Thermoelectric effects in superconductors: The role of fluctuations and weak links

A possible explanation of the well-known discrepancy between the observed and calculated enhancement of a thermal flux induced inside a superconducting loop is presented. The key point, as supposed, is the strong fluctuations nearT _c . An excess thermal flux due to weak links is discussed as well.The author is indebted to Prof. V. L. Ginzburg, whose recently published paper [4] stimulated the present communication, and to Profs. V. L. Aksenov and B. V. Vasiliev for their interest in the work and usefui discussions.     

BiSrCaCuO系超导材料微观结构研究

BiSrCaCuO系超导材料微观结构的研究结果表明,试样中存在两种超导相,即T_c为110K的Bi_2Sr_2Ca_2Cu_3O_3相和T_c为80K的Bi_2Sr_2Cu_2O_y相。超导转变温度的高低取决于两相含量比及其分布状态。掺杂少量Pb有利于高T_c相生成和稳定。烧结工艺影响试样的微观结构,从而影响到超导电性。     

First-principles calculations on superconductivity and H-diffusion kinetics in Mg–B–H phases under pressures

Focusing towards ternary metal hydrides has recently been regarded as a new avenue for research in pressure-dependent high-temperature superconductors, thanks highly to a fairly large number of permutations of alloying metals, even metalloids, with hydrogen. Herein, new phases of Mg ? B ? H ternary hydrides are predicted from the first-principles evolutionary techniques, as a result of which the corresponding phonon and electronic calculations for the three candidate phases are performed successively to confirm their dynamic stability and the possibility to become conductors. The metallic MgBH₉ undergoes a superconducting phase with a maximum T_c of 64 K at 110 GPa, with its spectral function predominantly active around optical modes. The significant increase in cumulative electron-phonon coupling constant is associated with a relatively low cutoff frequency according to the bandwidth function. As for the non-metallic candidate, hydrogen-vacancy diffusion kinetics of the MgB₂H₈ phase are determined by means of total energy calculations. Stable pathways at varying pressure are reported, suggesting that elevated pressure lowers the activation energy which is presumably due to an optimal level of average nearest H ? H(B) inter-fragment distances.     

Superconducting electrical motorsMoteurs électriques supraconducteurs

Superconductors have always been very promising materials because they offer the possibility of building lighter and more efficient electrical machines than by any other conventional solution. These characteristics make superconducting motors very attractive drives for embarked equipment with low weight requirements. The historical development of superconducting machines is quickly reviewed. The high critical temperature superconductors have renewed the works about superconducting machines. Different motor options are presented (fully superconducting machine and permanent magnet superconducting motor). For each, the choice of the superconductor (low or high critical temperature) is discussed, according to their performance and technological problems, especially about cryogenics. The different solutions are compared through a preliminary electromagnetic design for a 10 MW–100 rpm torque motor. We carry out a research and development program on the permanent magnet superconducting motors. After a first 15 kW–750 rpm successfully tested model a second 150 kW–400 rpm demonstrator is under development.     

Single-Step Synthesis of Sr4V2O6Fe2As2: The Blocking Layer Based Potential Future Superconductor

We report synthesis, structural details and transport measurements on Sr₄V₂O₆Fe₂As₂. Namely, the stoichiometric amounts of V₂O₅+1/2×SrO₂+7/2×Sr+2×FeAs are weighed mixed, ground thoroughly and palletized in rectangular form in a glove box in high purity Ar atmosphere. The pellet is further sealed in an evacuated (10^?5 torr) quartz tube and put for heat treatments at 750 and 1150°C in a single step for 12 and 36 hours respectively. Finally the quartz ampoule is allowed to cool naturally to room temperature. The as-synthesized sample is black in color. The compound crystallized in P4/nmm space group with lattice parameters a=b=3.925 Å and c=15.870 Å. Also seen are some small impurity lines. The compound did not exhibit superconductivity but instead a spin density wave (SDW) like metallic step at around 175 K is seen in R(T) measurements. Principally in [FeAs]^?1{Sr₄V₂O₆}^C[FeAs]^?1 the net value of blocking layer charge C must be either less or more than 2, to let it be electron or hole type superconductor respectively. Efforts are under way to achieve superconductivity in the studied system.     

Superconductivity in polycrystalline diamond thin films

Superconductivity was discovered in heavily boron-doped diamond thin films deposited by the microwave plasma assisted chemical vapor deposition (MPCVD) method. Advantages of the MPCVD deposited diamond are the controllability of boron concentration in a wide range, and a high boron concentration, especially in (111) oriented films, compared to that of the high-pressure high-temperature method. The superconducting transition temperatures are determined to be 8.7 K for T_c onset and 5.0 K for zero resistance by transport measurements. And the upper critical field is estimated to be around 7 T.     

Effect of polysilane addition on spark plasma sintering and superconducting properties of MgB2 bulks

Polycrystalline MgB₂ bulk samples were produced by ex-situ spark plasma sintering (SPS) using the oxygen-free preceramic polymer additive poly(dimethylsilane) as a source for carbon doping and as a sintering aid. Two major effects were identified. One is the significant enhancement of the densification kinetics during sintering for all tested compositions. The second one is the improvement of the high field critical current density for a certain level of the poly(dimethylsilane) addition.     

Improvement of flux pinning in GdBa2Cu3O7−δ thin film by nanoscale ferromagnetic La0.67Sr0.33MnO3 pretreatment of substrate surface

The present paper presents the effects of ferromagnetic La_0.67Sr_0.33MnO₃ (LSMO) nanoparticles on the pinning characteristics of an epitaxial GdBa₂Cu₃O_7?δ ((Gd) BCO) film deposited on top. LSMO nanoparticles with the size between 10 and 20 nm were obtained on a (001) STO substrate by RF sputtering method. The analyses of magnetic measurements revealed that the presence of a complex vortex pinning mechanism within the (Gd) BCO film deposited on the undecorated substrate. With respect to a reference (Gd) BCO film, two additional pinning effects in LSMO decorated (Gd) BCO film were observed. One is the effect of the threading dislocations due to LSMO nanoparticles, and the other one is the LSMO nanoparticles itself. Like other nanosized particles, the LSMO nanoparticles on substrate will induce threading dislocations passing through the (Gd) BCO film. These threading dislocations serve as correlated pinning centers along the c-axis, which lead to the improvement of flux pinning properties in LSMO decorated (Gd) BCO film. According to the ferromagnetic property of LSMO material, a magnetic pinning mechanism was proposed to explain the significant increase of Jc and Fp values. The flux pinning abilities of LSMO nanoparticles were evaluated. The enhanced pinning characteristics in the high field region and high temperature together with the movement of H _Fpmax (the corresponding field of F_pmax) also suggest a magnetic pinning mechanism for LSMO nanoparticles existed in LSMO decorated (Gd) BCO film.     

Transition of Cs3C60 crystals with Rb impurity to the superconducting state

The statistical theory of phase transition in fullerides of alkali metals to the superconducting state has been elaborated. The calculation of the free energy of crystals has been performed on the basis of molecular-kinetics and the equilibrium equations have been examined. The critical temperature of superconductivity has been estimated, and its concentration dependence has been ascertained. The calculation results have been compared with experimental data. The numerical estimation for energetic parameters of pairing of Cooper pairs electrons has been performed.