Spin dynamics and quantum transport in magnetic semiconductor quantum structures

Quantum structures derived from magnetic semiconductors serve as a powerful arena within which to study the interplay between quantum electronics and thin film magnetism. In particular, the semiconductor aspects of these flexible systems allow direct access to the electronic spin degrees of freedom using both magneto-optical as well as magneto-transport probes. Here we provide an overview of recent developments in the experimental study of II–VI magnetic semiconductor quantum structures, with particular emphasis on the dynamical behavior of field-tunable electronic spin states and spin-dependent quantum transport.     

基于金刚石NV色心的纳米尺度磁场测量和成像技术

纳米级分辨率的磁场测量和成像是磁学中的一种重要研究手段.金刚石中的单个氮-空位点缺陷电子自旋作为一种量子传感器,具有灵敏度高、原子级别尺寸、可工作在室温等诸多优势,灵敏度可以达到单核自旋级别,空间分辨率达到亚纳米.将这种磁测量技术与扫描成像技术结合,能够实现高灵敏度和高分辨率的磁场成像,定量地重构出杂散场.这种新型的磁成像技术可以给出磁学中多种重要的研究对象如磁畴壁、反铁磁序、磁性斯格明子的结构信息.随着技术的发展,基于氮-空位点缺陷的磁成像技术有望成为磁性材料研究的重要手段.     

反铁磁半金属 EuCd2Pn2（Pn = As,Sb）中磁交换诱导的外尔态

由于丰富的拓扑量子效应及巨大的潜在应用价值,拓扑材料逐渐成为凝聚态物理前沿的研究材料体系。其中,作为与石墨烯具有相似电子结构的材料,三维拓扑半金属吸引了越来越多的研究兴趣。目前已知的拓扑半金属大多为非磁性的,而磁性拓扑半金属数量有限,与非磁性拓扑半金属相比较,研究开展的还比较少。磁性与拓扑之间的相互作用能够导致非常规的物理性质,如反常霍尔效应甚至量子反常霍尔效应等。此外,在一些具有特殊磁结构的拓扑半金属中,施加外磁场能够调制其自旋结构,从而影响其拓扑能带结构。在该综述中,笔者将详细介绍利用外磁场在 EuCd2Pn2 (Pn = As, Sb) 反铁磁半金属材料中通过调制自旋结构从而改变晶体结构对称性来诱导拓扑相变。此外,笔者也将简单介绍包括 GdPtBi 和 MnBi2Te4 在内的几个相关材料。该综述中讨论的外磁场调控的磁交换诱导的拓扑相变不仅有望应用于拓扑器件,也有助于为理解磁性与拓扑态之间的紧密关联提供新的线索,对于设计新的磁性拓扑材料有启发意义。综述最后,笔者对发展磁性拓扑半金属做了一些简单展望。     

On the anomalous low-resistance state and exceptional Hall component in hard-magnetic Weyl nanoflakes

Magnetic topological materials, which combine magnetism and topology, are expected to host emerging topological states and exotic quantum phenomena. In this study, with the aid of greatly enhanced coercive fields in high-quality nanoflakes of the magnetic Weyl semimetal Co_3Sn_2S_2, we investigate anomalous electronic transport properties that are difficult to reveal in bulk Co_3Sn_2S_2 or other magnetic materials. When the magnetization is antiparallel to the applied magnetic field, the low longitudinal resistance state occurs, which is in sharp contrast to the high resistance state for the parallel case. Meanwhile, an exceptional Hall component that can be up to three times larger than conventional anomalous Hall resistivity is also observed for transverse transport. These anomalous transport behaviors can be further understood by considering nonlinear magnetic textures and the chiral magnetic field associated with Weyl fermions, extending the longitudinal and transverse transport physics and providing novel degrees of freedom in the spintronic applications of emerging topological magnets.     

Effect of very high pressure on the magnetic state of transition metal compounds

By simultaneously combining the methods of X-ray diffraction for structural phase transitions and EOS measurements, ⁵⁷Fe Mössbauer spectroscopy as a site-sensitive probe, and resistivity measurements for studying insulating-metal transitions, we are able to study the effect of extreme pressures and at varying temperature on magnetic and electronic properties of transition metal compounds. Studies are carried out with specially tailored diamond anvils and diamond anvil cells, reaching pressures beyond 100?GPa. From our studies, we can investigate the most basic phenomenon of the quantum effect of magnetism in insulating antiferromagnets, the Mott insulators, such as high to low spin crossovers, quenching of the magnetic moments' orbital term, and the collapse of the Mott–Hubbard state. Examples of these phenomena will be given in cases of ferrous and ferric oxides, ferrous-halides and the rare-earth iron perovskites.     

High-throughput identification of one-dimensional atomic wires and first principles calculations of their electronic states

Low dimensional materials are suitable candidates applying in next-generation high-performance electronic, optoelectronic, and energy storage devices because of their uniquely physical and chemical properties. In particular, onedimensional(1 D) atomic wires(AWs) exfoliating from 1 D van der Waals(vd W) bulks are more promising in next generation nanometer(nm) even sub-nm device applications owing to their width of few-atoms scale and free dandling bonds states. Although several 1 D AWs have been experimentally prepared, few 1 D AW candidates could be practically applied in devices owing to lack of enough suitable 1 D AWs. Herein, 367 kinds of 1 D AWs have been screened and the corresponding computational database including structures, electronic structures, magnetic states, and stabilities of these 1 D AWs has been organized and established. Among these systems, unary and binary 1 D AWs with relatively small exfoliation energy are thermodynamically stable and theoretically feasible to be exfoliated. More significantly, rich quantum states emerge,such as 1 D semiconductors, 1 D metals, 1 D semimetals, and 1 D magnetism. This database will offer an ideal platform to further explore exotic quantum states and exploit practical device applications using 1 D materials. The database are openly available at http://www.dx.doi.org/10.11922/sciencedb.j00113.00004.     

A local view of the Kondo effect: Scanning tunneling spectroscopy

The fascinating many-body physics involved in the interaction of a single magnetic impurity with the conduction electrons of its nonmagnetic metallic host is reflected in unconventional phenomena in magnetism, transport properties and the specific heat. Characteristic low-energy excitations, termed the Kondo resonance, are generally believed to be responsible for this striking behaviour. However, in spite of an intense research for over more than 30 years, a direct spectroscopic observation of the Kondo resonance on individual magnetic adatoms withstood experimental efforts hitherto. The development of low-temperature scanning tunneling microscopes (STM) operating under ultrahigh vacuum (UHV) conditions has provided new opportunities for investigating locally the electronic structure at surfaces. At low temperatures, due to the reduced broadening of the Fermi level of the STM tip and the sample, rather high energy resolution (≤ 1 meV) in scanning tunneling spectroscopy (STS) is achievable. Moreover, the absence of diffusion together with the spatial resolution of the STM enables detailed studies of electronic states on and near single adsorbed atoms and other nanoscale structures. Recently, for the first time, two such STS/STM experiments spatially resolved the electronic properties of individual magnetic adatoms displaying the Kondo effect. In particular, the observed Fano lineshape of the Kondo resonance reveals unambiguously the details of the quantum mechanical interference between the localized orbital and the conduction electrons on an atomic length scale [1,2]. This achievement of the detection of individual magnetic atoms with atomic resolution opens new perspectives for probing magnetic nanostructures.     

Magnetic 4d systems studied by implanted ions

By application of the perturbed -ray distribution method following heavy-ion reactions and recoil implantation techniques, we have found an experimental way of producing and investigating magnetic 4d states in metals. Strong 4d magnetism has been found for 4d ions in alkali metal hosts and in Pd hosts. In alkali metals, 4d ions reflect the phenomena of well-defined ionic ground states, orbital magnetism, mixed valence, and crystal field splittings smaller than theLS coupling. Magnetic 4d states in alkali metals cannot be described by one-electron approaches based on Anderson-type models, but requires an analysis in terms of many electron ionic configurations exhibiting basic features common to the physics of stable and unstable f stales in metals. In contrast, the local moment formation of 4d and 3d ions in Pd is governed by inter-atomic interactions of the magnetic d states with host d-band electrons, giving rise to spin magnetic behavior of the 4d impurity and to strong spin polarizations of the 4d electrons of the Pd host. Thus, the magnetism and electronic structure of 4d ions in metals exhibit qualitative differences in alkali metal hosts compared to Pd. The existence of magnetic 4d systems strongly depends on the 4d ion species and the host matrix, and on spin fluctuation rates or the corresponding Kondo temperatures. The results can be directly compared to theoretical work and also to the magnetic behavior of 3d ions in sp metal hosts and in hosts with d-band electrons.     

Quantum magnetism in minerals

The discovery of magnetism by the ancient Greeks was enabled by the natural occurrence of lodestone – a magnetized version of the mineral magnetite. Nowadays, natural minerals continue to inspire the search for novel magnetic materials with quantum-critical behaviour or exotic ground states such as spin liquids. The recent surge of interest in magnetic frustration and quantum magnetism was largely encouraged by crystalline structures of natural minerals realizing pyrochlore, kagome, or triangular arrangements of magnetic ions. As a result, names like azurite, jarosite, volborthite, and others, which were barely known beyond the mineralogical community a few decades ago, found their way into cutting-edge research in solid-state physics. In some cases, the structures of natural minerals are too complex to be synthesized artificially in a chemistry lab, especially in single-crystalline form, and there is a growing number of examples demonstrating the potential of natural specimens for experimental investigations in the field of quantum magnetism. On many other occasions, minerals may guide chemists in the synthesis of novel compounds with unusual magnetic properties. The present review attempts to embrace this quickly emerging interdisciplinary field that bridges mineralogy with low-temperature condensed-matter physics and quantum chemistry.     

Ce 基重费米子材料的电子态研究与维度调控

重费米子材料作为一类典型的强关联电子体系,蕴含着非常规超导、奇异金属、量子临界、 磁有序、重电子态、关联拓扑态等新奇的量子态,而4f 电子在其中扮演着重要的作用。随着高分 辨角分辨光电子能谱和薄膜生长技术的发展,精确探测重费米子材料中4f 电子在能量/动量空间 的色散和谱权重成为了可能,这为从微观上理解这类材料中的电子关联效应和新奇量子现象提供 了重要的基础。本论文总结了几个典型的重费米子单晶和薄膜体系的电子态研究,包括Ce-115 体 系、CeCu2Si2、CeRh6Ge4 以及单晶 Ce 膜等。这些结果为理解重费米子体系中重电子态的形成 和温度演化、近藤杂化的能带/动量依赖、重电子能带与超导的关系、近藤效应与磁性和其它量子 态的竞争、4f 电子的维度调控等重要物理问题提供了谱学证据。     

Tuning kinetic magnetism of strongly correlated electrons via a staggered flux

An interplay between kinetic process and magnetic ordering is manifested when strong correlation and electronic frustration are present: tuning a staggered flux phi from 0 to pi makes the ground state (GS) of an infinite-U Hubbard model change abruptly from a Nagaoka-type ferromagnet to a Haerter-Shastry-type antiferromagnet at a phi_(c), with both states being metallic and of kinetic origin. Intraplaquette spin correlation, as well as nonanalyticity in the GS energy, signals such a novel quantum criticality. This tunable kinetic magnetism is generic and may be experimentally realized.     

Intrinsic magnetic topological materials

Topological states of matter possess bulk electronic structures categorized by topological invariants and edge/surface states due to the bulk-boundary correspondence. Topological materials hold great potential in the development of dissipationless spintronics, information storage and quantum computation, particularly if combined with magnetic order intrinsically or extrinsically. Here, we review the recent progress in the exploration of intrinsic magnetic topological materials, including but not limited to magnetic topological insulators, magnetic topological metals, and magnetic Weyl semimetals. We pay special attention to their characteristic band features such as the gap of topological surface state, gapped Dirac cone induced by magnetization (either bulk or surface), Weyl nodal point/line and Fermi arc, as well as the exotic transport responses resulting from such band features. We conclude with a brief envision for experimental explorations of new physics or effects by incorporating other orders in intrinsic magnetic topological materials.     

Exotic ferromagnetism in the two-dimensional quantum material C<Subscript>3</Subscript>N

The search for and study of exotic quantum states in novel low-dimensional quantum materials have triggered extensive research in recent years. Here, we systematically study the electronic and magnetic structures in the newly discovered two-dimensional quantum material C₃N within the framework of density functional theory. The calculations demonstrate that C₃N is an indirect-band semiconductor with an energy gap of 0.38 eV, which is in good agreement with experimental observations. Interestingly, we find van Hove singularities located at energies near the Fermi level, which is half that of graphene. Thus, the Fermi energy easily approaches that of the singularities, driving the system to ferromagnetism, under charge carrier injection, such as electric field gating or hydrogen doping. These findings not only demonstrate that the emergence of magnetism stems from the itinerant electron mechanism rather than the effects of local magnetic impurities, but also open a new avenue to designing field-effect transistor devices for possible realization of an insulator–ferromagnet transition by tuning an external electric field.     

钙钛矿超晶格的演生磁电物性

钙钛矿过渡金属氧化物中存在诸多自由度（如晶格、电荷、自旋和轨道）,这些自由度 之间的相互耦合以及相互竞争会诱导出很多奇异物性,如高温超导、庞磁电阻效应、多铁性等, 这些物性在量子器件的发展过程中扮演了重要的角色。在此基础上,若再将不同特性的氧化物材 料耦合在一起形成超晶格,通过界面处的晶格重组与电子重组,体系可呈现出更加丰富的物理和 更多可调控的性能。本综述主要关注钙钛矿超晶格中的磁电物性。首先介绍了超晶格中磁性调控 的几种物理机制,然后对超晶格中的杂化非本征铁电性以及电子铁电性进行了重点讨论,最后围 绕超晶格中的磁电耦合效应进行了讨论和总结。     

On non-Fermi liquid quantum critical points in heavy fermion metals

Heavy electron metals on the verge of a quantum phase transition to magnetism show a number of unusual non-Fermi liquid properties which are poorly understood. This article discusses in a general way various theoretical aspects of this phase transition with an eye toward understanding the non-Fermi liquid phenomena. We suggest that the non-Fermi liquid quantum critical state may have a sharp Fermi surface with power law quasiparticles but with a volume not set by the usual Luttinger rule. We also discuss the possibility that the electronic structure change associated with the possible Fermi surface reconstruction may diverge at a different time/length scale from that associated with magnetic phenomena.     

Experimental Realization of an Intrinsic Magnetic Topological Insulator

An intrinsic magnetic topological insulator(TI) is a stoichiometric magnetic compound possessing both inherent magnetic order and topological electronic states. Such a material can provide a shortcut to various novel topological quantum effects but remained elusive experimentally for a long time. Here we report the experimental realization of thin films of an intrinsic magnetic TI, MnBi_2Te_4, by alternate growth of a Bi_2Te_3 quintuple layer and a MnTe bilayer with molecular beam epitaxy. The material shows the archetypical Dirac surface states in angle-resolved photoemission spectroscopy and is demonstrated to be an antiferromagnetic topological insulator with ferromagnetic surfaces by magnetic and transport measurements as well as first-principles calculations. The unique magnetic and topological electronic structures and their interplays enable the material to embody rich quantum phases such as quantum anomalous Hall insulators and axion insulators at higher temperature and in a well-controlled way.     

基于原子操纵技术的人工量子结构研究

扫描隧道显微镜原子操纵技术是指利用扫描探针在特定材料表面以晶格为步长搬运单个原子或分子的技术.它是纳米尺度量子物理与器件研究领域一种独特而有力的研究手段.利用这种手段,人们能够以原子或分子为单元构筑某些常规生长或微加工方法难以制备的人工量子结构,通过对格点原子、晶格尺寸、对称性、周期性的高度控制,实现对局域电子态、自旋序、以及能带拓扑特性等量子效应的设计与调控.原子操纵技术与超快测量及自动控制技术的结合,使得人们能够进一步研究原子级精准的量子器件,因而该技术成为探索未来器件新机理、新工艺的重要工具.本文首先简介原子操纵方法的发展过程和技术要点,然后分别介绍人工电子晶格、半导体表面人工量子点、磁性人工量子结构、人工结构中的信息存储与逻辑运算、单原子精度原型器件等方面的最新研究进展,以及单原子刻蚀和自动原子操纵等方面的技术进展,最后总结并展望原子操纵技术的应用前景和发展趋势.     

Optical and electronic properties of quantum dots with magnetic impurities

The article discusses some of the recent results on semiconductor quantum dots with magnetic impurities. A single Mn impurity incorporated in a quantum dot strongly changes the optical response of a quantum-dot system. A character of Mn-carrier interaction is very different for II-VI and III-V quantum dots (QDs). In the II-VI QDs, a Mn impurity influences mostly the spin-structure of an exciton. In the III-V dots, a spatial localization of hole by a Mn impurity can be very important, and ultimately yields a totally different spin structure. A Mn-doped QD with a variable number of mobile carriers represents an artificial magnetic atom. Due to the Mn-carrier interaction, the order of filling of electronic shells in the magnetic QDs can be very different to the case of the real atoms. The “periodic” table of the artificial magnetic atoms can be realized in voltage-tunable transistor structures. For the electron numbers corresponding to the regime of Hund's rule, the magnetic Mn-carrier coupling is especially strong and the magnetic-polaron states are very robust. Magnetic QD molecules are also very different to the real molecules. QD molecules can demonstrate spontaneous breaking of symmetry and phase transitions. Single QDs and QD molecules can be viewed as voltage-tunable nanoscale memory cells where information is stored in the form of robust magnetic-polaron states. To cite this article: A.O. Govorov, C. R. Physique 9 (2008).     

基于金刚石氮-空位色心的精密磁测量

磁是一种重要的物理现象,对其进行精密测量推动了许多科技领域的发展.各类测磁技术,包括霍尔传感器、超导量子干涉仪、自旋磁共振等,都致力于提升空间分辨率和灵敏度.近年来,金刚石中的氮-空位色心广受关注.这一固态单自旋体系具有许多优点,例如易于初始化和读出、可操控、具有较长相干时间等,这使得它不仅在量子信息、量子计算等领域崭露头角,而且在量子精密测量上显现出巨大的应用前景.基于氮-空位色心,利用动力学解耦、关联谱等技术,已实现若干高灵敏度、高分辨率的微观磁共振实验,其中包括纳米尺度乃至单分子、单自旋的核磁共振和电子顺磁共振.氮-空位色心也可以用于微波和射频信号的精密测量.本文对围绕上述主题开展的一系列研究工作进行综述.