Magnetic Proximity Effect in an Antiferromagnetic Insulator/Topological Insulator Heterostructure with Sharp Interface

We report an experimental study of electron transport properties of MnSe/(Bi,Sb)_2Te_3 heterostructures,in which MnSe is an antiferromagnetic insulator,and(Bi,Sb)_2Te_3 is a three-dimensional topological insulator(TI).Strong magnetic proximity effect is manifested in the measurements of the Hall effect and longitudinal resistances.Our analysis shows that the gate voltage can substantially modify the anomalous Hall conductance,which exceeds 0.1 e^(2)/h at temperature T=1.6 K and magnetic field μ_0H=5 T,even though only the top TI surface is in proximity to MnSe.This work suggests that heterostructures based on antiferromagnetic insulators provide a promising platform for investigating a wide range of topological spintronic phenomena.

Proximity effects in topological insulator heterostructures

Topological insulators （Tls） are bulk insulators that possess robust helical conducting states along their interfaces with conventional insulators. A tremendous research effort has recently been devoted to TI-based heterostructures, in which con- ventional proximity effects give rise to a series of exotic physical phenomena. This paper reviews our recent studies on the potential existence of topological proximity effects at the interface between a topological insulator and a normal insu- lator or other topologically trivial systems. Using first-principles approaches, we have realized the tunability of the vertical location of the topological helical state via intriguing dual-proximity effects. To further elucidate the control parameters of this effect, we have used the graphene-based heterostructures as prototypical systems to reveal a more complete phase diagram. On the application side of the topological helical states, we have presented a catalysis example, where the topo- logical helical state plays an essential role in facilitating surface reactions by serving as an effective electron bath, These discoveries lay the foundation for accurate manipulation of the real space properties of the topological helical state in TI- based heterostructures and pave the way for realization of the salient functionality of topological insulators in future device applications.

Ferromagnetic-insulators-modulated transport properties on the surface of a topological insulator

Transport properties on the surface of a topological insulator （TI） under the modulation of a two-dimensional （2D） ferromagnet/ferromagnet junction are investigated by the method of wave function matching. The single ferromagnetic barrier modulated transmission probability is expected to be a periodic function of the polarization angle and the planar rotation angle, that decreases with the strength of the magnetic proximity exchange increasing. However, the transmission probability for the double ferromagnetic insulators modulated n-n junction and n-p junction is not a periodic function of polarization angle nor planar rotation angle, owing to the combined effects of the double ferromagnetic insulators and the barrier potential. Since the energy gap between the conduction band and the valence band is narrowed and widened respectively in ranges of 0 ≤ 0 〈π/2 and r/2 〈 0 ≤ π, the transmission probability of the n-n junction first increases rapidly and then decreases slowly with the increase of the magnetic proximity exchange strength. While the transmission probability for the n-p junction demonstrates an opposite trend on the strength of the magnetic proximity exchange because the band gaps contrarily vary. The obtained results may lead to the possible realization of a magnetic/electric switch based on TIs and be useful in further understanding the surface states of TIs.

Growth and transport properties of topological insulator Bi2Se3 thin film on a ferromagnetic insulating substrate

Exchange coupling between topological insulator and ferromagnetic insulator through proximity effect is strongly attractive for both fundamental physics and technological applications. Here we report a comprehensive investigation on the growth behaviors of prototype topological insulator Bi2Se3 thin film on a single-crystalline LaCoO3 thin film on SrTiO3 substrate, which is a strain-induced ferromagnetic insulator. Different from the growth on other substrates, the Bi2Se3 films with highest quality on LaCoO3 favor a relatively low substrate temperature during growth. As a result, an inverse dependence of carrier mobility with the substrate temperature is found. Moreover, the magnetoresistance and coherence length of weak antilocalization also have a similar inverse dependence with the substrate temperature, as revealed by the magnetotransport measurements. Our experiments elucidate the special behaviors in Bi2Se3/LaCoO3 heterostructures, which provide a good platform for exploring related novel quantum phenomena, and are inspiring for device applications.

Ferromagnetic barrier-induced negative differential conductance on the surface of a topological insulator

The effect of the negative differential conductance of a ferromagnetic barrier on the surface of a topological insulat（ is theoretically investigated. Due to the changes of the shape and position of the Fermi surfaces in the ferromagnetic barrie the transport processes can be divided into three kinds： the total, partial, and blockade transmission mechanisms. The bias voltage can give rise to the transition of the transport processes from partial to blockade transmission mechanisms, which results in a considerable effect of negative differential conductance. With appropriate structural parameters, the currenl voltage characteristics show that the minimum value of the current can reach to zero in a wide range of the bias voltag and then a large peak-to-valley current ratio can be obtained.

Ferromagnetism on a paramagnetic host background in cobalt-doped Bi_2Se_3 topological insulator

Cobalt-doped Bi2Se3 topological insulators have been grown though melt-grown reaction. The Bi2Se3 matrix is diamagnetic and doped sample is a superposition of ferromagnetism （FM） and paramagnetism （PM） behavior at low tem- perature. The values of Msmol, Hc, and Mr increase as the Co concentration increases. Two possible explanations have been proposed for the origin of ferromagnetism in Co-doped Bi2Se3. One is the magnetic ordering from nanoclusters of Co-Se compound in the crystals, and the other is Ruderman-Kittel-Kasuya-Yosida （RKKY） interaction between magnetic impurities.

Visualization of ferromagnetic domains in vanadium-doped topological insulator thin films and heterostructures

Magnetically doped topological insulator(TI) thin films and related heterostructures have been extensively studied for years due to their exotic quantum transport properties and potential applications in low-dissipation electronic devices and quantum computation.The selection of magnetic dopants is crucial to realize a high-quality magnetic TI with a robust ferromagnetic ordering and a preserved topological band structure.In this paper,we briefly review the recent magnetic domain imaging works in vanadium-doped magnetic topological insulator thin films and heterostructures.Using cryogenic magnetic force microscopy and in situ transport measurements,a ferromagnetic domain behavior has been demonstrated in V-doped Sb2Te3(ST) and Cr,V co-doped(Bi,Sb)2Te3(BST) thin films.The direct visualization of long-range ferromagnetic ordering in a quantum anomalous Hall(QAH) system sheds light on enhancing the QAH temperature by improving the ferromagnetism.Taking advantage of the different coercivity of Cr-and V-doped BST films,an axion insulating state has been observed in Cr-doped BST/BST/V-doped BST sandwich heterostructures.The antiparallel magnetization alignment,which is the key ingredient for realization of axion insulating state,has been directly visualized via magnetic imaging at various magnetic fields.The V-doped ST/ST heterostructures also provide a platform for Berry phase engineering in momentum space.By suppressing the anomalous Hall effect in such heterostructures,an intrinsic topological Hall effect can be revealed,which resolved the long-term puzzle of the origin of THE in the ultrathin ferromagnetic thin films and two-dimensional ferromagnets.The review of magnetic domain imaging in vanadium-doped topological insulators and heterostructures inspires further exploration of quantum transport properties in magnetic topological insulators and deepens the understanding of the interplay between the magnetic ordering and topological electronic band structures in magnetic TIs and beyond.

From magnetically doped topological insulator to the quantum anomalous Hall effect

Quantum Hall effect （QHE）, as a class of quantum phenomena that occur in macroscopic scale, is one of the most important topics in condensed matter physics. It has long been expected that QHE may occur without Landau levels so that neither external magnetic field nor high sample mobility is required for its study and application, Such a QHE free of Landau levels, can appear in topological insulators （TIs） with ferromagnetism as the quantized version of the anomalous Hall effect, i.e., quantum anomalous Hall （QAH） effect. Here we review our recent work on experimental realization of the QAH effect in magnetically doped TIs. With molecular beam epitaxy, we prepare thin films of Cr-doped （Bi,Sb）2Te3 TIs with well- controlled chemical potential and long-range ferromagnetic order that can survive the insulating phase. In such thin films, we eventually observed the quantization of the Hall resistance at h/e2 at zero field, accompanied by a considerable drop in the longitudinal resistance. Under a strong magnetic field, the longitudinal resistance vanishes, whereas the Hall resistance remains at the quantized value. The realization of the QAH effect provides a foundation for many other novel quantum phenomena predicted in TIs, and opens a route to practical applications of quantum Hall physics in low-power-consumption electronics.

Topological transport in Dirac electronic systems： A concise review

Various novel physical properties have emerged in Dirac electronic systems, especially the topological characters pro- tected by symmetry. Current studies on these systems have been greatly promoted by the intuitive concepts of Berry phase and Berry curvature, which provide precise definitions of the topological phases. In this topical review, transport properties of topological insulator （Bi2Se3）, topological Dirac semimetal （Cd3As2）, and topological insulator-graphene heterojunc- tion are presented and discussed. Perspectives about transport properties of two-dimensional topological nontrivial systems, including topological edge transport, topological valley transport, and topological Weyl semimetals, are provided.

Field-effect modulation of anomalous Hall effect in diluted ferromagnetic topological insulator epitaxial films

High quality chromium （Cr） doped three-dimensional topological insulator （TI） Sb2Te3 films are grown via molecular beam epitaxy on heat-treated insulating SrTiO3 （111） substrates. We report that the Dirac surface states are insensitive to Cr doping, and a perfect robust long-range ferromagnetic order is unveiled in epitaxial Sb2 xCrxTe3 films. The anomalous Hall effect is modulated by applying a bottom gate, contrary to the ferromagnetism in conventional diluted magnetic semiconductors （DMSs）, here the coercivity field is not significantly changed with decreasing cartier density. Carrier-independent ferromag- netism heralds Sbz_xCrxTe3 films as the base candidate TI material to realize the quantum anomalous Hall （QAH） effect. These results also indicate the potential of controlling anomalous Hall voltage in future TI-based magneto-electronics and spintronics.

石墨烯-MnBi_(2)Te_(4)异质结室温高灵敏度太赫兹探测器

太赫兹技术因其在医学成像、深空探索、无线通信、无损检测等领域的应用前景而受到广泛关注。然而,常温条件下对太赫兹辐射进行高灵敏度检测仍然是一个极大的技术挑战。文中利用低热导率的磁性拓扑绝缘体MnBi_(2)Te_(4)与高热导率的石墨烯形成范德华异质结,进而构建了一种异质结结构的太赫兹探测器。室温条件下,在0.04 THz与0.12 THz的频率太赫兹辐射照射下,探测器展现出超快的光电响应速度(16μs)和较高的响应度(0.43 mA/W和4.61 mA/W),同时具备较低的噪声等效功率(2.04 nW/Hz^(1/2)和190.58 pW/Hz^(1/2))。结果表明,基于石墨烯-MnBi_(2)Te_(4)异质结的太赫兹探测器在太赫兹探测领域具有巨大的应用潜力。

Molecular beam epitaxy of superconducting PdTe_2 films on topological insulator Bi_2Te_3

Majorana fermions have been observed in topological insulator/s-wave superconductor heterostructures. To manipulate Majorana fermions, superconducting materials should be deposited on the surfaces of topological insulators. In this study, highquality superconducting PdTe_2 films are deposited on the topological insulator Bi_2Te_3 surface using molecular beam epitaxy. The surface topography and electronic properties of PdTe_2/Bi_2Te_3 heterostructures are investigated via in situ scanning tunneling microscopy/spectroscopy. Under Te-rich conditions, the Pd atoms presumably form PdTe_2 film on Bi_2Te_3 surface rather than diffuse into Bi_2Te_3. The superconductivity of the PdTe_2/Bi_2Te_3 heterostructure is detected at a transition temperature of ～1.4 K using the two-coil mutual inductance technique. This study proposes a method for fabricating superconducting materials on topological insulator surfaces at low doping levels, paving ways for designing nanodevices that can manipulate Majorana fermions.

InAs/(In)GaSb-superconductor heterostructures:Materials and devices

This article will briefly describe a Majorana platform made of InAs/GaSb(including InAs/(In)GaSb)semiconductor-superconductor heterostructures.A unique advantage of this platform is that the quantum spin Hall edge state realized in inverted InAs/GaSb is a topologically protected spinless single mode,and can be tuned by front-back dual gates.Similar to a number of other platforms the proximity effect of a conventional s-wave superconductor on the helical edge has been proposed to realize Majorana bound state.We will present an introduction to this platform with a focus on the materials and devices aspects and those points that are particularly illustrative.

Fermi level tuning in Sn_(1-x)Pb_(x)Te/Pb heterostructure via changing interface roughness

Superconducting SnTe-type topological crystalline insulators(TCIs)are predicted to host multiple Majorana zero modes(MZMs)which can coexist in a single vortex.Fermi level(FL)close to the Dirac points of topological surface states is helpful for detecting MZMs.However,the TCI SnTe is a heavily p-type semiconductor which is very difficult to modify to n-type via doping or alloying.In this work,we fabricate the atomically flat Sn_(1-x)Pb_(x)Te/Pb heterostructure by molecular beam epitaxy,and make the p-type Sn_(1-x)Pb_(x)Te become n-type through changing the interface roughness.Using scanning tunnelling microscope,we find the Dirac points of Sn_(1-x)Pb_(x)Te/Pb heterostructure are always above the FL due to the Fermi level pinning(FLP)induced by topological surface states at atomically flat interface.After increasing the interface roughness,the FLP effect is suppressed and then the Dirac points of p-type Sn_(1-x)Pb_(x)Te can be tuned very close to or even below the FL.Our work provides a new method for tuning the FL of SnTe-type TCI which has potential application in novel topological superconductor device.

磁控溅射法生长Bi2Te3/CoFeB双层异质结太赫兹发射

自旋太赫兹源作为一种新型太赫兹辐射源,以其高效率、超宽带、低成本、易集成等优点已成为太赫兹科学与应用领域的研究热点.本实验报道了晶圆级磁控溅射生长的多晶拓扑绝缘体Bi2Te3和铁磁体CoFeB双层异质结纳米薄膜发射太赫兹电磁波,并对太赫兹辐射特性进行了深入而系统的实验研究.在飞秒激光放大级脉冲作用下,该异质结呈现出高效率的太赫兹发射,且辐射偏振可通过外加磁场方向控制.通过与Pt/CoFeB对比,研究发现Bi2Te3/CoFeB的发射性能与Pt/CoFeB双层异质结相当.实验还对生长在不同衬底上的Bi2Te3/CoFeB的发射性能进行了对比研究,发现MgO衬底上制备的样品具有相对较好的太赫兹辐射性能.本实验研究不仅对自旋太赫兹发射机理有更加深入的认识,而且通过样品和结构的优化,有望获得更高的发射效率,且该发射器具有大尺寸批量生长、成本较低的优势,具备商业化应用的潜力.

磁性拓扑绝缘体中的量子反常霍尔效应

量子反常霍尔效应是一种不需要外磁场、具有手性边缘态的量子化霍尔效应,可以用于构建其他新奇量子态和发展未来低功耗电子学器件等.该全新的量子效应于2012年首先由中国科学家在五层Cr掺杂的(Bi,Sb)2Te3拓扑绝缘体薄膜中实现.在过去七年间,经过大家的努力,其观测温度从最初的30 mK已经提高到2 K左右,进一步提高量子反常霍尔效应的观测温度是目前该领域主要的研究方向之一,是许多拓扑量子效应走向应用的关键因素.本文主要总结了量子反常霍尔效应研究的实验进展,特别在提高其观测温度方面的研究进展.文章包括四个部分:前两个部分分别介绍磁性掺杂和磁性近邻拓扑绝缘体体系中量子反常霍尔效应研究,第三部分介绍最新发现的内禀磁性拓扑绝缘体体系,最后一部分对设计和构造高温量子反常霍尔效应系统的原理和路线图给出一些建议和展望.

铁磁/正常/铁磁结调制的拓扑绝缘体薄膜表面输运性质（英文）

研究了拓扑绝缘体薄膜表面态在铁磁/正常/铁磁结调制下的电子自旋相关输运.发现由于交换场与杂化带隙的竞争而产生量子相变,在结无门电压时电导行为类似于自旋阀,加门电压后为自旋场效应管.有趣的是,无门电压且交换场能是杂化带隙的两倍时出现一个电导平台,磁阻比率可达100%.

EuS/Ta异质结的极大磁电阻效应

在铁磁/超导异质结中,铁磁体的交换场通过近邻效应将导致超导体准粒子态密度的塞曼劈裂.基于该效应,在外磁场不强的情况下,通过外加磁场可以有效地调节铁磁/超导界面处的交换作用,从而实现超导体在正常态和超导态之间转换,产生极大磁电阻.本文利用脉冲激光沉积方法制备了EuS/Ta异质结并研究了其电磁特性.Ta在3.6 K以下为超导态,EuS在20 K以下为铁磁态.在2 K时,EuS/Ta异质结中可观测蝴蝶型磁滞回线,证明在低磁场下(<±0.18 T)异质结中EuS铁磁态和Ta超导态共存.磁输运测试表明,通过施加外磁场可以有效调节EuS的交换场,随着交换场的增大,同时也加强了界面处的交换作用,从而抑制Ta的超导态,实现了Ta在超导态和正常态之间的转变,在EuS/Ta异质结中观测到了高达144000%的磁电阻.本文制备的EuS/Ta异质结具有极大磁电阻效应,在自旋电子学器件中有潜在的应用前景.

(SrVO_(3))_(5)/(SrTiO_(3))_(1)(111)异质结金属-绝缘体转变和磁性调控的第一性原理研究

(111)取向的钙钛矿异质结具有独特的六角蜂窝状双层结构,展现出丰富独特的物理现象,因而近年来得到越来越多的关注.本文利用第一性原理计算研究了(111)取向的(SrVO_(3))_(5)/(SrTiO_(3))_(1)异质结,计算结果表明该体系为半金属铁磁体.进一步的研究表明该体系的电、磁性质可以通过施加面内应变和界面元素掺杂进行调控:在4%的面内压缩应变到2%的面内拉伸应变范围内,该体系保持铁磁半金属性质,V 3d电子是体系半金属性的主要来源;当面内压缩应变增加到8%或面内拉伸应变增加到4%时,该体系的基态变为反铁磁绝缘体;通过异质结界面处Ti-V阳离子的混合掺杂,该体系可以实现从铁磁半金属向铁磁绝缘体的转变.本文的研究结果表明,该体系在自旋电子学领域具有很高的应用潜力,本文研究为利用(SrVO_(3))_(5)/(SrTiO_(3))_(1)(111)异质结探索量子相变提供了理论参考.

二维锡烯材料的若干进展

二维锡烯是一种潜在的可室温工作的拓扑绝缘体,有望进行无损耗的电子输运,在未来高集成度的电子学器件和量子器件领域具有重要的意义.理论方面,研究人员系统模拟了锡烯物理性质,发现其具有若干新奇的特性.实验方面,主要集中在单层锡烯的制备.系统梳理金属锡到二维锡烯的发展及制备进程,介绍锡烯的主要物理性质、相关结构及调控、锡烯复合及功能材料,最后简要总结锡烯面临的机遇和挑战.