We perform linear and non-linear photon absorption calculations in topological insulator ultra-thin films on a substrate. Due to the unique band structure of the coupled topological surface states, novel features are observed for suitable photon frequencies, including a divergent edge singularity in one-photon absorption process and a significantly enhancement in two-photon absorption process. The resonanct frequencies can be controlled by tuning the energy difference and coupling of the top and bottom surface states. Such unique linear and nonlinear optical properties make ultra-thin films of topological insulators promising material building blocks for tunable high-efficiency nanophotonic devices.
We theoretically investigate the exchange interaction between magnetic impurities in ultrathin Bi$_2$Se$_3$ topological insulator films by taking into account the low-energy states produced by the impurities. We find that the locally induced impurity resonances strongly influence the exchange interaction between magnetic moments. In particular, we find a non-collinear alignment being more favorable than the collinear ferromagnetic alignment preferred when impurity states are ignored and only the pristine topological insulator band structure is considered. Moreover, we show that by applying of …
Uniform and large area synthesis of bulk insulating ultrathin films is an important subject toward applications of a surface of three dimensional topological insulators (3D-TIs) in various electronic devices. Here we report epitaxial growth of bulk insulating three dimensional topological insulator (3D-TI) Bi2-xSbxTe3-ySey (BSTS) ultrathin films, ranging from a few quintuple to several hundreds of layaers, on mica in a large-area (1 cm2) via catalyst free physical vapor deposition. These films can nondestructively be exfoliated using deionized water and transferred to various kinds of substrat…
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.
In an ultrathin topological insulator (TI) film, a hybridization gap opens in the TI surface states, and the system is expected to become either a trivial insulator or a quantum spin Hall insulator when the chemical potential is within the hybridization gap. Here we show, however, that these insulating states are destroyed by the presence of a large and long-range-correlated disorder potential, which converts the expected insulator into a metal. We perform transport measurements in ultrathin, dual-gated topological insulator films as a function of temperature, gate voltage, and magnetic field,…
Van der Waals epitaxial growth of topological insulator Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ ultrathin nanoplate on electrically insulating fluorophlogopite mica
我们报道了高质量的Bi $_{2-x}$ Sb $_x$ Te $_{3-y}$ Se $_y$纳米片（BSTS-NPs）的电绝缘氟金云母衬底上使用无催化剂的气相固体方法的生长。在优化的气压和合适的氩气流量下，我们控制了BSTS-NP的厚度、尺寸和组成。拉曼光谱显示系统的变化表明，BSTS-NP的厚度和组成确实是精确控制。电输运表明，在BSTS-NP中的鲁棒狄拉克锥载流子输运。由于BSTS-NP提供了可调谐狄拉克锥表面态的优越的主导表面输运，而体态的导电贡献可以忽略不计，因此BSTS-NP为探索三维拓扑绝缘体的内在物理现象以及未来的技术应用提供了理想的平台。
Quantum capacitance of an ultrathin topological insulator film in a magnetic field
本文研究了外磁场作用下双拓扑绝缘体薄膜的量子磁电容。本研究探讨塞曼作用与薄膜上下表面间杂化的相互作用。确定态密度，我们发现，电子空穴对称性被打破时，塞曼和杂化能量相对于彼此变化。这导致电荷中性点处的磁电容特性的变化。我们进一步表明，在塞曼相互作用和杂化的存在下，磁电容表现出拍在低和分裂的Shubnikov de Haas振荡在高垂直磁场。此外，我们解决了交叉从垂直到平行的磁场，并找到最近的实验数据的一致性。
The helicity of 3D topological insulator surface states has drawn significant attention in spintronics owing to spin-momentum locking where the carriers' spin is oriented perpendicular to their momentum. This property can provide an efficient method to convert charge currents into spin currents, and vice-versa, through the Rashba-Edelstein effect. However, experimental signatures of these surface states to the spin-charge conversion are extremely difficult to disentangle from bulk state contributions. Here, we combine spin- and angle-resolved photo-emission spectroscopy, and time-resolved THz …
Ultrafast carrier dynamics in the topological insulator Bi2Se3 have recently been intensively studied using a variety of techniques. However, we are not aware of any successful experiments exploiting transient absorption (TA) spectroscopy for these purposes. Here we demonstrate that if the ~730 nm wavelength pumping (~1.7 eV photon energy) is applied to ultrathin Bi2Se3 films, TA spectra cover the entire visible region, thus unambiguously pointing to two-photon excitation (~3.4 eV). The carrier relaxation dynamics is found to be governed by the polar optical phonon cascade emission occurring i…
Thermal Strain-Driven Periodic Ripples in Ultrathin Sb2Te3/Graphene Heterostructures and their Implications for Strain Engineering
Universidad de Chile（智利大学）；CEDENNA；Universidad Adolfo Ibáñez（阿道夫·伊巴涅斯大学）；Stanford University（斯坦福大学）；Universidad Técnica Federico Santa María（弗雷德里科·圣塔玛利亚技术大学）
We report a magnetotransport study of spin relaxation in 1.4-21.2 nm epitaxial SrIrO$_3$ thin films coherently strained on SrTiO$_3$ substrates. Fully charge compensated semimetallic transport has been observed in SrIrO$_3$ films thicker than 1.6 nm, where the charge mobility at 10 K increases from 45 cm$^2$/Vs to 150 cm$^2$/Vs with decreasing film thickness. In the two-dimensional regime, the charge dephasing and spin-orbit scattering lengths extracted from the weak localization/anti-localization effects show power-law dependence on temperature, pointing to the important role of electron-elec…
Interface effects and dielectric mismatch in ultrathin silicon on insulator films
CNR-IMM（意大利国家研究委员会材料研究所）；Università degli Studi di Milano-Bicocca（米兰比可卡大学）；Institut für Experimentelle Physik, TU Bergakademie Freiberg（弗莱贝格矿业科技大学实验物理研究所）；Università del Piemonte Orientale ‘‘A. Avogadro’’（皮埃蒙特东方大学）；Institut für Angewandte Physik, TU Bergakademie Freiberg（弗莱贝格矿业科技大学应用物理研究所）；University of Torino（都灵大学）
Observation of the surface hybridization gap in the electrical transport properties of the ultrathin topological insulator (Bi$_{1-x}$Sb$_{x}$)$_2$Te$_3$
Novel quantum phenomena, including high-temperature superconductivity, topological properties, and charge/spin density waves, appear in low-dimensional conductive materials. It is possible to artificially create low-dimensional systems by fabricating ultrathin films, quantum wires, or quantum dots with flat interfaces. Some experiments have been performed on ultrathin compounds of strongly correlated electron systems. However, since it is technically difficult to control multiple elements precisely, most of the properties of artificially fabricated low-dimensional compounds fall into uncharted…
As a promising candidate system to realize topological superconductivity, the system of a 3D topological insulator (TI) grown on top of the s-wave superconductor has been extensively studied. To access the topological superconductivity experimentally, the 3D TI sample must be thin enough to allow for Cooper pair tunneling to the exposed surface of TI. The use of magnetically ordered dopants to break time-reversal symmetry may allow the surface of a TI to host Majorana fermion, which are believed to be a signature of topological superconductivity. In this work, we study a magnetically-doped thi…
High harmonic generation (HHG) from semiconductors and insulators has become a very active area of research due to its great potential for developing compact HHG devices. Here we show that by growing monolayers (ML) of insulators on single-crystal metal surfaces, one can tune the harmonic spectrum by just varying the thickness of the ultrathin layer, not the laser properties. This is shown from numerical solutions of the time-dependent Schrödinger equation for $n$ML NaCl/Cu(111) systems ($n=1-50$) based on realistic potentials available in the literature. Remarkably, the harmonic cutoff increa…
We report the epitaxial growth of PdTe$_2$ ultrathin films on topological insulator Bi$_2$Se$_3$. A prominent Moiré pattern was observed in STM measurements. The Moiré periodicity increases as film thickness decreases, indicating a lattice expansion of epitaxial PdTe$_2$ thin films with lower thicknesses. In addition, our simulations based on Moiré Metrology reveal uniaxial lattice strains at the edge of PdTe$_2$ domains, and anisotropic strain distributions throughout the Moiré supercell with a net change in lattice strain up to ~2.9%. Our DFT calculations show that this strain effect leads t…
Insulator-to-Metal Transition via Magnetic Reconstruction at Oxide Interfaces
CAS Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences（中国科学院磁性材料与器件重点实验室，宁波材料技术与工程研究所）；Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences（浙江省磁性材料与应用技术重点实验室，宁波材料技术与工程研究所）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；Eastern Institute of Technology（东方理工学院）；International Research Centre Magtop, Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所磁顶国际研究中心）；CNR-SPIN, UOS Salerno（意大利国家研究委员会自旋中心萨莱诺联合实验室）；Photon Science Division, Paul Scherrer Institut（保罗谢尔研究所光子科学部）；Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences（中国科学院大学材料科学与光电工程技术中心）
The inverse spinel ferrimagnetic NiCo$_2$O$_4$ possesses high magnetic Curie temperature $T_C$, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo$_2$O$_4$ thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo$_2$O$_4$ films on MgAl$_2$O$_4$ substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films …
Circular dichroism (CD) observed by photoemission, being sensitive to the orbital and spin angular momenta of the electronic states, is a powerful probe of the nontrivial surface states of topological insulators, but the experimental results thus far have eluded a comprehensive description. We report a study of Bi2Te3 films with thicknesses ranging from one quintuple layer (two-dimensional limit) to twelve layers (bulk limit) over a wide range of incident photon energy. The data show complex variations in magnitude and sign reversals, which are nevertheless well described by a theoretical calc…
We determine the optical response of ultrathin film topological insulators in the presence of a quantizing external magnetic field taking into account both hybridization between surface states, broken inversion symmetry and explicit time reversal symmetry breaking by the magnetic field. We find that breaking of inversion symmetry in the system, which can be due to interaction with a substrate or electrical gating, results in Landau level crossings which lead to additional optical transition channels that were previously forbidden. We show that by tuning the hybridization and symmetry breaking …
Probing Dirac Fermion Dynamics in Topological Insulator Bi$_2$Se$_3$ Films with Scanning Tunneling Microscope
利用扫描隧道显微镜和光谱学研究了拓扑绝缘体Bi $_2$ Se $_3$薄膜中狄拉克费米子的飞秒动力学.在二维极限下，体电子被量子化，量子化可以通过薄膜厚度控制。通过研究驻波（准粒子干涉图样）的空间衰减，直接测量了拓扑表面态电子的相位弛豫长度$l_ϕ$和非弹性散射寿命$τ$与能量和膜厚的关系.我们发现，$τ$表现出显着的$(E-E_F)^{-2}$能量依赖性，并增加与膜厚度。我们发现，所揭示的功能是典型的表面和体状态之间的电子-电子散射。
We study an unconventional quantum Hall effect for the surface states of ultrathin Floquet topological insulators in a perpendicular magnetic field. The resulting band structure is modified by photon dressing and the topological property is governed by the low-energy dynamics of a single surface. An exchange of symmetric and antisymmetric surface states occurs by reversing the light's polarization. We find a novel quantum Hall state in which the zeroth Landau level undergoes a phase transition from a trivial insulator state, with Hall conductivity $σ_{yx}=0$ at zero Fermi energy, to a Hall ins…
Further development of the field of all-electric spintronics requires the successful integration of spin transport channels with spin injector/generator elements. While with the advent of graphene and related 2D materials high performance spin channel materials are available, the use of nanostructured spin generators remains a major challenge. Especially promising for the latter purpose are 3D topological insulators, whose 2D surface states host massless Dirac fermions with spin-momentum locking. Here, we demonstrate injection of spin-polarized current from a topological insulator into graphen…
Conductivities are key material parameters that govern various types of transport (electronic charge, spin, heat etc.) driven by thermodynamic forces. Magnons, the elementary excitations of the magnetic order, flow under the gradient of a magnon chemical potential in proportion to a magnon (spin) conductivity $σ_{m}$. The magnetic insulator yttrium iron garnet (YIG) is the material of choice for efficient magnon spin transport. Here we report an unexpected giant $σ_{m}$ in record-thin YIG films with thicknesses down to 3.7 nm when the number of occupied two-dimensional (2D) subbands is reduced…
The effect of magnetic field on an ultrathin magnetic topological insulator film with structural inversion asymmetry is investigated. We introduce the phase diagram, calculate the Landau-level spectrum analytically and simulate the transport behavior in Landauer-Büttiker formalism. The quantum anomalous Hall phase will survive increasing magnetic field. Due to the two spin polarized zero modes of Landau levels a nontrivial phase similar with quantum spin Hall effect can be induced by magnetic field which is protected by structural inversion symmetry. Some exotic longitudinal and Hall resistanc…
We explore the electronic structure and transport properties of a metal on top of a (weakly coupled) two-dimensional topological insulator. Unlike the widely studied junctions between topological non-trivial materials, the systems studied here allow for a unique bandstructure and transport steering. First, states on the topological insulator layer may coexist with the gapless bulk and, second, the edge states on one edge can be selectively switched-off, thereby leading to nearly perfect directional transport of charge and spin even in the zero bias limit. We illustrate these phenomena for Bern…
Bulk 1T$^\prime$-MoTe$_2$ shows a structural phase transition from 1T$^\prime$ to Weyl semimetallic (WSM) $ T_{d} $ phase at $\sim$ 240 K. This phase transition and transport properties in the two phases have not been investigated on ultra-thin crystals. Here we report electrical transport, $1/f$ noise and Raman studies in ultra-thin 1T$^\prime$-MoTe$_2$ ($\sim$ 5 to 16 nm thick) field-effect transistors (FETs) devices as a function of temperature. The electrical resistivities for thickness 16 nm and 11 nm show maxima at temperatures 208 K and 178 K, respectively, making a transition from semi…
