Magnetization study of ITER-type internal-Sn Nb3Sn superconducting wire

Through magnetization measurement with a SQUID magnetometer the heat treatment optimization of an international thermonuclear experimental reactor （ITER）-type internal-Sn Nb3Sn superconducting wire has been investigated. The irreversibility temperature T^＊ （H）, which is mainly dependent on A15 phase composition, was obtained by a warming and cooling cycle at a fixed field. The hysteresis width △M（H） which reflects the flux pinning situation of the A15 phase is determined by the sweeping of magnetic field at a constant temperature. The results obtained from differently heat-treated samples show that the combination of T^＊ （H） with AM（H） measurement is very effective for optimizing the heat reaction process. The heat treatment condition of the ITER-type wire is optimized at 675℃/128 h, which results in a composition closer to stoichiometric Nb3Sn and a state with best flux pinning.

Fabrication and Microstructure of BSCCO Superconductive Wires Made by LHPG Method

Laser heated pedestal growth technique (LHPG) has been used to fabricate BSCCO (BiSrCaCuO) superconductive wires.The critical current density of the wires has surpassed 5000A/cm^2 (78K, OT;cross area=1.04 mm^2).The fiber rods obtained directly from LHPG method are semiconductive and after heat treatment at temperature range from 780 to 870℃,they transformed into superconductive wires.Triple junctions and sharp grain boundary exist in the sample.HREM studies showed the intergrowth of (010) and (110),the occasional missing of CaCuO_2 layer in the 2212 phase.

Penalty-Finite Element Analysis of the Eddy Current Loss in a Fully-Stabilized Multifilamentary Superconducting Wire

The paper is mainly concerned with the penalty incurred in finite element analysis of the eddy-current loss problem in a fully established multi-filamentary superconducting wire. A finite element model with 4-node quadrilateral isoparametric elements is formulated for the present problem. Unlike the conventional vector potential scheme in use for electromagnetic field problem, the present work features a direct computational approach to eddy current loss. Simplicity and remarkable enhancement in computational accuracy can be obtained with the proposed method.

Electron-phonon coupling in cuprate and iron-based superconductors revealed by Raman scattering

Electron-phonon coupling （EPC） in cuprate and iron-based superconducting systems, as revealed by Raman scat- tering, is briefly reviewed. We introduce how to extract the coupling information through phonon lineshape. Then we discuss the strength of EPC in different high-temperature superconductor （HTSC） systems and possible factors affecting the strength. A comparative study between Raman phonon theories and experiments allows us to gain insight into some crucial electronic properties, especially superconductivity. Finally, we summarize and compare EPC in the two existing HTSC systems, and discuss what role it may play in the HTSC.

A Comparative Study on the Microstructures and Mechanical Properties of Two Kinds of Iron-Based Alloys by WAAM

In order to adapt to the high temperature and heavy load process environment of large forgings,a novel die with"fist-like"structure is designed.The“fist-like”die mainly consists of“skin”layer,“bone”layer and matrix.To obtain the material with good supportability and good bonding strength with the“skin”layer,iron-based alloys RMD248 and CN72 were selected to make the"bone"layer,and the properties were compared.In this paper,the"bone"layer and the"skin"layer(CHN327)were surfaced on 5CrNiMo matrix by wire arc additive manufacture(WAAM).Then,cyclic heating to 500℃and thermal compression with a maximum deformation of 30%were adapted to test the high temperature mechanical properties.The microstructure changes before and after thermal cycles and compressions were observed by optical microscopy(OM),X-ray diffraction(XRD),energy dispersive spectrometer(EDS)and scanning electron microscopy(SEM).The results show that CN72 has more carbides than RMD248 at the joint surface,which makes it easy to form brittle fracture at the joint.Mechanical properties were tested by using microhardness machine.Meanwhile,hot tensile tests were performed to study bonding strength between the“skin”layer and the“bone”layer.The results show that the RMD248 has stable microhardness distribution while the microhardness of CN72 decreases with the distance from the interface.And the ultimate tensile strength between CN72 and CHN327 is higher than RMD248 in the temperature range of 400-450℃.It can be inferred that CN72 has higher inter-layer wear resistance and RMD248 has more stable high temperature performance.

Nodal superconducting gap in LiFeP revealed by NMR:Contrast with LiFeAs

Identifying the uniqueness of FeP-based superconductors may shed new lights on the mechanism of superconductivity in iron-pnictides.Here,we report nuclear magnetic resonance(NMR) studies on LiFeP and LiFeAs which have the same crystal structure but different pnictogen atoms.The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state and can provide the information on the superconducting pairing symmetry through the temperature dependence of London penetration depth λ_(L).We find that λ_(L) saturates below T~0.2 T_(C) in LiFeAs,where T_(C) is the superconducting transition temperature,indicating nodeless superconducting gaps.Furthermore,by using a two-gaps model,we simulate the temperature dependence of λ_(L) and obtain the superconducting gaps of LiFeAs,as Δ_(1)=1.2 kB T_(C) and Δ_(2)=2.8 k_(B)T_(C),in agreement with previous result from spin-lattice relaxation.For LiFeP,in contrast,λ_(L) does not show any saturation down to T~0.03 T_(C),indicating nodes in the superconducting gap function.Finally,we demonstrate that strong spin fluctuations with diffusive characteristics exist in LiFeP,as in some cuprate high temperature superconductors.

Hofstadter's Butterfly and Phase Transition of Checkerboard Superconducting Network in a Magnetic Field

我们学习棋盘的磁性的效果超导的电线网络。基于 de GennesAlexader 理论，我们在电线网络为超导的顺序参数获得差别方程。通过解决这些差别方程，我们获得特征值，连接了到连贯长度，，磁场的功能。特征值的图显示出分数维的结构，是所谓的霍夫施塔特的蝴蝶。我们也计算并且讨论棋盘的转变温度的依赖超导的电线在应用磁场上联网，它与霍夫施塔特的蝴蝶光谱的起来边有关。

Electrochemical synthesis of alkali-intercalated iron selenide superconductors

Electrochemical method has been used to insert K/Na into FeSe lattice to prepare alkali-intercalated iron selenides at room temperature. Magnetization measurement reveals that KxFe2Se2 and NaxFe2Se2 are superconductive at 31 K and 46 K, respectively. This is the first successful report of obtaining metal-intercalated FeSe-based high-temperature superconductors using electrochemical method. It provides an effective route to synthesize metal-intercalated layered compounds for new superconductor exploration.

Doping effects of transition metals on the superconductivity of (Li,Fe)OHFeSe films

The doping effects of transition metals(TMs = Mn, Co, Ni, and Cu) on the superconducting critical parameters are investigated in the films of iron selenide(Li,Fe)OHFe Se. The samples are grown via a matrix-assisted hydrothermal epitaxy method. Among the TMs, the elements of Mn and Co adjacent to Fe are observed to be incorporated into the crystal lattice more easily. It is suggested that the doped TMs mainly occupy the iron sites of the intercalated(Li,Fe)OH layers rather than those of the superconducting Fe Se layers. We find that the critical current density J_(c) can be enhanced much more strongly by the Mn dopant than the other TMs, while the critical temperature T_(c) is weakly affected by the TM doping.

Band Structure of New ReFeAsO Superconductors

We investigate the band structure of Fe-based superconductors using the first-principle method of density-functional theory. We calculated the band structure and the density of states at the Fermi level for ReFeAsO (Re = Sm, Er) superconductors. Our calculations indicate that the maximum critical superconducting transition temperature Tc will be observed for compounds with Sm and Er at 55 and 46 K, respectively.

STUDY OF EFFECT OF SURFACE-OPTICAL PHONON ON BIPOLARON IN QUANTUM-WELL WIRE

ＳＴＵＤＹＯＦＥＦＦＥＣＴＯＦＳＵＲＦＡＣＥ┐ＯＰＴＩＣＡＬＰＨＯＮＯＮＯＮＢＩＰＯＬＡＲＯＮＩＮＱＵＡＮＴＵＭ┐ＷＥＬＬＷＩＲＥＣ．ＭＬｅ（李泽民）ＧｕＳｈｉｗｅｉ＊（顾世洧）Ｃ．ＣＬａｍ（林长净）（Ｄｅｐｔ．ｏｆＰｈｙｓｉｃｓａｎｄＭａｔｅｒｉａ...

强电用超导材料的发展现状与展望

超导材料具有常规材料不具备的零电阻、完全抗磁性等宏观量子现象,是典型的量子材料。在强电应用领域,使用超导材料可以实现常规技术无法实现的超强磁场、大容量储能等诸多颠覆性技术,因此,强电用超导材料制备技术一直是国际高技术竞争前沿。本文通过梳理国内外强电用超导材料及其制备技术的发展现状,系统分析和阐明了包括低温超导材料NbTi、Nb_(3)Sn和高温超导材料YBCO涂层导体、Bi-2223带材、Bi-2212线材以及MgB_(2)线材等实用化超导材料在强电应用领域的发展趋势。分析我国强电用超导材料发展存在的问题,我国需要以开发出面向不同强电应用需求的高性能超导材料体系为基础,实现超导材料和强电应用产品的协同发展,推动强电用超导材料制备技术和应用技术的创新水平提升和产业化规模。研究建议,通过国家层面组织“产学研用”联合攻关,实现低温超导材料产业升级,突破高温超导材料批量化制备关键技术的发展思路,实现强电用超导材料的快速发展和应用。

Exploring FeSe-based superconductors by liquid ammonia method

（Received 8 June 2013） Our recent progress on the preparation of a series of new FeSe-based superconductors and the clarification of SC phases in potassium-intercalated iron selenides are reviewed here. By the liquid ammonia method, metals Li, Na, Ca, Sr, Ba, Eu, and Yb are intercalated in between FeSe layers and form superconductors with transition temperatures of 30 K^46 K, which cannot be obtained by high-temperature routes. In the potassium-intercalated iron selenides, we demonstrate that at least two SC phases exist, KxFe2Se2（NH3）y （x 0.3 and 0.6）, determined mainly by the concentration of potassium. NH3 has little, if any, effect on superconductivity, but plays an important role in stabilizing the structures. All these results provide a new starting point for studying the intrinsic properties of this family of superconductors, especially for their particular electronic structures.

Revealing the A1g-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 B1g-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 Ts)in the FeSe thin flake.Our results reveal a predominant A1g-type strain effect on T_(c).Meanwhile,Ts exhibits a monotonic anti-correlation with T_(c)and the maximum T_(c)reaches to 12 K when Ts 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 B1g-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.

Anomalous spectral weight transfer in the nematic state of iron-selenide superconductor

Nematic phase intertwines closely with high-Tc superconductivity in iron-based superconductors.Its mechanism,which is closely related to the pairing mechanism of superconductivity,still remains controversial.Comprehensive characterization of the electronic state reconstruction in the nematic phase is thus crucial.However,most experiments focus only on the reconstruction of band dispersions.Another important characteristic of electronic state,the spectral weight,has not been studied in details so far.Here,we studied the spectral weight transfer in the nematic phase of FeSe0.9S0.1 using angle-resolved photoemission spectroscopy and in-situ detwinning technique.There are two elliptical electron pockets overlapping with each other orthogonally at the Brillouin zone corner.We found that,upon cooling,one electron pocket loses spectral weight and fades away,while the other electron pocket gains spectral weight and becomes pronounced.Our results show that the symmetry breaking of the electronic state is manifested by not only the anisotropic band dispersion but also the band-selective modulation of the spectral weight.Our observation completes our understanding of the nematic electronic state,and put strong constraints on the theoretical models.It further provides crucial clues to understand the gap anisotropy and orbital-selective pairing in iron-selenide superconductors.

Magnetoresistivity and filamentary superconductivity in nickel-doped BaFe_2As_2

We present magnetotransport studies on a series of BaFe2_xNixAs2 （0.03 〈 x 〈 0.10） single crystals. In the un- derdoped （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 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 trig- gers 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 corelated heavy-fermion （HF） metal with Kondo scale TK 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 CeFeAsOl_yFy （y = 0 and 0.05）. This P/As-doping in CeFeAsO serves as an effective controlling parameter which leads to two magnetic critical points, Xcl -- 0.4 and Xc2 - 0.92, associated with suppression of 3d and 4f magnetism, respectively. We also observe a turning point of AFM-FM ordering of Ce3＋ moment at Xc3 - 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 Xcl. We continue to investigate CeFeAsl-xPxO0.95Fo.os. With the separation of xcl and xc3, 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.

Comparison of band structure and superconductivity in FeSe_(0.5)Te_(0.5) and FeS

The isovalent iron chalcogenides, FeSe0.5Te0.5 and FeS, share similar lattice structures but behave very differently in superconducting properties. We study the underlying mechanism theoretically. By first principle calculations and tight-binding fitting, we find the spectral weight of the dX2-Y2 orbital changes remarkably in these compounds. While there are both electron and hole pockets in FeSe0.5Te0.5 and FeS, a small hole pocket with a mainly dX2-Y2 character is absent in FeS. We find the spectral weights of dX2-Y2 orbital change remarkably, which contribute to electron and hole pockets in FeSe0.5Te0.5 but only to electron pockets in FeS. We then perform random-phase-approximation and unbiased singular-mode functional renormalization group calculations to investigate possible superconducting instabilities that may be triggered by electron-electron interactions on top of such bare band structures. For FeSe0.5Te0.5, we find a fully gapped s-wave pairing that can be associated with spin fluctuations connecting electron and hole pockets. For FeS, however, a nodal dxy （or dx2-y2 in an unfolded Broullin zone） is favorable and can be related to spin fluctuations connecting the electron pockets around the corner of the Brillouin zone. Apart from the difference in chacogenide elements, we propose the main source of the difference is from the dX2-Y2 orbital, which tunes the Fermi surface nesting vector and then influences the dominant pairing symmetry.

The Superconductivity in Fe-Based Family of Superconductors and Its Electronic Structure Analysis in Presence of Dopants Rh and Pd

The discovered in 2008 Fe-based superconductors (SC) are a paramagnetic semimetal at ambient temperature and in some cases they become superconductor upon doping. In spite of so many years since its discovery it is still not known the mechanism that leads to superconductivity. The electronic structure study is used for determining key features of the SC mechanism in these materials. The calculations were performed using the modern suite of programs MOLPRO 2021. We performed quantum calculations of a cluster embedded in a background charge distribution that represents the infinite crystal. The Natural Population Analysis was used for determining the charge and spin distribution in the studied materials. As follows from our results, the possible mechanism for superconductivity corresponds to the RVB theory proposed by Anderson for high Tc superconductivity in cuprates.

标准化助力国内低温超导线材产业化

2008年6月,西部超导建成了国内首条低温超导线材产业化生产线。经过两年多的工程化试制,于2010年底开始向国际热核聚变实验堆(ITER)项目批量供货,拉开了国内低温超导线材产业化的序幕。近几年来随着我国低温超导线材工艺的不断优化改进,性能和质量水平的持续提高,国内国际市场逐步拓宽,使得低温超导线材产业化得以迅猛发展,特别在2022年,低温超导线材突破了1000吨产量大关,跻身于国际前列。在国内整个低温超导线材产业化的发展过程中,标准化起到了至关重要的作用。本文以《实用超导线的扭距测量Nb-Ti与Nb_(3)Sn复合超导体的扭距测量方法》标准为例,介绍其制定工作和应用情况,以此来阐明标准化对低温超导线材产业发展的助推作用。