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.     

基于量子自旋-量子反常霍尔效应的单层锑异质结构的热电性质研究（英文）北大核心CSCD

我们研究了一个基于单层氢化锑的异质结构材料的热电性质,该材料有着新颖的量子自旋-量子反常霍尔效应.我们计算了这种新的拓扑相材料的锯齿形结构纳米带的电导、热导、塞贝克系数以及热电效益指数.通过我们的研究,表明该材料在低温下有着较好的热电性能.此外,我们通过改变样品最外层原子的交换场的方式来调控样品的边缘态,并计算样品热电性质的变化.最后,我们发现随着纳米带宽度的增加,系统的热电性能也会增加.     

磁性拓扑绝缘体中的量子化反常霍尔效应

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.     

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.     

从反常霍尔效应到量子反常霍尔效应

美国物理学家霍尔 (Edwin H.Hall) 在1879 年发现,当电流垂直于外磁场通过导体时,在导体的垂直于磁场和电流方向的两个边界之间会出现电势差(图1)。这个现象被称作霍尔效应。在当时要理解这一重要的现象还非常困难,因为电子的概念在18年后才被提出来。     

“量子反常霍尔效应”研究取得重大突破

由中国科学院物理研究所和清华大学物理系的科研人员组成的联合攻关团队,经过数年不懈探索和艰苦攻关,最近成功实现了"量子反常霍尔效应".这是国际上该领域的一项重要科学突破.     

反常霍尔效应理论的研究进展

文章介绍了在铁磁性材料中反常霍尔效应的发现及其机制研究的历史；阐述了反常霍尔效应理论研究最近取得的重大进展，即倒空间中布洛赫态的贝里曲率（规范场）特性决定了霍尔电导率；同时指出，建立系统地解释反常霍尔效应机制的理论仍然是一个挑战性的任务．     

反常霍尔效应理论的研究进展

文章介绍了在铁磁性材料中反常霍尔效应的发现及其机制研究的历史;阐述了反常霍尔效应理论研究最近取得的重大进展,即倒空间中布洛赫态的贝里曲率(规范场)特性决定了霍尔电导率;同时指出,建立系统地解释反常霍尔效应机制的理论仍然是一个挑战性的任务.     

从磁性掺杂拓扑绝缘体到内禀磁性拓扑绝缘体——通往高温量子反常霍尔效应之路

量子反常霍尔效应被认为是已知的拓扑量子效应中最有希望获得广泛实际应用的一个。阻碍其应用的主要障碍是其很低的实现温度。文章介绍了在磁性拓扑绝缘体中量子反常霍尔效应的机理和决定其实现温度的因素,回顾了过去几年在提高量子反常霍尔效应实现温度方面的研究进展,尤其是最近内禀磁性拓扑绝缘体MnBi₂Te₄的相关工作。在此基础上提出在磁性拓扑绝缘体系统中进一步提高量子反常霍尔效应温度的路线图。

     

自旋陈数理论和时间反演对称破缺的量子自旋霍尔效应

一般认为,量子自旋霍尔效应只有受到时间反演对称性的保护才是稳定的。但是,因为在 实际材料中破坏时间反演对称性的微扰往往无法避免,这种受时间反演对称性保护的量子自旋霍 尔效应在真实环境中并不稳定。本综述将介绍近期在寻找无需时间反演对称性保护的量子自旋霍 尔效应方向上的系列研究进展。我们将证明量子自旋霍尔体系的非平庸拓扑性质在时间反演对称 性被破坏后仍然可以完好存在,并通过一个规范讨论,将边缘态一般性质和体能带的非平庸拓扑 性质联系起来。进一步,将探讨通过人工消除边缘态时间反演对称性而实现稳定的量子自旋霍尔 效应的方案。此外,我们还将介绍自旋陈数理论,自旋陈数是在没有时间反演对称性存在时,表 征量子自旋霍尔体系所处不同拓扑相的有效工具。     

异常霍尔效应和自旋霍尔效应

异常霍尔效应和自旋霍尔效应是在常规霍尔效应的基础上引发出的2种新现象.本文介绍了这2种现象及其原理和潜在的应用.     

Anomalous quantum Hall effect induced by nearby quantum dots

Transport measurements in high magnetic fields have been performed on two-dimensional electron system (2DES) separated by a thin barrier layer from a layer of InAs self-assembled quantum dots (QDs). Clear feature of quantum Hall effect was observed in spite of presence of QDs nearby 2DES. However, both magnetoresistance, ρ_xx, and Hall resistance, ρ_xy, are suppressed significantly only in the magnetic field range of filling factor in 2DES ν<1 and voltage applied on a front gate . The results indicate that the electron state in QDs induces spin-flip process in 2DES.     

Quantum Hall skyrmions with higher topological charge

We have investigated quantum Hall skyrmions with more than one unit of topological, and hence electric, charge. Using a combination of analytic and numerical methods we find the counterintuitive result that when the Zeeman energy is tuned to values much smaller than the interaction energy ( ), the creation energy of a charge two skyrmion becomes less than twice the creation energy of a charge one skyrmion, i.e. skyrmions bind in pairs. The doubly charged skyrmions are stable to further accretion of charge and exhibit a larger spin per unit charge than charge one skyrmions which would, in principle, signal this pairing. 1997 Elsevier Science B.V. All rights reserved.     

An Extended Extrinsic Mechanism for Anomalous Hall Effect

The extrinsic mechanism for anomalous Hall effect in ferromagnets is extended to include the contributions both from spin-orbit-dependent impurity scattering and from the spin-orbit coupling induced by external electric fields. The results obtained suggest that, within the framework of the extrinsic mechanisms, the anomalous Hall current in a ferromagnet may also contain a substantial amount of dissipationless contribution independent of impurity scattering. After the contribution from the spin-orbit coupling induced by external electric fields is included, the total anomalous Hall conductivity is about two times larger than that due to soin-orbit dependent impurity scatterings.     

An Extended Extrinsic Mechanism for Anomalous Hall Effect

The extrinsic mechanism for anomalous Hall effect in ferromagnets is extended to include the contributions both from spin-orbit-dependent impurity scattering and from the spin-orbit coupling induced by external electric fields. The results obtained suggest that, within the framework of the extrinsic mechanisms, the anomalous Hall current in a ferromagnet may also contain asubstantial amount of dissipationless contribution independent of impurity scattering. After the contribution from the spin-orbit coupling induced by external electric fields is included, the total anomalous Hall conductivity is about two times larger than that due to spin-orbit dependent impurity scatterings.     

Quantum critical Hall exponents

We investigate a finite size “double scaling” hypothesis using data from an experiment on a quantum Hall system with short range disorder ,  and . For Hall bars of width w at temperature T   the scaling form is w^−μT^−κ$w^{-\mu }{T}^{-\kappa }$, where the critical exponent μ≈0.23$\mu \approx 0.23$ we extract from the data is comparable to the multi-fractal exponent _α0−2${\alpha }_0-2$ obtained from the Chalker–Coddington (CC) model [4]. We also use the data to find the approximate location (in the resistivity plane) of seven quantum critical points, all of which closely agree with the predictions derived long ago from the modular symmetry of a toroidal σ-model with m matter fields [5]. The value _ν8=2.60513…${\nu }_8=2.60513\dots$ of the localisation exponent obtained from the m=8$m=8$ model is in excellent agreement with the best available numerical value _νnum=2.607±0.004${\nu }_{\mathrm{num}}=2.607\pm 0.004$ derived from the CC-model [6]. Existing experimental data appear to favour the m=9$m=9$ model, suggesting that the quantum Hall system is not in the same universality class as the CC-model. We discuss the reason this may not be the case, and propose experimental tests to distinguish between the two possibilities.     

Anomalous Hall effect of heavy holes in III-V semiconductor quantum wells

The anomalous Hall effect of heavy holes in semiconductor quantum wells is studied in the intrinsic transport regime, where the Berry curvature governs the Hall current properties. Based on the first--order perturbation of wave function the expression of the Hall conductivity the same as that from the semiclassical equation of motion of the Bloch particles is derived. The dependence of Hall conductivity on the system parameters is shown. The amplitude of Hall conductivity is found to be balanced by a competition between the Zeeman splitting and the spin--orbit splitting.     

Anomalous Hall effect in perpendicular CoFeB thin films

Our recent research achievements in the perpendicular magnetic anisotropy （PMA） properties of the CoFeB sand- wiched by MgO and tantalum layers are summarized. We found that the PMA behaviors of Ta/CoFeB/MgO and MgO/CoFeB/Ta thin films are different. The larger PMA in the latter film is related to the lower magnetization of CoFeB deposited on MgO. Furthermore, we have demonstrated a large anomalous Hall effect in perpendicular CoFeB thin fihn. Our results show large anomalous Hall resistivity, large longitudinal resistivity, and low switching field can be achieved, all at the same time, in the perpendicular CoFeB thin film. Anomalous Hall effect with high and linear sensitivity is also found in an MgO/CoFeBFFa thin film with a thick MgO layer, which opens a door tbr future device applications of perpendicular ferromagnetic thin films.     

Quantum Hall effect: The resistivity of a 2D electron gas—a thermodynamic approach

Based on a thermodynamic approach, we have calculated the resistivity of a 2D electron gas, assumed dissipationless in a strong quantum limit. Standard measurements, with extra current leads, define the resistivity caused by a combination of Peltier and Seebeck effects. The current causes heating (cooling) at the first (second) sample contacts, due to the Peltier effect. The contact temperatures are different. The measured voltage is equal to the Peltier effect-induced thermoemf which is linear in current. As a result, the resistivity is non-zero as I→0. The resistivity is a universal function of magnetic field and temperature, expressed in fundamental units h/e². The universal features of magnetotransport data observed in the experiment confirm our predictions.     

Mg0.3Fe2.7GeTe2 单晶的磁性及霍尔效应研究

Fe3GeTe2 是一种具有稳定长程磁有序的准二维范德瓦尔斯磁性材料, 范德瓦尔斯材料的稳定性和可调性使其在自旋电子器件的应用方面具有巨大潜力. 本文用助熔剂法生长了 Mg 原子掺杂Fe2 位的 Mg0.3Fe2.7GeTe2单晶样品, 并对 Mg 掺杂Fe3GeTe2 的结构、磁性和输运性质的影响进行了研究. 磁性数据表明 Mg 掺杂后铁磁转变温度不变, 但样品的饱和磁矩减小. 输运性质的测量中观察到各向异性的反常霍尔效应, 与Fe3GeTe2 相比, Mg掺杂后的反常霍尔电阻率减小, 同时各向异性发生了变化.