Exploration of iron-chalcogenide superconductors

Iron-chalcogenide compounds with FeSe（Te, S） layers did not attract much attention until the discovery of high-Tc superconductivity （SC） in the iron-pnictide compounds at the begining of 2008. Compared with FeAs-based superconductors, iron-chalcogenide superconductors have aroused enormous enthusiasm to study the relationship between SC and magnetisms with several distinct features, such as different antiferromagnetic ground states with relatively large moments in the parents, indicating possibly different superconducting mechanisms, the existence of the excess Fe atoms or Fe vacancies in the crystal lattice. Another reason is that the large single crystals are easily grown for the iron-chalcogenide compounds. This review will focus on our exploration for the iron-chalcogenide superconductors and discussion on several issues, including the crystal structure, magnetic properties, superconductivity, and phase separation. Some of them reach a consensus but some important questions still remain to be answered.     

超导材料的发展与研究现状

新型超导材料一直是人类追求的目标。该文主要从超导材料的探索与发现、制备技术、基础研究面临的挑战等几个方面来探讨超导材料的发展与研究现状。     

二维过渡金属硫族化合物中的缺陷和相关载流子动力学的研究进展

原子级厚度的单层或者少层二维过渡金属硫族化合物因其独特的物理特性而被寄希望成为下一代光电子器件的重要组成部分.然而,二维材料的缺陷在很大程度上影响着材料的性质.一方面,缺陷的存在降低了材料的荧光量子效率、载流子迁移率等重要参数,影响了器件的性能.另一方面,合理地调控和利用缺陷催生了单光子源等新的应用,因此,表征、理解、...     

美妙的超导应用

 超导态是物质的一种独特状态,它的新奇性,立刻使人想到将它用到技术上。自20年代起,人们对超导应用的热情总比对超导机理研究要高;到80年代高温超导材料问世后,这种热情就更为炽烈。目前超导的应用正在许多领域里迅速发展,其景况十分美妙和诱人。     

醇体系中合成CuGaS2纳米晶及其形貌演变

以CuCl2·2H2O,自制的GaCl3和(NH2)2CS为原料,在乙二醇体系中合成了花状结构的CuGaS2纳米晶．产物分别用X射线粉末衍射仪、透射电子显微镜、场发射扫描电子显微镜、高分辩透射电子显微镜和X射线光电子能谱仪进行了表征．实验结果表明,220℃反应24 h得到均匀的花状纳米结构CuGaS2,它是由厚度80-100 nm的片晶组成．同时,通过反应时间的控制,可以清楚的看到由纳米颗粒到纳米球以及花状纳米结构的演变过程．另外,研究了反应温度、反应时间、溶剂等对产物和形貌的影响．此外,对花状结构纳米晶的生长机理进行了初步的探讨．室温荧光光谱表明,随粒径的降低,发光位发生了部分蓝移．     

Crystal chemistry and structural design of iron-based superconductors

The second class of high-temperature superconductors （HTSCs）, iron-based pnictides and chalcogenides, necessarily contain Fe2X2 （＂X＂ refers to a pnictogen or a chalcogen element） layers, just like the first class of HTSCs which possess the essential CuO2 sheets. So far, dozens of iron-based HTSCs, classified into nine groups, have been discovered. In this article, the crystal-chemistry aspects of the known iron-based superconductors are reviewed and summarized by employing ＂hard and soft acids and bases （HSAB）＂ concept. Based on these understandings, we propose an alternative route to exploring new iron-based superconductors via rational structural design.     

金属硫族簇基半导体中的Mn2+发光

锰离子相对稳定的发光特性及锰掺杂荧光粉的成功使得人们对锰离子发光的研究热情不减。本文综述了Mn~(2+)掺杂金属硫族簇基半导体中团簇结构、组分关联的Mn~(2+)发光特性。金属硫族簇基半导体具有原子级的精确结构,为探究Mn~(2+)发光的精确"构效关系"提供了理想模型。在Mn~(2+)掺杂的金属硫族簇基半导体中,Mn~(2+)附近键长和团簇组装方式的差异决定了Mn~(2+)配位场强的变化,进而影响Mn~(2+)发光波长的变化。在Mn~(2+)掺杂的金属硫族纳米团簇中,Mn~(2+)的聚集形式和聚集体中Mn~(2+)的数量决定了Mn-Mn耦合相互作用的大小,直接影响Mn~(2+)发光效率、寿命及激发特性。     

超导简史

 超导现象最初是1911年由荷兰物理学家昂内斯(Onnes)发现的。1908年,昂内斯首次获得液化的氦,并且在液氦温度(4.2K)下研究各种物质的电学特性。他发现,在温度为4.2K时,汞的电阻突然消失。1933年,迈斯纳(Meissner)和奥森菲尔德(Os-chenfeld)发现,处于弱磁场中的超导体会将磁场从内部排斥出来(见图1),这就是迈斯纳效应。1945年,俄罗斯物理学家阿卡迪也夫(Arkadiev)利用这一特性首次演示了将一块小的条形磁铁悬浮于超导体的上方的实验(见图2)。随后而来的几十年,其他超导材料--金属、合金、化合物的超导材料相继找到。     

永磁体结构对高温超导推力轴承静态特性的影响

基于商用电磁场有限元软件以及M e issner效应假设,提出了对由块状高温超导体和永磁体组成的高温超导推力轴承静态特性进行分析的方法,分析了永磁体的结构对高温超导推力轴承悬浮力的影响。结果发现:永磁体的结构及磁极排列方式对高温超导推力轴承悬浮力的影响很大,适当地选择永磁体的结构和磁极排列方式可以显著地提高高温超导推力轴承的悬浮力。     

Interplay of superconductivity and d-f correlation in CeFeAs_(1-x)P_xO_(1-y)F_y

The recent discovery of high-temperature superconductivity in iron-based pnictides (chalcogenides) not only triggers tremendous enthusiasm in searching for new superconducting materials, but also opens a new avenue to the study of the Kondo physics. CeFeAsO is a parent compound of the 1111-type iron-based superconductors. It shows 3d-antiferromagnetic (AFM) ordering below ～ 139 K and 4f-AFM ordering below ～ 4 K. On the other hand, the phosphide CeFePO is a ferromagnetically correlated heavy-fermion (HF) metal with Kondo scale T K ～ 10 K. These properties set up a new platform for research of the interplay among magnetism, Kondo effect, and superconductivity (SC). In this review, we present the recent progress in the study of chemical pressure effect in CeFeAsO 1-y F y (y = 0 and 0.05). This P/As-doping in CeFeAsO serves as an effective controlling parameter which leads to two magnetic critical points, x c1 0.4 and x c2 0.92, associated with suppression of 3d and 4f magnetism, respectively. We also observe a turning point of AFM-FM ordering of Ce 3+ moment at x c3 0.37. The SC is absent in the phase diagram, which is attributed to the destruction to Cooper pair by Ce-FM fluctuations in the vicinity of x c1 . We continue to investigate CeFeAs 1-x P x O 0.95 F 0.05 . With the separation of x c1 and x c3 , this chemical pressure results in a broad SC region 0≤ x ≤ 0.53, while the original HF behavior is driven away by 5% F doping. Different roles of P and F dopings are addressed, and the interplay between SC and Ce-4f magnetism is also discussed.     

Magnetoresistivity and filamentary superconductivity in nickel-doped BaFe_2As_2

We present magnetotransport studies on a series of BaFe_(2-x)Ni_xAs_2(0.03 ≤ x ≤ 0.10) single crystals. In the underdoped(x = 0.03) non-superconducting sample, the temperature-dependent resistivity exhibits a peak at 22 K, which is associated with the onset of filamentary superconductivity(FLSC). FLSC is suppressed by an external magnetic field in a manner similar to the suppression of bulk superconductivity in an optimally-doped(x = 0.10) compound, suggesting the same possible origin as the bulk superconductivity. Our magnetoresistivity measurements reveal that FLSC persists up to the optimal doping and disappears in the overdoped regime where the long-range antiferromagnetic order is completely suppressed, pointing to a close relation between FLSC and the magnetic order.     

Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu2Si2 versus YbRh2Si2

In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu₂Si₂, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu₂Si₂. In YbRh₂Si₂, superconductivity appears to be suppressed at T???10?mK by AF order (T_N?=?70?mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at T_A slightly above 2?mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at T_c?=?2?mK. Like the pressure – induced QCP in CeRhIn₅, the magnetic field – induced one in YbRh₂Si₂ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-T unconventional heavy – fermion superconductors and other families of unconventional superconductors with higher T_cs, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon.     

Finite cluster studies of electron correlation effects

A review of the physical properties of small systems described by various model Hamiltonians is given. Their physical properties are found either exactly or by using Monte-Carlo methods, and are compared with properties of bulk systems. It is shown that (i) clusters differ in an essential way from the bulk, having electronic states strongly dependent on the geometry and cluster size, but, at the same time, (ii) important insights into the nature of electron correlation and low-energy excitations in solids may be gained by cluster studies. In particular, the cluster calculations allow one to identify the tendencies to various kinds of symmetry-broken states in the (extended) Hubbard model and give a simple qualitative understanding of the low temperature properties of the Kondo and the heavy-fermion systems. Furthermore, cluster calculations allow one to study such problems as the development of magnetic moment at chemisorbed atoms for strong correlations and the suppression of frustration by quantum fluctuations. Recently, cluster calculations gave important information about the possible role of charge transfer processes and magnetic interactions in the microscopic mechanism of pairing in high temperature superconducting oxides.     

压力环境下重费米子体系的物性探索

@@@@重费米子体系是近藤晶格中的强关联电子系统,它由于电、磁等相互作用的竞争呈现出丰富的电子基态,而且由于各种相互作用的能量尺度较小,它会对外界参量(压力、掺杂、磁场)的调控比较敏感。文章论述了重费米子体系的基本性质和它在压力环境下的物性演变以及相关的量子临界现象。超导态一般会出现在量子临界点附近,文章特别以“115”超导体系为例,讲述了超导态与其他长程序共存时可能出现的纹络化结构,介绍了如何运用压力下的谱学测量(如转角比热、软点接触隧道谱)来研究压力下重费米子体系的物性特别是超导序参量的对称性。     

压力环境下重费米子体系的物性探索

重费米子体系是近藤晶格中的强关联电子系统,它由于电、磁等相互作用的竞争呈现出丰富的电子基态,而且由于各种相互作用的能量尺度较小,它会对外界参量(压力、掺杂、磁场)的调控比较敏感。文章论述了重费米子体系的基本性质和它在压力环境下的物性演变以及相关的量子临界现象。超导态一般会出现在量子临界点附近,文章特别以“115”超导体系为例,讲述了超导态与其他长程序共存时可能出现的纹络化结构,介绍了如何运用压力下的谱学测量(如转角比热、软点接触隧道谱)来研究压力下重费米子体系的物性特别是超导序参量的对称性。     

重费米子超导与竞争序

与其他非常规超导系列相比, 重费米子超导体往往具有丰富多样的竞争序, 超导与各种竞争序相伴而生, 电子配对与反铁磁涨落、铁磁涨落、价态涨落、电四极矩涨落等量子临界涨落密切相关, 扩充了非常规超导的研究内容. 重费米子材料中的f电子往往同时参与超导与各种竞争序的形成, 表现出局域与巡游的二重性. 重费米子二流体理论为理解重费米子超导与竞争序的关系提供了新的思路.     

CrAs——第一个Cr 基化合物超导体的发现

非常规超导体自发现以来便引起了凝聚态物理学家的广泛兴趣,诸如重费米子超导体,铜基高温超导体和铁基高温超导体等。这类超导体的一个基本特征是,当长程反铁磁序被抑制时会出现超导现象。而要破坏长程磁有序,除了掺杂不同元素以引入电荷载流子或者化学压力外,物理加压也是一种有效的调控手段。文章将介绍第一个Cr基化合物超导体——CrAs单晶通过物理加压首次实现超导电性的过程。当压力为8 kbar 时,超导临界温度出现在2 K,这时发生在常压下265 K的一级反铁磁相变被完全压制。在CrAs体系中,超导与反铁磁序之间的竞争关系说明CrAs是非常规超导体。CrAs的超导也为发现新型超导体打开了一扇大门。     

Resistance minimum and heavy fermions

The phenomenon of the resistance minimum in dilute magnetic alloys is explained in terms of the s-d interaction which takes account of scattering of the conduction electron off the magnetic impurities in metals. Some of the intermetallic compounds which involve rare earth elements or uranium show a very large electronic specific heat and remain non-magnetic even though they show a Curie-like susceptibility at higher temperatures. These phenomena are also explained based on the s-d interaction model.     

Revealing the A_(1g)-type strain effect on superconductivity and nematicity in FeSe thin flake

The driving mechanism of nematicity and its twist with superconductivity in iron-based superconductors are still under debate.Recently,a dominant B_(1 g)-type strain effect on superconductivity is observed in underdoped iron-pnictides superconductors Ba(Fe_(1-x)Co_x)_2 As_2,suggesting a strong interplay between nematicity and superconductivity.Since the long-range spin order is absent in FeSe superconductor,whether a similar strain effect could be also observed or not is an interesting question.Here,by utilizing a flexible film as substrate,we successfully achieve a wide-range-strain tuning of FeSe thin flake,in which both the tensile and compressive strain could reach up to ～0.7%,and systematically study the strain effect on both superconducting and nematic transition(T_c and T_s) in the FeSe thin flake.Our results reveal a predominant A_(1 g)-type strain effect on T_c.Meanwhile,T_s exhibits a monotonic anti-correlation with T_c and the maximum T_c reaches to 12 K when T_s is strongly suppressed under the maximum compressive strain.Finally,in comparison with the results in the underdoped Ba(Fe_(1-x)Co_x)_2 As_2,the absence of B_(1 g)-type strain effect in FeSe further supports the role of stripe-type spin fluctuations on superconductivity.In addition,our work also supports that the orbital degree of freedom plays a key role to drive the nematic transition in FeSe.