长盛不衰的超导研究

 新年刚刚开始,中国超导界就迎来了喜讯。中国科学院物理研究所研究员、中国科学院院士赵忠贤以其在钇钡铜氧的发现和铁基超导体等方面的突出贡献获得了国家科技成就奖。赵忠贤院士获奖是对他几十年来在高温超导体研究中孜孜以求、不断求索的认可,是实至名归。     

铁基超导体的远红外光谱学研究

铁基超导体是由日本科学家Hosono发现的具有层状结构和多带特征的非常规超导体,虽然人们利用各种实验手段对不同家族的铁基超导体进行了详尽的研究,最受关注的超导机理的问题尚未解决,也吸引着人们进行更加深入的探索.红外光谱是研究费米面附近的电子态激发以及晶格振动动力学的重要实验探测手段,在早期的超导研究中对于超导能隙的确定和电声相互作用的探测起到了重要作用.本文将主要介绍近些年国际上利用傅里叶变换红外光谱学研究铁基超导体方面的主要进展,并结合作者近些年的工作详细地阐述红外光谱的具体应用,主要包括以下几个方面的内容:(1)铁基超导体中的超导能隙;(2)(Ba,K)Fe_2As_2中的量子临界与非费米液体行为;(3)KFe_2As_2中的轨道选择性行为;(4)铁基超导材料中的电声相互作用;(5)铁基超导材料中的向列相.最后,在总结已有工作的基础上,我们对未来红外光谱在铁基超导材料中的研究做了进一步的展望.     

铁基超导材料研究进展

铁基超导体(iron-based superconductors)被认为是继铜基超导材料后的第二类高温超导家族,具有丰富的物理特性和广阔的应用前景,同时也为探究高温超导机制提供了全新的平台.利用33篇文献综述了几种典型的铁基超导体研究现状,同时简述了近期铁基超导体研究领域取得的一些进展,最后对其应用前景进行了展望.     

自旋三重态超导

超导体中的库珀对通常是由一对自旋相反总自旋为零(自旋单态)的电子组成.然而,在某些特殊情况下,超导体中的库珀对可以是自旋三重态,这类超导体被称之为自旋三重态超导体.自旋三重态超导是非常罕见的量子现象,在已经发现的上万种超导体中,仅有几个超导体可能具有三重态配对.近几年,随着拓扑物态研究的深入,三重态超导体因为可能是拓扑超导的载体而越来越多地引起关注.本文简要地总结了几类可能的自旋三重态超导体的物理性质.     

发现新的铁基超导材料

<正>中国科学技术大学合肥微尺度物质科学国家实验室教授陈仙辉研究组发现了一种新的铁基超导材料锂铁氢氧铁硒化合物(Li0.8Fe0.2)OHFe Se,其超导转变温度高达40 K(零下233.15摄氏度)以上,并确定了该新材料的晶体结构。相关成果在线发表在12月15日的《自然-材料》上。铁基高温超导体是目前凝聚态物理领域的研究热点,铁硒(Fe Se)类超导体以其诸多独特的性质被认为是研究铁基超导机理的理想材料体系。尤其是近期报道的生长于钛酸锶衬底上的铁硒单层薄膜的零电阻转变温度高达100K以上,更加激起了科学家们的浓厚兴趣。     

超导研究与未来超导技术

综述了超导体的发现和各个时期的研究成果,特别是金属间化合物MgB2研究的新成果.认为由于氧化物陶瓷材料的固有属性及一些具体技术的限制,高温超导的应用还十分有限,MgB2不久将成为有真正应用价值的超导体,低温超导技术将领先于高温超导技术.     

超导材料的超快光谱研究

电子、晶格、自旋和轨道微观自由度对超导材料的宏观特性起到至关重要的作用.在超导体系中,特别是非常规超导材料,这些自由度衍生出具有不同能量尺度的玻色激发和有序态.前者如声子、磁振子、电荷密度波、自旋密度波、自旋涨落、向列涨落等;后者如超导态、赝能隙态、向列相、反铁磁/铁磁等.前者与后者的形成密切相关.尤其是,不同的玻色激发在频域内纠缠在一起彼此相互作用,同时又与电子(或准粒子)耦合,构建出复杂而又丰富的平衡态和非平衡态物理过程.超快光谱技术的独特性在于具有宽能量范围和高时间分辨率的特点,利用光(电磁波)与超导材料相互作用中的线性和非线性响应,可以共振或非共振地探测与调控这类材料中的准平衡或非平衡态动力学属性.因为桌面超快光谱系统功能全面且具有很大的灵活性,它不仅被应用于超导体系,而且被广泛应用于其他各种无机和有机材料.由于非平衡态理论,特别是与关联电子体系相关的,目前还处在快速发展的阶段,所以本综述主要介绍了常用的桌面超快光谱技术和目前被广泛使用的相关分析理论,聚焦于讨论超导材料中超快光谱实验数据涌现出来的一些普适性趋势及进展.所涉及的超导材料包含了常规超导体、铜氧化物超导体、铁基超导体和重费米子超导体.     

超导综述

引言超导体是人们急需研究和开发的一种理想电磁材料,凡涉及到电和磁或电磁辐射的领域,超导体具有十分重要的应用。例如:磁流发电,电磁推进、超导储能、核磁共振、高能物理,受控热核反应、激光等。自1911年发现超导现象以来,超导材料的临界温度平均每年仅提高1K,而87年2月以来中、美、日、德、苏、匈等国相继宣布得到大于90K 的超导材料,最近美国宣布     

重大科技成就奖获奖代表简介

正陈仙辉中国科学院院士,现任中国科学技术大学教授,长期坚持新型非常规超导体的探索及超导和强关联物理的研究,发现的新型超导体涵盖铜氧化合物超导体、富勒烯超导体、铁基超导体和有机超导体等多种体系。在铁基超导体的研究中取得突破性成果:首次在铁基超导体(常压下)实现40开以上的     

122型铁基高温超导体的研究进展

零电阻和迈斯纳效应使超导材料在许多领域都发挥着重要作用,因此超导电性100年来一直是研究的热点。本文系统总结了122型铁基超导体的研究状况,包括用化学掺杂和化学压力等不同方法获得的超导材料。此外,也总结了铁基超导线材和带材的制备,简短的介绍了目前的工艺和对未来的展望。     

Antiferromagnetic order as the competing ground state in electron-doped Nd1.85Ce0.15CuO4

Superconductivity in the high-transition-temperature (high-T(c)) copper oxides competes with other possible ground states. The physical explanation for superconductivity can be constrained by determining the nature of the closest competing ground state, and establishing if that state is universal among the high-T(c) materials. Antiferromagnetism has been theoretically predicted to be the competing ground state. A competing ground state is revealed when superconductivity is destroyed by the application of a magnetic field, and antiferromagnetism has been observed in hole-doped materials under the influence of modest fields. None of the previous experiments have revealed the quantum phase transition from the superconducting state to the antiferromagnetic state, because they failed to reach the upper critical field B(c2). Here we report the results of transport and neutron-scattering experiments on electron-doped Nd1.85Ce0.15CuO4 (refs 13, 14), where B(c2) can be reached. The applied field reveals a static, commensurate, anomalously conducting long-range ordered antiferromagnetic state, in which the induced moment scales approximately linearly with the field strength until it saturates at B(c2). This and previous experiments on the hole-doped materials therefore establishes antiferromagnetic order as a competing ground state in the high-T(c) copper oxide materials, irrespective of electron or hole doping.     

Spatially resolved electronic structure inside and outside the vortex cores of a high-temperature superconductor

Puzzling aspects of high-transition-temperature (high-Tc) superconductors include the prevalence of magnetism in the normal state and the persistence of superconductivity in high magnetic fields. Superconductivity and magnetism generally are thought to be incompatible, based on what is known about conventional superconductors. Recent results, however, indicate that antiferromagnetism can appear in the superconducting state of a high-Tc superconductor in the presence of an applied magnetic field. Magnetic fields penetrate a superconductor in the form of quantized flux lines, each of which represents a vortex of supercurrents. Superconductivity is suppressed in the core of the vortex and it has been suggested that antiferromagnetism might develop there. Here we report the results of a high-field nuclear-magnetic-resonance (NMR) imaging experiment in which we spatially resolve the electronic structure of near-optimally doped YBa2Cu3O7-delta inside and outside vortex cores. Outside the cores, we find strong antiferromagnetic fluctuations, whereas inside we detect electronic states that are rather different from those found in conventional superconductors.     

Quantum magnetic excitations from stripes in copper oxide superconductors

In the copper oxide parent compounds of the high-transition-temperature superconductors the valence electrons are localized--one per copper site--by strong intra-atomic Coulomb repulsion. A symptom of this localization is antiferromagnetism, where the spins of localized electrons alternate between up and down. Superconductivity appears when mobile 'holes' are doped into this insulating state, and it coexists with antiferromagnetic fluctuations. In one approach to describing the coexistence, the holes are believed to self-organize into 'stripes' that alternate with antiferromagnetic (insulating) regions within copper oxide planes, which would necessitate an unconventional mechanism of superconductivity. There is an apparent problem with this picture, however: measurements of magnetic excitations in superconducting YBa2Cu3O6+x near optimum doping are incompatible with the naive expectations for a material with stripes. Here we report neutron scattering measurements on stripe-ordered La1.875Ba0.125CuO4. We show that the measured excitations are, surprisingly, quite similar to those in YBa2Cu3O6+x (refs 9, 10) (that is, the predicted spectrum of magnetic excitations is wrong). We find instead that the observed spectrum can be understood within a stripe model by taking account of quantum excitations. Our results support the concept that stripe correlations are essential to high-transition-temperature superconductivity.     

S-P波混合超导的平均场理论谨以此文深切怀念敬爱的魏荣爵先生.他的关怀、支持和帮助,铭刻于心

对于由4个费米子相互作用系统组成的哈密顿量的平均场理论,当只有S波超导时,由SU(2)(局域同构于SO(3))代数表达,P波超导可以通过SO(5)表达,而对于S-P混合波超导的情况则由SU(4)(局域同构于SO(6))代数表达.本文通过在SO(6)代数中引入了反映S波和P波混合超导的SO (4)子集,从而可以通过对相干...     

离子液体调控超导研究

载流子在超导材料中扮演关键角色,其浓度调控对研究超导特性及相关量子器件至关重要.然而载流子浓度通常与其他自由度相互耦合,难以做到单一物理量变化,例如,化学掺杂同时会导致晶格等发生变化.最近迅速发展的离子液体调控技术,兼具了传统的化学掺杂和场效应晶体管优点——大范围、原位、可逆地调节载流子浓度.随着这项技术的发展,逐渐演变出两类调控思路:静电场效应和电化学调控.本文从这两个方面,回顾了离子液体调控在诱导新奇超导态和调控高温超导薄膜物性上的应用:静电场效应对绝缘或半导体体系十分有效,而对于本身载流子浓度较高的材料(如高温超导体等),电化学调控则发挥了重要作用.离子液体因其强大的原位调控能力和易于与其他手段结合的特点,正逐渐成为超导研究中不可或缺的手段,在构建精确相图、突破高温超导机理等方面发挥不可替代的作用.     

Superconductivity at 52 K in hole-doped C60

Superconductivity in electron-doped C60 was first observed almost ten years ago. The metallic state and superconductivity result from the transfer of electrons from alkaline or alkaline-earth ions to the C60 molecule, which is known to be a strong electron acceptor. For this reason, it is very difficult to remove electrons from C60--yet one might expect to see superconductivity at higher temperatures in hole-doped than in electron-doped C60, because of the higher density of electronic states in the valence band than in the conduction band. We have used the technique of gate-induced doping in a field-effect transistor configuration to introduce significant densities of holes into C60. We observe superconductivity over an extended range of hole density, with a smoothly varying transition temperature Tc that peaks at 52 K. By comparison with the well established dependence of Tc on the lattice parameter in electron-doped C60, we anticipate that Tc values significantly in excess of 100 K should be achievable in a suitably expanded, hole-doped C60 lattice.     

Magnetic enhancement of superconductivity from electron spin domains

Since the discovery of superconductivity, there has been a drive to understand the mechanisms by which it occurs. The BCS (Bardeen-Cooper-Schrieffer) model successfully treats the electrons in conventional superconductors as pairs coupled by phonons (vibrational modes of oscillation) moving through the material, but there is as yet no accepted model for high-transition-temperature, organic or 'heavy fermion' superconductivity. Experiments that reveal unusual properties of those superconductors could therefore point the way to a deeper understanding of the underlying physics. In particular, the response of a material to a magnetic field can be revealing, because this usually reduces or quenches superconductivity. Here we report measurements of the heat capacity and magnetization that show that, for particular orientations of an external magnetic field, superconductivity in the heavy-fermion material CeCoIn(5) is enhanced through the magnetic moments (spins) of individual electrons. This enhancement occurs by fundamentally altering how the superconducting state forms, resulting in regions of superconductivity alternating with walls of spin-polarized unpaired electrons; this configuration lowers the free energy and allows superconductivity to remain stable. The large magnetic susceptibility of this material leads to an unusually strong coupling of the field to the electron spins, which dominates over the coupling to the electron orbits.     

Hidden magnetism and quantum criticality in the heavy fermion superconductor CeRhIn5

With only a few exceptions that are well understood, conventional superconductivity does not coexist with long-range magnetic order (for example, ref. 1). Unconventional superconductivity, on the other hand, develops near a phase boundary separating magnetically ordered and magnetically disordered phases. A maximum in the superconducting transition temperature T(c) develops where this boundary extrapolates to zero Kelvin, suggesting that fluctuations associated with this magnetic quantum-critical point are essential for unconventional superconductivity. Invariably, though, unconventional superconductivity masks the magnetic phase boundary when T < T(c), preventing proof of a magnetic quantum-critical point. Here we report specific-heat measurements of the pressure-tuned unconventional superconductor CeRhIn5 in which we find a line of quantum-phase transitions induced inside the superconducting state by an applied magnetic field. This quantum-critical line separates a phase of coexisting antiferromagnetism and superconductivity from a purely unconventional superconducting phase, and terminates at a quantum tetracritical point where the magnetic field completely suppresses superconductivity. The T --> 0 K magnetic field-pressure phase diagram of CeRhIn5 is well described with a theoretical model developed to explain field-induced magnetism in the high-T(c) copper oxides, but in which a clear delineation of quantum-phase boundaries has not been possible. These experiments establish a common relationship among hidden magnetism, quantum criticality and unconventional superconductivity in copper oxides and heavy-electron systems such as CeRhIn5.     

量子场论中的涡旋解与偏微分方程

涡旋解在量子场论中有着广泛的应用和重要的理论意义.这篇综述简要介绍在超导理论、宇宙学、弱电统一理论、夸克禁闭等领域的研究中出现的各种涡旋解的结构以及所涉及的非线性偏微分方程问题.     

Local switching of two-dimensional superconductivity using the ferroelectric field effect

Correlated oxides display a variety of extraordinary physical properties including high-temperature superconductivity and colossal magnetoresistance. In these materials, strong electronic correlations often lead to competing ground states that are sensitive to many parameters--in particular the doping level--so that complex phase diagrams are observed. A flexible way to explore the role of doping is to tune the electron or hole concentration with electric fields, as is done in standard semiconductor field effect transistors. Here we demonstrate a model oxide system based on high-quality heterostructures in which the ferroelectric field effect approach can be studied. We use a single-crystal film of the perovskite superconductor Nb-doped SrTiO3 as the superconducting channel and ferroelectric Pb(Zr,Ti)O3 as the gate oxide. Atomic force microscopy is used to locally reverse the ferroelectric polarization, thus inducing large resistivity and carrier modulations, resulting in a clear shift in the superconducting critical temperature. Field-induced switching from the normal state to the (zero resistance) superconducting state was achieved at a well-defined temperature. This unique system could lead to a field of research in which devices are realized by locally defining in the same material superconducting and normal regions with 'perfect' interfaces, the interface being purely electronic. Using this approach, one could potentially design one-dimensional superconducting wires, superconducting rings and junctions, superconducting quantum interference devices (SQUIDs) or arrays of pinning centres.