122 type pnictide superconductors are of particular interest for high-field applications because of their large upper critical fields H c2 (> 100 T) and low anisotropy γ (<2). Successful magnet applications require fabrication of polycrystalline superconducting wires that exhibit large critical current density J c , which is limited by poor grain coupling and weak-link behavior at grain boundaries.Here we report our recent achievement in the developing Sr 0.6 K 0.4 Fe 2 As 2 tapes with transport J c up to 0.1 MA/cm 2 at 10 T and 4.2 K. This value is by far the highest ever recorded for iron based superconducting wires and has surpassed the threshold for practical application. The synergy effects of enhanced grain connectivity, alleviation of the weak-link behavior at grain boundaries, and the strong intrinsic pinning characteristics led to the superior J c performance exhibited in our samples. This advanced J c result opens up the possibility for iron-pnictide superconducting wires to win the race in high-field magnet applications.
A 25 T cryogen-free superconducting magnet (25T-CSM) was developed and installed at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University. The 25T-CSM consists of a high-temperature superconducting (HTS) coil and a low-temperature superconducting (LTS) coil. A high-strength CuNb/Nb3Sn Rutherford cable with a reinforcing stabilizer CuNb composite is adopted for the middle LTS section coil. All the coils were impregnated using an epoxy resin for conduction cooling. Initially, a GdBa2Cu3Oy (Gd123) coil was designed as the HTS insert coil, and then a Bi2Sr2Ca2Cu3Oy (Bi2223) coil was also developed. The HTS insert and the LTS (CuNb/Nb3Sn and NbTi) outsert coils are cooled by two 4K GM and two GM/JT cryocoolers, respectively. The LTS coils successfully generated a central magnetic field of 14 T at an operating current of 854 A without any training quench. The Gd123 coil generated 10.15 T at an operating current of 132.6 A in the absence of a background field. Subsequently, the operating current of the Gd123 insert was increased in a step-by-step manner under a background field of 14 T. The Gd123 coil could be operated up to 124.0 A stably, which corresponds to 23.55 T, but quenched at around 124.6 A (23.61 T). The Bi2223 insert coil using a Ni-alloy reinforced Bi2223 tape successfully generated 11.48 T at an operation current of 204.7A in a stand-alone test and 24.57 T in a background field of 14 T. The differences between the calculated and the measured values of the central magnetic fields are about 0.4 T for the Gd123 insert and 0.1 T for the Bi2223 insert around 24 T.
An SDW antiferromagnetic (SDW-AF) low temperature phase transition is generally observed and the AF spin fluctuations are considered to play an important role for the superconductivity paring mechanism in FeAs superconductors. However, a similar magnetic phase transition is not observed in FeSe superconductors, which has caused considerable discussion. We report on the intrinsic electronic states of FeSe as elucidated by electric transport measurements under magnetic fields using a high quality single crystal. A mobility spectrum analysis, an ab initio method that does not make assumptions on the transport parameters in a multicarrier system, provides very important and clear evidence that another hidden order, most likely the symmetry broken from the tetragonal C4 symmetry to the C2 symmetry nematicity associated with the selective dorbital splitting, exists in the case of superconducting FeSe other than the AF magnetic order spin fluctuations. The intrinsic low temperature phase in FeSe is in the almost compensated semimetallic states but is additionally accompanied by Dirac cone like ultrafast electrons ∼ 10 4 cm 2 (VS) −1 as minority carriers.
Abstract-Iron-pnictides are hotly studied since 2008 in the superconducting materials research area, due to their special properties and unclear mechanism. Big achievement has been made in the pnictide research during the past years. For practical uses, pnictide superconductor should be fabricated in a long wire form which can be used for different devices. In this work, 100 m class 7-core Sr0.6K0.4Fe2As2 (Sr122) tapes have been made using the powder-in-tube technique, which is reported for the first time. Clearly, an average Jc of 1.3×10 4 A/cm 2 at 10 T was reached over the 115 meter length, showing a high property and good uniformity of 100 meter level Sr122 tapes. Using the 10 m long Sr122 tapes, two double-pancake coils were fabricated by a wind and reaction technique. No transport current could be measured for the coil made from the mono-filamentary tape. However, transport Ic was obtained in the coil made from the 7-filamentary tape. The factors which affect the superconducting property of the coil were discussed in this work. Index Terms-Coil, critical current density, pnictide, long tape.
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