高温陶瓷超导体市场展望

一、应用概况 1986年镧钡铜氧及其后钇钡铜氧的相继问世,将高温超导推上了世界舞台。与常规超导体一样,这类新材料传输电流没有电阻,两者的区别在于低温超导合金必须冷却到低于-250C,而陶瓷超导体仅需冷却到-140C。

Liquid nitrogen level meter for high-temperature superconductor (HTS)

Many kinds of high temperature superconductor （HTS） power machines such as HTS cable, HTS fault current limitcr and HTS magnet are cooled by liquid nitrogen. The level of liquid nitrogen should be monitored and controlled to ensure the thermal stability and the dielectric strength as well. To measure the level, capacitance method and differential pressure method are usually used. However, each method has installation difficulties and measurement errors for unsteady state operation with varying system pressure. A new liquid level meter using a 2G HTS conductor is described, which has similar structure with the liquid helium level meter with NbTi filament. The level meter is fabricated with a parallel connected heater, which helps the separation of the superconducting region and normal region, considering the critical temperature, large heat capacity of conductor and cooling characteristics. The level of liquid nitrogen can be obtained from the measured voltage signal along the 2G HTS conductor. Design, fabrication and test results of the new liquid nitrogen level meter are presented.

The c—Axis Infrared Conductivity of a dx^2—y^2— Wave Superconductor with the Hopping Assisted by the Spin Fluctuations

A theory of the c-axis infrared conductivity of a dx2-y2-wave superconductor due to the competition between the interlayer direct hopping and the hopping assisted by the spin fluctuations has been developed. The prediction of our theory captures the main feature of the experiment. Thus we argue that the anomalous behavior of the c-axis infrared conductivity of the underdoped cuprates in superconducting state may be properly understood within the theory.

MOSSBAUER SPECTROSCOPY OF HIGH T_c YBa_2 (Cu_(3-x)Fe_x )O_y

YBa2(Cu3-xFex)Oy (x is 0.001, 0.005, 0.01, 0.1 and 0.3 respectively), of whichCu is replaced with 57Fe are studied using M0ssbauer spectroscopy to understand thecrystal lattice property, the effects of the replacement on superconductivity of theHigh To materials.

Potts Model on Maple Leaf Lattice with Pure Three-Site Interaction

We use Monte Carlo method to study three-state Potts model on maple leaf lattice with pure three-site interaction. The critical behavior of both ferromagnetic and antiferromagnetic cases is studied. Our results confirm that the critical behavior of the ferromagnetic model is independent of the lattice details and lies in the universality class of the three-state ferromagnetic Potts model. For the antiferromagnetic case the transition is of the first order. We have calculated the energy jump and critical temperature in this area. We find there is a tricritical point separating the first order and second order phases for this system.

NiFe alloy particles doping effect of Gd-Ba-Cu-O bulks processed by a new cold-seeding technology

The process of cold seeding melt growth of GdBa2Cu3Oy （Gd123） bulk superconductors using NdBa2Cu3Oy （Nd123） thin films was reported. In addition, a novel cold seeding concept of combining MgO crystal and buffer pellet was also introduced. The misorientation caused by the lattice mismatch between MgO and Gd123 melt was overcome by choosing suitable heat treatment program and Gd2BaCuO5 （Gd211） content of the buffer pellet. The doping effect of soft ferromagnetic NiFe alloy particles was also reported. The bulk sample with 0.4% （mole fraction） doping amount shows the best performance on the flux trapping. The critical current density is largely enhanced under the external field of 1-2 T, which is promising for large-scale applications. This effect is originated from the substitution of Fe and Ni ions for the Cu sites contributing to magnetic flux pinning.

Experimental Investigation and Modelling for the Optimisation of Conduction Cooled HTS Hybrid Current Leads for SMES

It′s important that HTS tapes have lower thermal conductivity and higher transversal resistivity in order to reduce the heat leaks conducted along the tapes and AC losses in the high temperature superconducting system conduction cooled by GM coolers. This paper presents an experimental investigation into the effects of pure Ag and AgAu alloys sheath materials on the properties of Bi(2223) multifilamentary tapes and the optimisation of conduction cooled hybrid current leads made from copper and Bi(2223)/Ag or Bi(2223)/AgAu tapes. The thermal conductivity of the tapes were measured by cryogenic steady heat flux method and the resistance was measured by using standard DC four probe method at low temperature. The results showed that the reduction of thermal conductivity by the addition of Au into the sheath material of Bi(2223) tapes was 65 0 0, 75 0 0 and 85 0 0 lower than that of pure Ag sheathed Bi(2223) tapes and the increase of resistivity was 4.9 , 10 and 19.4 times higher than that of pure Ag for the addition of 2.2 0 0, 5.7 0 0 and 10.7 0 0Au(atom ratio) respectively. And the study also attempts to optimise thermodynamically the conduction cooled hybrid current lead by using a developed model, which took the irreversibility of commercial GM coolers, the contact resistance and thermal conductance into account. Predictions from the model showed that AgAu alloys were suitable candidate materials to replace Ag as sheath material of Bi(2223) tapes applied in HTS current leads. In addition, Bi(2223)/AgAu was a suitable material to be applied as the HTS section of hybrid current leads in conduction cooled superconducting electric systems.

The Critical Properties of One—Dimensional Extended Hubbard Model

In the framework of nonperturbative quantum field theory, the critical phenomena of one-dimensionalextended Hubbard model (EHM) at half-filling are discussed from weak to intermediate interactions. After the EHMbeing mapped into two decoupled sine-Gordon models, the ground state phase diagram of the system is derived in anexplicit way. It is confirmed that the coexisting phases appear in different interaction regimes which cannot be foundby conventional theoretical methods. The diagram shows that there are seven different phase regions in the groundstate, which seems not to be the same as previous discussions, especially the boundary between the phase separationand condensed phase regions. The phase transition properties of the model between various phase regions are studied indetail.

Essence of Pseudogap and Oscillation in High-Temperature Cuprate Superconductor Junction of Pseudogap State

This paper gives methods to calculate the pairing temperature T＊, at which a pseudogap is opened, and the superconducting temperature Tc, at which superconductivity appears, in the high-Te cuprates, and demonstrates directly that at Tc 〈 T 〈 T＊ the pseudogap is the gap of Cooper pair without long-range phase coherence, and at T 〈 Tc there is long-range phase coherence between Cooper pairs. Based on the above clear physical picture on the pseudogap state and our mechanism for the ac Josephson effect, this paper proposes that there should he a novel oscillatory current in P-I-P junction, induced by a constant bias on the junction. Here, P represents the high-Tc curates in the pseudogap state, where Cooper pairs do not have long-range phase coherence, and I represents the thin insulating barrier. This paper conjectures that there is a possible high-temperature superconductivity in the heavily underdoped high-Tc cuprates.

A Ship Deperming Coil System Using High Temperature Superconducting Cable Technology Refrigeration--

The purpose of ship deperming is to reduce its permanent magnetism to avoid the threat of magnetic sea-mines which appeared first in battlefields in World War II and stored by many navies and unanimous groups since then. Magnetic fields for deperming are mostly generated by electric current through conductor cable and the intensity of such fields decreases with the distance from the cable. In order to impose sufficient field to the ship, the deperming cable is tightly wrapped around the hull of the ship. A cable with superconducting material as the conductor is expected to pass high electric current because of its zero-electric resistivity and has potential to make deperming coil system more separated from the ship hull. In the previous study, we designed superconducting coil system set flat on the seabed for ship deperming by calculating magnetic field generated by the coil based on the characteristic of HTS （high temperature superconducting） tape seen in published papers. This time we have kept our design to utilize HTS tape conductors that are existing and readily available in the open market. In addition, the limitations of the manufacturing potential and capacity of the HTS tape conductor industry have been taken into account for the design. Then we designed the refrigerating system which is to keep the superconducting property of the cable materials.

Modelling and Analysis of Distribution Network with HTS Transformer

Transformers utilizing HTS （high temperature superconductors） are considered as a timely invention. The number of power transformers age more than 30 years old and nearing retirement is increasing. If this window of opportunity is not grabbed, there would be great reluctance to replace recently installed highly priced capital asset. Major projects of developing HTS transformers are well making progress in the United States, Europe, Japan, Korea and China which indicate the interest. The efforts must have been appropriately verified through the economic interest of the discounted losses. Consequently, it is very important to develop an understanding of the fundamental HTS transformer design issues that can provide guidance for developing practical devices of interest to the electric utility industry. The parameters of HTS transformer need to be validated before any effort is to carry out to model the behaviour of a distribution network under a range of conditions. The predicted performance and reliability of HTS transformers can then be verified through the network modelling and analysis calculation. The ultimate purpose is to furnish electric utilities precise information as to which HTS transformers work under various applications with greater technical efficiency and proven reliability.

Numerical analysis of magnetization AC losses in high-temperature superconducting slabs subjected to uniform strains

High-temperature superconductors in superconductor apparatuses are subjected to mechanical strains under operating conditions.These strains cause the degradation of the critical current densities and influence AC losses in the superconductors.Based on the dynamic process of thermomagnetic interaction,we report the results of numerical analysis of AC losses in an infinite high-temperature superconducting slab subjected to a uniform in-plane strain in an alternating external magnetic field parallel to the sample surface.The numerical analysis shows the details of electromagnetic phenomena in the slab and the dependences of AC loss on various external parameters including the uniform strain in the slab and the amplitude and frequency of the external magnetic field.In this paper,we find that whether the magnetic field fully penetrates the superconductor is the key factor that influences the features of AC loss.When the magnetic field cannot fully penetrate the superconductor,the loss rises with increasing strain or decreasing frequency.When the magnetic field can fully penetrate the superconductor,the feature is just opposite.We also analyze the effects of periodic strain on AC loss.It is interesting to find that when the periodic strain frequency equals the external magnetic field frequency,the AC loss reaches the maximum,regardless if the magnetic field fully penetrates the superconductor or not.

Identifying the genes of unconventional high temperature superconductors

We elucidate a recently emergent framework in unifying the two families of high temperature （high To） superconductors, cuprates and iron-based superconductors. The unification suggests that the latter is simply the counterpart of the former to realize robust extended s-wave pairing symmetries in a square lattice. The unification identifies that the key ingredients （gene） of high Tc superconductors is a quasi two dimensional electronic environment in which the d-orbitals of cations that partic- ipate in strong in-plane couplings to the p-orbitals of anions are isolated near Fermi energy. With this gene, the superexchange magnetic interactions mediated by anions could maximize their contributions to superconductivity. Creating the gene requires special arrangements between local electronic structures and crystal lattice structures. The speciality explains why high Tc superconductors are so rare. An explicit prediction is made to realize high Tc superconductivity in Co/Ni-based materials with a quasi two dimensional hexagonal lattice structure formed by trigonal bipyramidal complexes.

FeTe1-xSex monolayer films： towards the realization of high-temperature connate topological superconductivity

We performed angle-resolved photoemission spectroscopy studies on a series of FeTe_(1-x)Se_x monolayer films grown on Sr TiO_3.The superconductivity of the films is robust and rather insensitive to the variations of the band position and effective mass caused by the substitution of Se by Te.However,the band gap between the electron-and hole-like bands at the Brillouin zone center decreases towards band inversion and parity exchange,which drive the system to a nontrivial topological state predicted by theoretical calculations.Our results provide a clear experimental indication that the FeTe_(1-x)Se_x monolayer materials are high-temperature connate topological superconductors in which band topology and superconductivity are integrated intrinsically.

Relationship between the parent charge transfer gap and maximum transition temperature in cuprates

One of the biggest puzzles concerning the cup- rate high temperature superconductors is what determines the maximum transition temperature （Tc,max）, which varies from less than 30 to above 130 K in different compounds. Despite this dramatic variation, a robust trend is that within each family, the double-layer compound always has higher Tc,max than the single-layer counterpart. Here we use scanning tunneling microscopy to investigate the electronic structure of four cuprate parent compounds belonging to two different families. We find that within each family, the double layer compound has a much smaller charge transfer gap size （ACT）, indicating a clear anticorrelation between AcT and Tc,max. These results suggest that the charge transfer gap plays a key role in the superconducting physics of cuprates, which shed important new light on the high To mechanism from doped Mott insulator perspective.

Phase diagrams on composition-spread Fe_(y)Te_(1-x)Se_(x) films

Fe_(y)Te_(1-x)Se_(x),an archetypical iron-based high-temperature superconductor with a simple structure but rich physical properties,has attracted lots of attention because the two end compositions,Se content x=0 and 1,exhibit antiferromagnetism and nematicity,respectively,making it an ideal candidate for studying their interactions with superconductivity.However,what is clearly lacking to date is a complete phase diagram of Fe_(y)Te_(1-x)Se_(x)as functions of its chemical compositions since phase separation usually occurs from x~0.6 to 0.9 in bulk crystals.Moreover,fine control of its composition is experimentally challenging because both Te and Se are volatile elements.Here we establish a complete phase diagram of Fe_(y)Te_(1-x)Se_(x),achieved by high-throughput film synthesis and characterization techniques.An advanced combinatorial synthesis process enables us to fabricate an epitaxial composition-spread Fe_(y)Te_(1-x)Se_(x)film encompassing the entire Se content x from 0 to 1 on a single piece of CaFsubstrate.The micro-region composition analysis and X-ray diffraction show a successful continuous tuning of chemical compositions and lattice parameters,respectively.The micro-scale pattern technique allows the mapping of electrical transport properties as a function of relative Se content with an unprecedented resolution of0.0074.Combining with the spin patterns in literature,we build a detailed phase diagram that can unify the electronic and magnetic properties of Fe_(y)Te_(1-x)Se_(x).Our composition-spread Fe_(y)Te_(1-x)Se_(x) films,overcoming the challenges of phase separation and precise control of chemical compositions,provide an ideal platform for studying the relationship between superconductivity and magnetism.

Observation of a bi-critical point between antiferromagnetic and superconducting phases in pressurized single crystal Ca0.73La0.27FeAs2

One of the most strikingly universal features of the high-temperature superconductors is that the super- conducting phase emerges in the close proximity of the antiferromagnetic phase, and the interplay between these two phases poses a long-standing challenge. It is commonly believed that, as the antifer- romagnetic transition temperature is continuously suppressed to zero, there appears a quantum critical point, around which the existence of antiferromagnetic fluctuation is responsible for the development of the superconductivity. In contrast to this scenario, we report the observation of a bi-critical point identified at 2,88 GPa and 26.02 K in the pressurized high-quality single crystal Cao.73Lao.27FeAs2 by com- plementary in-situ high pressure measurements. At the critical pressure, we find that the antiferromag- netism suddenly disappears and superconductivity simultaneously emerges at almost the same temperature, and that the external magnetic field suppresses the superconducting transition temperature but hardly affects the antiferromagnetic transition temperature.

A possible new family of unconventional high temperature superconductors

We suggest a new family of Co[Ni-based materials that may host unconventional high temperature superconductivity （high-To）. These materials carry layered square lattices with each layer being formed by vertex-shared transition metal tetrahedra cation-anion complexes. The electronic physics in these materials is determined by the two dimensional layer and is fully attributed to the three near degenerated t2g d-orbitals close to a d7 filling configuration in the d-shell of CoJNi atoms. The electronic structure meets the necessary criteria for unconventional high Tc materials proposed recently by us to unify the two known high-Tc families, cuprates and iron-based superconductors. We predict that they host superconducting states with a d-wave pairing symmetry with Tc potentially higher than those of iron-based superconductors. These materials, if realized, can be a fertile new ground to study strongly correlated electronic Physics and provide decisive evidence for superconducting pairing mechanism.

In situ carrier tuning in high temperature superconductor Bi2Sr2CaCu2O(8+δ)by potassium deposition

We report a successful tuning of the hole doping level over a wide range in high temperature superconductor Bi2Sr2CaCu2O8＋δ （Bi2212） through successive in situ potassium （K） deposition. By taking high resolution angleresolved photoemission measurements on the Fermi surface and band structure of an overdoped Bi2212 （To = 76 K） at different stages of K deposition, we found that the area of the hole-like Fermi surface around the Brillouin zone corner （n,n） shrinks with increasing K deposition. This indicates a continuous hole concentration change from initial - 0.26 to eventual 0.09 after extensive K deposition, a net doping level change of 0.17 that makes it possible to bring Bi2212 from being originally overdoped, to optimally-doped, and even- tually becoming heavily underdoped. The electronic behaviors with K deposition are consistent with those of Bi2212 samples with different hole doping levels. These results demonstrate that K deposition is an effective way of in situ controlling the hole concentration in Bi2212. This work opens a good way of studying the doping evolution of electronic structure and establishing the electronic phase diagram in Bi2212 that can be extended to other cuprate superconductors.

A possible family of Ni-based high temperature superconductors

We suggest that a family of Ni-based compounds, which contain [Ni_2M_2O]~2à(M = chalcogen) layers with an antiperovskite structure constructed by mixed-anion Ni complexes, Ni M_4O_2, can be potential high temperature superconductors(high-Tc) upon doping or applying pressure. The layer structures have been formed in many other transitional metal compounds such as La_2B_2Se_2O_3(B = Mn, Fe, Co). For the Ni-based compounds, we predict that the parental compounds host collinear antiferromagnetic states similar to those in iron-based high temperature superconductors. The electronic physics near Fermi energy is controlled by two egd-orbitals with completely independent in-plane kinematics. We predict that the superconductivity in this family is characterized by strong competition between extended s-wave and d-wave pairing symmetries.