薛其坤等《科学》发文,首次在实验上发现量子反常霍尔效应或将推动信息技术进步

由清华大学薛其坤院士领衔,清华大学、中科院物理所和斯坦福大学的研究人员联合组成的团队在量子反常霍尔效应研究中取得重大突破,从实验上首次观测到量子反常霍尔效应,在美国物理学家霍尔于1880年发现反常霍尔效应133年后终于实现了反常霍尔效应的量子化.这是我国     

磁性拓扑绝缘体中的量子化反常霍尔效应——无需外磁场的量子化霍尔效应

文章从平常霍尔效应出发,介绍了反常霍尔效应及其内秉物理机制,并在此基础上介绍了其量子化版本——量子化反常霍尔效应.然后从拓扑有序态的角度,重点讨论了量子化反常霍尔效应与量子霍尔效应、量子自旋霍尔效应、拓扑绝缘体等之间的区别与内在联系.最后介绍了通过在拓扑绝缘体(Bi2Se3,Bi2Te3和Sb2Te3)薄膜中掺杂过渡金属元素(Cr或Fe)实现量子化反常霍尔效应的方法.     

Heusler合金Co2TiSn的磁性与输运性能

通过对Heusler合金Co₂TiSn磁性和输运性能的实验研究，发现低场中Co_{2< /sub>TiSn呈现出亚铁磁性；电阻和霍尔电阻率在温度低于8K时均出现异常.同时分析了电输 运的散射机制，并对反常霍尔效应产生的机制进行了探讨. 关键词： Heusler合金 磁性 输运性能}     

量子反常霍尔效应研究进展

在没有外加磁场的作用下就能表现出量子化霍尔电导的量子反常霍尔效应已经成为霍尔家族中的重要一员, 其物理起源是体能带反转结构和铁磁性相互作用. 量子反常霍尔效应最重要的表现是在边缘态处具有无耗散运动的手性电流, 这种性质拥有可以改变未来量子电子学的潜力, 极大推动器件小型化、 低损耗、 高速率发展. 近年来,基于理论指导, 人们在实验上已多次观察到量子反常霍尔效应. 在本文中, 从实验层面上重点回顾了量子反常霍尔效应在铬(Cr) 、钒(V) 掺杂的(Bi,Sb)2Te3 体系的研究进展, 以及目前量子反常霍尔效应在其它体系中的研究现状, 深入理解量子反常霍尔效应的起源和机理, 最后对量子反常霍尔效应进行总结和展望     

Heusler合金Co2 TiSn的磁性与输运性能

通过对Heusler合金Co2TiSn磁性和输运性能的实验研究,发现低场中Co2TiSn呈现出亚铁磁性;电阻和霍尔电阻率在温度低于8K时均出现异常.同时分析了电输运的散射机制,并对反常霍尔效应产生的机制进行了探讨.     

量子霍尔效应及量子反常霍尔效应的探索历程

回顾了霍尔效应、量子霍尔效应及量子反常霍尔效应的探索历程.着重叙述了1985年、1998年获得诺贝尔物理学奖的量子霍尔效应研究成果,以及在我国实验室首次观测到量子反常霍尔效应的重大成就.     

磁性拓扑绝缘体与量子反常霍尔效应

量子反常霍尔绝缘体,有时也被称为陈数绝缘体,是不同于普通绝缘体和拓扑绝缘体的一类新的二维绝缘体,该体系具有可被实验观测的特殊物理性质—量子反常霍尔效应。该体系的物态不能用朗道对称性破缺理论来描写,而要用到拓扑物态的概念。它的发现也经历了从反常霍尔效应的内秉物性阐释,到量子自旋霍尔效应与拓扑绝缘体的发现,再到磁性拓扑绝缘体的理论预测与实现,并最终成功实验观测的漫长过程。由于量子反常霍尔效应的实现不需要外加磁场,而此时样品的边缘态可以被看成一根无能耗的理想导线,因此人们对于其将来可能的应用充满了期待。本文将从理论的角度简单综述该领域的发展历程、基本概念、以及相关的材料系统。     

分子束外延Mn4N单晶薄膜的反常霍尔效应研究

Mn₄N是立方相的反钙钛矿型晶体,具有显著的亚铁磁性和反常霍尔效应.该文利用等离子辅助分子束外延在MgO(100)衬底上生长厚度为40 nm的Mn₄N(100)单晶薄膜,通过X射线衍射θ扫描和φ扫描,证实外延层的结构符合Mn₄N单晶的空间结构特征;化学态测试结果表明Mn₄N(100)薄膜内部存在Mn⁰、Mn²⁺和Mn⁴⁺等几种价态,其实际化学式为Mn_3.6N,薄膜中存在富余的N元素;电学测试数据表明Mn₄N(100)薄膜具有以电子为载流子的反常霍尔效应(在正磁场中得到负的霍尔电阻率),正常霍尔效应的贡献约占千分之六,其反常霍尔电阻率随着测试温度升高5 K～350 K单调增大,说明温度升高导致电子散射现象加剧.通过对测试数据分析可以推断,在5 K～50 K和50 K～75 K温度范围,反常霍尔效应的来源可分别归结为电子斜散射机制和电子边跳机制.在75 K～350 K这一温度范围内,反常霍尔效应...     

磁子霍尔效应

霍尔效应是凝聚态领域中古老却又极具潜力的研究领域,其起源可以追溯到数百年前. 1879年,霍尔发现将载流导体置于磁场中时,磁场带来的洛伦兹力将使得电子在导体的一侧积累,这一新奇的物理现象被命名为霍尔效应.之后,一系列新的霍尔效应被发现,包括反常霍尔效应、量子霍尔效应、自旋霍尔效应、拓扑霍尔效应和平面霍尔效应等.值得注意的是,霍尔效应能够实现不同方向的粒子流之间的相互转化,因此在信息传输过程中扮演着重要的角色.在玻色子体系(如磁子)中,相应的一系列磁子霍尔效应也被发现,他们共同推动了以磁子为基础的自旋电子学的发展.本文回顾了近年来在磁子体系中的霍尔效应,简述其现代半经典的处理方法,包括虚拟电磁场理论和散射理论等.并进一步介绍了磁子霍尔效应的物理起源,概述了不同类型磁子的霍尔效应.最后,对磁子霍尔效应的发展趋势进行了展望.     

Pt/La0.67Sr0.33MnO3异质结中的反常霍尔效应

非磁/铁磁异质结构中存在很多有趣的演生现象,特别是,铂/铁磁异质结构中的反常霍尔效应是一个研究热点.采用脉冲激光沉积技术和射频磁控溅射技术制备出具有原子级接触界面的铂/锰酸锶镧异质结,并对异质结的电输运性能进行了系统的研究.实验发现,铂/锰酸锶镧异质结中存在由铂贡献的反常霍尔效应,这是由磁近邻效应诱导铂表现出铁磁性造成的.反常霍尔电阻随着温度的降低而急剧增加,并且在低于40 K时改变符号.反常霍尔电阻随铂厚度的增加而急剧降低,证实了铂的铁磁性起源于异质结界面.此外,异质结在低外加磁场下可能产生了拓扑霍尔效应,这是由异质结界面处的Dzyaloshinskii-Moriya相互作用诱导产生手性磁畴壁结构引起的.上述研究结果为进一步理解非磁/铁磁异质结构中的电子自旋和电荷输运之间的相互作用提供了实验基础.     

Scaling law of anomalous Hall effect in Fe/Cu bilayers

The scaling of anomalous Hall resistivity on longitudinal resistivity has been intensively studied in different magnetic systems, including multilayer and granular films, to examine whether a skew scattering or a side jump mechanism dominates in the origin of anomalous Hall effect (AHE). The scaling law is based on the premise that both resistivities are a consequence of electron scattering by the imperfections in the materials. By studying the anomalous Hall effect in the simple Fe/Cu bilayers, it was demonstrated that the measured anomalous Hall effect should not follow the scaling laws derived from skew scattering or side jump mechanism due to the short-circuit and shunting effects of the non-magnetic layers.     

THEORETICAL AND EXPERIMENTAL INVESTIGATION ON ANOMALOUS HALL EFFECT OF HETEROGENOUS SEMICONDUCTOR SAMPLES

Based on the theoretical results of inversion layers model and interpenetrating network model, we studied the anomalous Hall effect of heterogenous semiconductor samples, and obtained the following results: (1) the wider the sample energy gap, the higher the temper-ature at which anomalous Hall effect begins to appear; (2) the lower the sample resistivity, the higher the temperature at which anomalous Hall effect begins to appear, which is further verified in our experiment; (3) mobility is the main factor which determines what role p-n junction plays in sample Hall voltage; (4) for n-Ge sample with inversion layers structure, with the increase of p-type region, the Hall effect changes from normal n-type Hall effect to anomalous Hall effect of the single dip type, then to anomalous Hall effect of the second reversal type.     

On the origin of the anomalous sign reversal in the Hall effect in Nb thin films

We re-visit the anomalous sign reversal problem in the Hall effect of the sputtered Nb thin films. We find that the anomalous sign reversal in the Hall effect is extremely sensitive to a small tilting of the magnetic field and to the magnitude of the applied current. Large anomalous variations are also observed in the symmetric part of the transverse resistance R_xy. We suggest that the surface current loops on superconducting grains at the edges of the superconducting thin films may be responsible for the Hall sign reversal and the accompanying anomalous effects in the symmetric part of R_xy.     

Spin hall effect in the presence of spin diffusion

Hirsch [Phys. Rev. Lett. 83, 1834 (1999)] recently proposed a spin Hall effect based on the anomalous scattering mechanism in the absence of spin-flip scattering. Since the anomalous scattering causes both anomalous currents and a finite spin-diffusion length, we derive the spin Hall effect in the presence of spin diffusion from a semiclassical Boltzmann equation. When the formulation is applied to certain metals and semiconductors, the magnitude of the spin Hall voltage due to the spin accumulation is found to be much larger than that of magnetic multilayers. An experiment is proposed to measure this spin Hall effect.     

Mn_(3)Sn的高压物性研究

非共线三角晶格的反铁磁体Mn_(3)Sn,由于其动量空间中非零的贝里曲率而表现出反常霍尔效应.利用金刚石对顶砧研究压力对Mn_(3)Sn低温电输运性质和反常霍尔效应的影响.结果表明:Mn_(3)Sn在0~60GPa压力范围内保持金属导电行为;从非共线磁有序到螺旋磁有序的转变温度随压力增加先降低,在5.5GPa以上迅速升高,对应一个等结构电子拓扑转变;Mn_(3)Sn的反常霍尔效应在5.5GPa被完全抑制.     

Anomalous Hall effect in a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit interaction

We discuss the mechanism of the anomalous Hall effect in a Rashba-Dresselhaus two-dimensional electron gas subjected to a homogeneous out-of-plane magnetization. On the basis of a systematic treatment of the kinetic equations for the spin-density matrix, results are derived for the dynamic Hall conductivity in a closed form. Its nonanalytic dependence on both the scattering time and the frequency of the applied electric field is discussed. Except for in a special Rashba-Dresselhaus model, there is a finite intrinsic anomalous Hall effect, which is extremely sensitive to short-range elastic scattering.     

Anomalous Hall effect in ferromagnetic semiconductors

We present a theory of the anomalous Hall effect in ferromagnetic (III, Mn)V semiconductors. Our theory relates the anomalous Hall conductance of a homogeneous ferromagnet to the Berry phase acquired by a quasiparticle wave function upon traversing closed paths on the spin-split Fermi surface. The quantitative agreement between our theory and experimental data in both (In, Mn)As and (Ga, Mn)As systems suggests that this disorder independent contribution to the anomalous Hall conductivity dominates in diluted magnetic semiconductors. The success of this model for (III, Mn)V materials is unprecedented in the longstanding effort to understand origins of the anomalous Hall effect in itinerant ferromagnets.     

The quantum anomalous Hall effect on a star lattice with spin-orbit coupling and an exchange field

We study a star lattice with Rashba spin-orbit coupling and an exchange field and find that there is a quantum anomalous Hall effect in this system, and that there are five energy gaps at Dirac points and quadratic band crossing points. We calculate the Berry curvature distribution and obtain the Hall conductivity (Chern number ν) quantized as integers, and find that ν?=-?1,2,1,1,2 when the Fermi level lies in these five gaps. Our model can be viewed as a general quantum anomalous Hall system and, in limit cases, can give what the honeycomb lattice and kagome lattice give. We also find that there is a nearly flat band with ν?=?1 which may provide an opportunity for realizing the fractional quantum anomalous Hall effect. Finally, the chiral edge states on a zigzag star lattice are given numerically, to confirm the topological property of this system.     

Weak localization corrections to the anomalous Hall effect

We calculate localization corrections to the anomalous Hall conductivity in the framework of side-jump and skew scattering mechanisms. In contrast to the ordinary Hall effect, there exists a nonvanishing localization correction to the anomalous Hall resistivity. The correction to the anomalous Hall conductivity vanishes in case of side-jump mechanism, but is nonzero for the skew scattering.