity remains finite. In the gapless region near the band gap spin conductivity is enhanced. These findings make a thin film of a topological insulator as a promising material for spintronic applications.
The purpose of this article is to develop a systematic approach to the solution of the Landau-Lifshitz equation in terms of the magnon wave function ψ (r) and apply it to physical phenomena in a thin ferromagnetic film. This article has also a purpose to represent the modern state of art for the properties of ferromagnetic films and the pumping-induced Bose-Einstein condensation of magnons in them at room temperature. Thus, it can be considered as a review on basic principles and the recent advances in the field.
Ultrafast carrier dynamics in thin-films of the topological insulator Bi2Se3
瞬态反射率测量的薄膜，从6到40 nm的厚度范围内，的拓扑绝缘体Bi 2 Se 3的载流子弛豫时间上的膜厚度显示出很强的依赖性。对于较厚的膜，弛豫动力学与体相Bi2Se3的弛豫动力学相似，其中体绝缘相的贡献大于表面金属相的贡献。载流子弛豫时间随着膜厚度的减小而缩短，达到与贵金属相当的值。这种效应可能是由于狄拉克锥态在相对表面的最薄的膜的杂交。
Axion insulator is an exotic magnetic topological insulator with zero Chern number but a nonzero quantized Chern-Simons magnetoelectric coupling. A conclusive experimental evidence for axion insulators is still lacking due to the small signal of topological magnetoelectric effect (TME). Here we show that the dynamical magnetoelectric coupling can be induced by the \emph{out-of-plane} surface magnetization dynamics in axion insulator thin films, which further generates a polarization current in the presence of an external magnetic field in the same direction. Such a current is finite in the bul…
Transport Properties of Topological Insulators Films and Nanowires
近几年来，人们对拓扑绝缘体的研究和认识发展迅速。本文在简要总结了拓扑绝缘体的发展历史后，重点介绍了拓扑绝缘体薄膜和纳米线输运实验的最新进展。一些量子现象，包括弱反局域化，Aharonov玻姆效应和Shubnikov de Haas振荡，在这些纳米结构中观察到的描述。此外，超导邻近效应的电子输运证据，以及在拓扑绝缘体/超导体混合结构的异常电阻增强包括在内。
Electric Field Tuning of the Surface Band Structure of Topological Insulator Sb2Te3 Thin Films
用低温扫描隧道显微镜测量了外延Sb_2Te_3薄膜的表面态谱对外加栅电场的响应。费米能级的栅极相关位移和体载流子的屏蔽效应随膜厚的变化而变化。我们观察到的狄拉克点的薄膜薄于四五层，由于顶部和底部表面的耦合的间隙开口。此外，三个五层膜的顶面态带隙被发现是可调的背栅，表明在该系统中观察到拓扑相变的可能性。我们的结果很好地解释了一个有效的模型的三维拓扑绝缘体薄膜的结构反转不对称性，表明三个五层Sb 2 Te 3膜是拓扑非平凡的，属于量子自旋霍尔绝缘体类。
We theoretically show that the three-dimensional (3D) topological insulator (TI)/thin-film ferromagnetic metal (FMM) bilayer structure is possible to be a quantum anomalous Hall (QAH) insulator with a wide global band gap. Studying the band structure and the weight distributions of eigenstates, we demonstrate that the attachment of a metallic thin-film on the 3DTI can shift the topologically non-trivial state into the metal layers due to the hybridization of bands around the original Dirac point. By introducing the magnetic exchange interaction in the thin-film layers, we compute the anomalous…
Twisting the stacking of layered materials leads to rich new physics. A three dimensional (3D) topological insulator film host two dimensional gapless Dirac electrons on top and bottom surfaces, which, when the film is below some critical thickness, will hybridize and open a gap in the surface state structure. The hybridization gap can be tuned by various parameters such as film thickness, inversion symmetry, etc. according to the literature. The 3D strong topological insulator Bi(Sb)Se(Te) family have layered structures composed of quintuple layers (QL) stacked together by van der Waals inter…
Propagation of Surface Plasmon Polaritons in Thin Films of Topological Insulators
研究了表面等离子激元在拓扑绝缘体薄膜中的传播。所考虑的材料是第二代三维拓扑绝缘体Bi $_2$ Se $_3$、Bi $_2$ Te $_3$和Sb $_2$ Te $_3$。色散关系和传播长度的数值估计，考虑到拓扑绝缘体以及基板使用Drude-Lorentz模型的体介电函数的变化。确定了影响传播长度的关键因素，并提出了修改色散关系的方法。对Bi $_2$ Se $_3$的实验数据与理论值之间的明显差异提出了解释。
Magnon transport through a magnetic insulator can be controlled by current-biased heavy-metal gates that modulate the magnon conductivity via the magnon density. Here, we report nonlinear modulation effects in 10$\,$nm thick yttrium iron garnet (YIG) films. The modulation efficiency is larger than 40\%/mA. The spin transport signal at high DC current density (2.2$\times 10^{11}\,$A/m$^{2}$) saturates for a 400$\,$nm wide Pt gate, which indicates that even at high current levels a magnetic instability cannot be reached in spite of the high magnetic quality of the films.
Electron-phonon scattering in topological insulator thin films
本文从理论上研究了强拓扑绝缘体薄膜中的电子-声子散射效应。声子模型由各向同性弹性连续介质理论与应力自由边界条件，和螺旋表面狄拉克费米子的相互作用是介导的变形势。我们确定了随温度变化的电阻率$ρ(T)$和准粒子衰减率$Γ(T)$可观察到的光电发射。这两个数量的低温和高温幂律得到解析。提供了Bi $_2$ Se $_3$的全部温度范围的详细估算值。
Floquet engineering is an important way to manipulate the electronic states of condensed matter physics. Recently, the discovery of the magnetic topological insulator MnBi$_2$Te$_4$ and its family provided a valuable platform to study magnetic topological phenomena, such as, the quantum anomalous Hall effect, the axion insulator state and the topological magnetoelectric effect. In this work, based on the effective model and first-principles calculations in combination with the Floquet theory, we reveal that the circularly polarized light (CPL) induces the sign reversal of the Chern number of o…
Large Chern Number Quantum Anomalous Hall Effect In Thin-film Topological Crystalline Insulators
量子反常霍尔绝缘体是一种二维绝缘态，在没有外加磁场的情况下，表现出与量子霍尔态相似的性质。它们具有量子化的霍尔电导$σ^H=Ce^2/h$，其中整数$C$被称为陈数，并且表示无间隙边缘模式的数量。最近的实验表明，掺铬薄膜（Bi，Sb）$_2$ Te $_3$是一种QAH绝缘体，具有陈数$C=\pm1$。在这里，我们从理论上预测，薄膜拓扑晶体绝缘体（TCI）可以托管各种QAH相，当掺杂铁磁有序掺杂剂。$\pm4$之间的任何陈氏数原则上可以作为（a）取决于磁掺杂浓度的感应塞曼场，（b）固有的或由压电材料通过邻近效应感应的结构变形，以及（c）薄膜的厚度之间的相互作用的结果而达到。在TCI中发现的可调陈氏数在超低功耗信息处理应用中具有重要的潜力。
Topological phases of topological insulator thin films
我们研究了一种拓扑绝缘体材料薄膜的性质。我们处理的螺旋状态之间的耦合在相对的表面的膜在适当的适应隧道近似，并表明隧道矩阵元振荡作为函数的膜厚度和动量在平面的膜Bi $_2$ Se $_3$和Bi $_2$ Te $_3$。因此，虽然表面布里渊区中心的矩阵元的大小给出了能谱中的间隙，但矩阵元的符号唯一地确定了膜的拓扑性质，如通过明确计算赝自旋纹理和陈数所证明的。我们发现一个序列的拓扑和非拓扑阶段之间的过渡，分离的半金属状态，作为薄膜厚度的变化。在拓扑相，薄膜的边缘态总是存在的，但只有在边缘势打破粒子-空穴对称性时才携带自旋流。边缘状态从Bi $_2$ Se $_3$中的边界衰减非常缓慢，使得Bi $_{2}$ Te $_{3}$，其中该尺度较短，是观察这些状态的更有希望的候选者。我们的研究结果适用于独立的薄膜以及具有大间隙绝缘体的异质结构。
Topological insulator (TI) thin films with surface magnetism are expected to exhibit a quantized anomalous Hall effect (QAHE) when the magnetizations on the top and bottom surfaces are parallel, and a quantized topological magnetoelectric (QTME) response when the magnetizations have opposing orientations (axion insulator phase) and the films are sufficiently thick. We present a unified picture of both effects that associates deviations from exact quantization of the QTME caused by finite thickness with non-locality in the side-wall current response function. Using realistic tight-binding model…
We consider a disordered topological insulator thin film placed on the top of a ferromagnetic insulator with a perpendicular exchange field $M$ and subjected to a perpendicular electric field. The presence of ferromagnetic insulator causes that bottom surface states of the topological insulator thin film become spin polarized and the electric field provides a potential difference $V$ between the two surface states, resulting in breaking of time-reversal and inversion symmetry in the system. Using Kubo formalism and employing the first Born approximation as well as the self-consistent Born appr…
Motivated by recent transport experiments, we theoretically study the quantum Hall effect in topological semimetal films. Owing to the confinement effect, the bulk subbands originating from the chiral Landau levels establish energy gaps that have quantized Hall conductance and can be observed in relatively thick films. We find that the quantum Hall state is strongly anisotropic for different confinement directions not only due to the presence of the surface states but also because of the bulk chiral Landau levels. As a result, we re-examine the quantum Hall effect from the surface Fermi arcs a…
Granularity Controlled Non-Saturating Linear Magneto-resistance in Topological Insulator Bi2Te3 Films
我们报告的磁输运性质的化学气相沉积生长的互连Bi 2 Te 3纳米片的薄膜。类似于许多其他拓扑绝缘体（TI）材料，这些颗粒状Bi 2 Te 3薄膜表现出线性磁阻（LMR）效应，这最近受到了很多关注。研究了不同粒度的样品，我们发现LMR的大小与薄膜的平均迁移率（μ）在近两个数量级的变化范围内具有普遍的相关性。根据Parish-Littlewood理论，颗粒控制的LMR效应归因于迁移率涨落引起的经典LMR。这些发现对TI和小带隙半导体材料的基本理解和磁阻器件应用都有影响。
