μ−SR studies of high-T c oxide-superconductor systems

Application of negative muons to studies of electronic states at oxygen sites of high-T _c copper-oxide superconductors as well as 3d transition metal oxides are described. A characteristic feature of the muonic oxygen probes in oxides is discussed. Muonic oxygen can be considered as a nitrogen impurity in the oxides. Around muonic oxygen one hole seems to be introduced and the behavior of the hole depends on the properties of the host oxides. A new impurity problem in strongly correlated electron systems is formulated.     

液氮温区镍氧化物高温超导体的发现

非常规超导材料探索和机理研究是物理学中重要的研究内容和科学问题。目前主要的非常规超导体材料包括重费米子超导体、铜氧化物高温超导体和铁基超导体。其中只有铜氧化物超导临界温度高于液氮沸点77 K,处于液氮温区。镍氧化物与铜氧化物具有相似的材料结构体系,其中+1价镍离子与+2价铜离子具有相同的电子填充数,是此前研究人员探索超导电性的重点。具有钙钛矿结构单元、双镍氧层的La₃Ni₂O₇中镍的价态为+2.5,常压下呈现顺磁性的金属态。近期作者在14 GPa压力下的La₃Ni₂O₇单晶样品中发现了高达80 K的超导临界温度,超出此前理论预期。文章将简要介绍非常规超导材料体系的研究历史、La₃Ni₂O₇单晶样品的生长方法、常压下的物理性质和高压下的结构表征、以及通过电学和磁性测量对超导电性的确认。     

New gap equation for strongly correlated metals

Assuming a phenomenological self-energy ImΣ(ω)|ω|^β, (β=1), which becomes gapped below T_c, we derived a new gap equation. The new gap equation contains the effect of the kinetic energy gain upon developing a superconducting order parameter. However, this new kinetic energy gain mechanism works only for a repulsive pairing potential leading to a s-wave state. In this case, compared to the usual potential energy gain in the superconducting state as in the BCS gap equation, the kinetic energy gain is more effective to easily achieve a high critical temperature T_c, since it is naturally Fermi energy scale. In view of the experimental evidences of a d-wave pairing state in the hole-doped copper-oxide high-T_c superconductors, we discuss the implications of our results.     

Effects of anisotropy and layering in the high-temperature superconductors

The key building blocks of the high-temperature copper-oxide superconductors are the CuO₂ layers, on which superconductivity tends to be localized and along which the normal-state electrical conductivity is highest. Separating these layers (or bilayers, trilayers, etc.) are locally nonsuperconducting layers, which can be modeled as proximity junctions or tunnel junctions. In this paper I summarize some consequences of a theoretical model for strongly anisotropic high-temperature superconductors in which the interlayer regions are treated as Josephson junctions. In such a model, the vortex lines threading through the structure are best visualized as stacks of two-dimensional pancake vortices connected by Josephson strings. The two-dimensional pancake vortices are centered on the layers and have Abrikosov cores, while the Josephson strings are confined to the junctions and have Josephson cores. Outside the cores, the field and current distributions can be calculated from the anisotropic Ginzburg-Landau (or London) theory. Various features of the flux-pinning anisotropy can be explained using these approaches.     

铁基超导的前世今生

高温超导机理被誉为凝聚态物理学前沿研究皇冠上的明珠,铁基超导的发现和研究极大地促进了人们对高温超导电性的理解。文章将简要回顾超导研究的历史和基本概念,介绍铜基高温超导研究的现状及困难,详述铁基超导的发现及其在高温超导研究中的关键桥梁作用,着重介绍中国科学家在铁基超导洪流中做出的重要贡献。     

Ba1-xKxFe2As2单晶(Tc=38.5K)磁通钉扎力与钉扎机理研究

铁基超导体是在2008年由Hosono发现的一种新型超导材料, 由于其具有上临界场高、各向异性小、临界电流密度大等优点, 在世界范围内引起了广泛关注. 以Ba_1-xK_xFe₂As₂为代表的FeAs-122系超导体具有结构简单、合成温度低、单晶容易制备等优点, 是物理学家和材料学家关注的焦点. 本工作在获得最优化掺杂的Ba_1-xK_xFe₂As₂单晶(T_c = 38.5 K)基础上, 通过分析其在不同磁场条件下电阻温度变化关系、不同温度条件下的磁滞回线等数据, 系统的研究了Ba_1-xK_xFe₂As₂单晶磁通钉扎力和磁通钉扎机理. 研究发现Ba_1-xK_xFe₂As₂超导体具有非常高的磁通钉扎势, 其中9 T的外场条件下, 其在H//c轴和H//ab面的钉扎势分别为5800 K和8100 K, 展示出良好的应用前景; 通过进一步分析发现, 其磁通钉扎机理应是由于晶格内部的小尺寸缺陷引起的电子平均自由程变化而导致的δl钉扎.     

Study of the magnetic properties of single-crystal Ba0.6K0.4BiO3

The magnetic properties of single-crystal Ba_0.6K_0.4BiO₃ were studied. The results show that this isotropic superconductor (cubic structure with T_c ≈ 32 K) exhibits irreversibility and relaxation properties similar to those observed in the layered, high temperature superconductors. For fields above 0.1 T, an irreversibility line B_irr = B_irr (0) (1 − t)ⁿ with n = 3/2 and B_irr (0) = 20 T is observed. The comparison among several superconducting systems with different anisotropies suggests that the irreversibility line is unlikely to be the melting line for the YBa₂Cu₃O_7−δ.     

Direct observation of vortex motion in high-Tc superconductors by Lorentz microscopy

The behavior of vortices in superconductors, especially how vortices are depinned to start moving is closely related to practical applications of superconductors. However, it has hitherto been difficult to directly observe the motion of vortices especially in high-T_c superconductors. In the present experiments, the vortex motion peculiar to high-T_c superconductors was observed by improving the observation method using Lorentz electron microscopy in our 300 kV field-emission transmission electron microscope.     

准一维Cr基超导体RbCr3As3的超导能隙

RbCr₃As₃是具有[（Cr₃As₃）^-]_∞线性链的准一维超导体,超导转变温度约为6.6 K.对RbCr₃As₃单晶进行了电输运和极低温热输运性质的研究.低温下,拟合了RbCr₃As₃正常态电阻率随温度的变化,发现其满足费米液体行为.通过拟合超导转变温度随磁场的关系,得到RbCr₃As₃单晶的上临界场约为25.6 T.对RbCr₃As₃进行了零场下的极低温热导率测量,得到其剩余线性项为7.5 μW·K^-2·cm^-1,占正常态热导率值的24%.测量不同磁场下RbCr₃As₃的热导率,发现与单带s波超导体相比较,RbCr₃As₃剩余线性项随磁场增加相对较快.这些结果表明RbCr₃As₃单晶很可能是有节点的非常规超导体.     

Optical conductivity in superconducting layered cuprate oxides

Various properties of the cuprate oxides are explained by a layer model in which normal and superconducting layers are alternately stacked and the bulk superconductivity is maintained by the proximity effect. By using this model, the optical conductivity of the cuprate oxides is calculated. The result for YBa₂Cu₃O₇ is that the optical conductivity for the light polarized parallel to the CuO chain has finite values even for frequencies below the gap, although the conductivity for the light perpendicular to the chain vanishes below the gap, as in the conventional superconductors. The character of the optical conductivities is seen in the results of the optical conductivities measured by Schlesinger et al. using untwinned YBa₂Cu₃O₇ single crystals.     

插层FeSe高温超导体的高压研究进展

通过对FeSe进行化学插层可以将其超导转变温度（T_c）从约8 K提高到40 K以上,实现高温超导电性.最近,我们对两种插层FeSe高温超导材料（Li_0.84Fe_0.16）OHFe_0.98Se和Li_0.36（NH₃）_yFe₂Se₂开展了高压调控研究,发现压力会首先抑制高温超导相（称为SC-I相）,然后在临界压力P_c以上诱导出第二个高温超导相（称为SC-Ⅱ相）,呈现出双拱形T-P超导相图.这两个体系的P_c分别约为5和2 GPa,两个体系SC-Ⅱ相的最高T_c分别可以达到约52和55 K,比相应SC-I相的初始T_c提高了10 K.对（Li_0.84Fe_0.16）OHFe_0.98Se的正常态电输运性质分析表明,SC-I和SC-Ⅱ相的正常态分别具有费米液体和非费米液体行为,意味着这两个超导相可能存在显著差异.此外,还发现这两个体系的SC-Ⅱ相的T_c与霍尔系数倒数1/R_H（∝载流子浓度n_e）具有很好的线性依赖关系.对（Li_0.84Fe_0.16）OHFe_0.98Se的高压X射线衍射测量排除了其在10 GPa以内发生结构相变的可能,因此P_c以上SC-Ⅱ相的出现和载流子浓度的增加很可能起源于压力导致的费米面重构.     

High pressure study on the superconducting transition temperature and Hall coefficient of Ba1−xKxBiO3 up to 8 GPa

The superconducting transition temperature (T_c) of Ba_0.62K_0.38Bio₃ (T_c=30 K) has been measured under high pressure up to 8 GPa. It is observed that T_c increases initially with pressure, as reported by Uwe et al., Shirber et al. and Huang et al., but decreases above 4 GPa. The Hall coefficient of Ba_0.62K_0.38BiO₃ has been measured up to 1.2 GPa. The absolute value of the Hall coefficient decreases with pressure by 10% GPa, the value of which is almost the same as that obtained in most CuO-based high-temperature superconductors.     

Electrostatic modulation of the superfluid density in an ultrathin film

By capacitively charging an underdoped ultrathin La2-xSrxCuO4 film with an electric field applied across a gate insulator with a high dielectric constant, relative changes of the areal superfluid density ns of unprecedented strength were observed in measurements of the film kinetic inductance. Although ns appears to be substantially reduced by disorder, the data provide, for the first time on the same sample, direct compelling evidence for the Uemura relation Tc proportional to ns(T=0) in the underdoped regime of copper-oxide superconductors.     

Normal state magnetism of the high Tc cuprate superconductors

Magnetic measurements of various types have played an essential role in establishing the novel normal state characteristics of high transition temperature (T_c) superconductors with T_c > 23 K. Among these materials, the highest T_c's ( 125 K) are exhibited by the layered cuprates. In this paper, the normal state magnetic susceptibilities of the cuprates are reviewed and interpreted in the context of magnetic neutron scattering and other magnetic measurements, using the La_2−xM_xCuO₄-type and YBa₂Cu₃O_6+x-type materials as prototypical examples. The evolution of the magnetism upon doping the insulating antiferromagnetic “parent” compounds with x = 0 to form the high temperature superconductors is described. A recurrent property which differentiates these materials from conventional superconductors is the existence of strong antiferromagnetic correlations in the metallic state on the same sublattice of the structure in which the itinerant carriers reside.     

Thermodynamics of vortices in layered superconductors and the dependence of Tc on anisotropy

Data on multilayered superconductors such as YBa₂Cu₃O₇PrBa₂Cu₃O₇ show a strong dependence of T_c on anisotropy. In particular, the ratio T_cτ, where τ is the effective XY coupling constant, is found to vary by much more than the theoretical XY limit of 2.4. Layered superconductors allow for an additional anisotropy due to the core energy E_c of vortices perpendicular to the layers. It is shown that if E_c is large, such that the anisotropy is larger than exp(-E_cT_c), T_c is near a fluxon transition which is described by fluctuations of flux loops parallel to the layers. In the latter case T_cτ can vary by more than 2.4, accounting for the data.     

热处理对钙钛矿锰氧化物La_(0.95)Sr_(0.05)MnO_3离子价态和磁结构的影响

对于磁性氧化物的磁有序,传统的观点用超交换相互作用(SE)和双交换相互作用(DE)模型进行解释,其出发点都建立在全部氧离子是-2价的基础上.例如,对于LaMnO_3,认为其中的La和Mn都处于+3价,用SE模型解释相邻Mn~(3+)离子间的反铁磁序;当以二价的Sr离子替代一部分La离子后,认为等量的Mn~(3+)离子变为Mn~(4+)离子,用DE模型解释相邻Mn~(3+)和Mn~(4+)离子间的铁磁序.然而,事实上在氧化物中存在一部分负一价氧离子.Cohen[Nature 358 136]利用密度泛函理论计算了BaTiO_3的价电子态密度,结果得到只有Ba离子的化合价与传统观点相同,为+2价;而Ti和0分别为+2.89价和-1.63价,不是传统观点的+4价和-2价,但是与多年来关于氧化物电离度的研究[Rev.Mod.Phys.42 317]和X射线光电子谱(XPS)的研究结果相符合.本文经过不同热处理条件制备了名义成分为La_(0.95)Sr_(0.05)MnO_3的三个样品,通过对样品的XPS分析,发现样品中不存在Mn~(4+)离子,只存在Mn2+和Mn~(3+)离子,平均价态随热处理程序的增加而升高.尽管三个样品有相同的晶体结构,但磁矩明显不同.对于这样的性能,不能用SE和DE模型解释其磁结构.利用本课题组最近在研究尖晶石结构铁氧体磁有序过程中提出的O 2p巡游电子模型解释了这种现象,利用样品在10 K的磁矩估算出的Mn离子平均价态变化趋势与XPS分析结果一致.O 2p巡游电子模型的出发点建立在氧化物中存在一部分负一价氧离子的基础上,这是其与SE和DE模型的根本区别.     

压力下碱金属铁硒基超导体中的现象与物理

在凝聚态物理研究中, 压力作为对物质状态调控的独立变量得到了广泛的应用. 压力对发现物质的新现象、新规律及对其形成机理的理解和对相关理论的验证起到了重要的作用, 尤其在超导电性的研究中取得了巨大的成功. 文章简要的介绍了通过利用压力手段对具有相分离结构的碱金属铁硒基超导体A_xFe_2-ySe₂ (A=K, Rb, Tl/Rb)开展的系列研究所取得的实验结果, 以及其他一些文献中报道的在此方面的主要实验与理论研究工作, 包括压力导致的超导再进入现象和其产生的量子临界机理、其特有的反铁磁绝缘体相在该类超导体实现超导电性中的作用、化学负压力对超导电性的影响、构成该类超导体的反铁磁序与其寄居的超晶格的关系等.     

High Tc superconductivity in layered hydrides XH15 (X = Ca,Sr, Y,La) under high pressures

The theoretical predictions and experimental synthesis of H₃S and LaH₁₀ superconductors with record high superconducting transition temperatures (T_c) have promoted the hydrogen-based superconducors to be a research hotspot in the field of solid-state physics. Here, we predict an unprecedented layered structure CaH₁₅, with high T_c of 189 K at 200 GPa using ab initio calculations. As concerns the novel structure, one layer is made of a hydrogen nonagon, the other layer consists of a Ca atom and six H₂ molecular units surrounding the Ca atom. This layered structure was also found in SrH₁₅, YH₁₅, and LaH₁₅ at high pressures, each materials exhibit high T_c especially YH₁₅ can reach above 200 K at 220 GPa. It represents the second class of layered superhydrides with high value of T_c after pentagraphene like HfH₁₀.     

Magnetic impurities in amorphous superconductors:TKondo>Tc

Measurements of upper critical field as a function of temperature and Fe-concentration are reported for the amorphous superconductors ((Mo_.6Ru_.4)_.80G_.20)_1−cFe_c, where G is a glass-former. The linear shapes of the H_c2 vs. T curves near T_c agree well with predictions made by Matsura, Ichinose, and Nagaoka, for T_Kondo > T_c.