Kazumasa Togano

Superconducting properties of MgB2 bulk materials prepared by high-pressure sintering

High-density bulk materials of a recently discovered 40 K intermetallic MgB2 superconductor were prepared by high-pressure sintering. Superconducting transition with the onset temperature of 39 K was confirmed by both magnetic and resistive measurements. Magnetization versus field (M–H) curve shows the behavior of a typical type-II superconductor and the lower critical field Hc1(0) estimated from the M–H curve is 0.032 T. The bulk sample shows good connection between grains and critical current density Jc estimated from the magnetization hysteresis using sample size was 5×104 A/cm2 at 20 K and 1 T. Upper critical field Hc2(0) determined by extrapolating the onset of resistive transition and assuming a dirty limit is 18 T.

High transport critical current density obtained for powder-in-tube-processed MgB2 tapes and wires using stainless steel and Cu–Ni tubes

MgB2 tapes and wires were fabricated by the powder-in-tube method. Stainless steel and Cu–Ni tubes were used as sheath materials, and no heat treatment was applied. The tapes made of stainless steel showed transport critical current density Jc of about 10 000 A/cm2 at 4.2 K and 5 T. A high Jc of about 300 000 A/cm2 was obtained by extrapolating the Jc–B curves to zero field. Multifilamentary (seven-core) MgB2 wire was successfully fabricated using Cu–Ni tubes. For both tapes and wires, the grain connectivity of MgB2 was as good as a high-pressure sintered bulk sample. However, the Jc of the Cu–Ni sheathed wire was lower than the stainless steel sheathed tape due to the lower packing density of MgB2.

Partial melt growth process of Bi2Sr2Ca1Cu2Ox textured tapes on silver

Bi2Sr2Ca1Cu2Ox textured tapes with an excellent Jc were synthesized by doctor-blade casting and a melt process. Green tape was laid on a silver foil and oxide/silver composite tape was heat treated. A highly textured microstructure was formed by slow cooling from 890°C to 870°C with the oxide in a partially melted state. The highest Tc and Jc (77 K, 0 T), 89 K and 1.3×104 A/cm2, respectively, were obtained by quenching from 870°C. However, Jc at 4.2 K was higher for the sample slowly cooled to room temperature and reached 1.4×105 A/cm2 at 25 T. These results indicate the potential for the practical application of Bi-based oxide superconductors at both 77 K and 4.2 K.

S-wave spin-triplet order in superconductors without inversion symmetry: Li2Pd3B and Li2Pt3B.

We investigate the order parameter of noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B via the behavior of the penetration depth lambda(T). The low-temperature penetration depth shows BCS-like behavior in Li2Pd3B, while in Li2Pt3B it follows a linear temperature dependence. We propose that broken inversion symmetry and the accompanying antisymmetric spin-orbit coupling, which admix spin-singlet and spin-triplet pairing, are responsible for this behavior. The triplet contribution is weak in Li2Pd3B, leading to a wholly open but anisotropic gap. The significantly larger spin-orbit coupling in Li2Pt3B allows the spin-triplet component to be larger in Li2Pt3B, producing line nodes in the energy gap as evidenced by the linear temperature dependence of lambda(T). The experimental data are in quantitative agreement with theory.

Superconductivity in a New Pseudo-Binary Li2B(Pd1-xPtx)3 (x=0–1) Boride System

Recently we have found superconductivity in a cubic antiperovskite-like compound Li 2 BPd 3 . A new pseudo-binary complete solid solution Li 2 B(Pd 1- x Pt x ) 3 , x =0–1 with similar structure has been synthesized and observation of superconductivity in the entire x -range is reported. Our results strongly suggest that superconductivity is of bulk type. Critical temperature T c is decreasing approximately linearly with amount ( x ) of Pt from 7.2–8 K for Li 2 BPd 3 to 2.2–2.8 K for Li 2 BPt 3 . From isothermal magnetization ( M – H ) measurements, lower critical fields H c1 (138 Oe/ x =0 , 38 Oe/ x =1 ), upper critical fields H c2 WHH (3.4 T/ x =0 , 1 T/ x =1 ), coherence length ξ(0) (9.8 nm/ x =0 , 17.9 nm/ x =1 ) and penetration depth λ(0) (190 nm/ x =0 , 364 nm x =1 ) were estimated and shown to follow approximately linear dependencies with x , either. Structure and superconducting similarities with MgCNi 3 , viewed as a bridge between low and high T c superconductors are increasing the expectations t...

Improvement in critical current density of Bi2Sr2Ca1Cu2Ox tapes synthesized by doctor‐blade casting and melt growth

Bi2Sr2Ca1Cu2Ox superconducting tape with an excellent Jc at 4.2 K has been synthesized by a modified doctor‐blade process. A green tape prepared by doctor‐blade casting is laid on a silver foil and heat treated at various temperatures below and above its partial melting temperature. The Jc of the tape increases dramatically when heat treated in a narrow temperature range just above the melting temperature. A highly oriented layer structure with c‐axis alignment is obtained by the growth of aligned Bi2Sr2Ca1Cu2Ox grains in the partially melted state. This oriented microstructure produces an exceptional increase in Jc . Typical Jc ’s of the tape at 4.2 K are 50 000 A/cm2 at 0 T, 22 000 A/cm2 at 10 T, and 17 000 A/cm2 at 23 T. Note the small magnetic field dependence in Jc above 10 T.

Enhancement of critical current densities of powder-in-tube processed MgB2 tapes by using MgH2 as a precursor powder

We investigated the effect of starting powder on the microstructure and superconducting properties of MgB2 tapes prepared by a powder-in-tube technique. Two kinds of powder mixtures, Mg + B and MgH2 + B, were used as precursors. When using the mixture of Mg + B, carbon-steel (CS) sheaths reacted with B, and FexB (x = 1 and 2) was formed. This brought about a decrease in the density of the MgB2 core and, hence, a low critical current density (Jc). On the other hand, we found that the mixture of MgH2 + B was effective in suppressing the formation of FexB and obtaining a high density of the core layer. The Jc values of the tapes prepared using the mixture of MgH2 + B were about twice as large as those prepared using the mixture of Mg + B and reached 1600 A cm−2 at 4.2 K and 10 T. This is higher than that of the CS-sheathed MgB2 tapes prepared with MgB2 powder.

ANISOTROPY OF CRITICAL CURRENT DENSITY IN TEXTURED BI2SR2CA1CU2OX TAPES

Anisotropy in Jc with respect to field direction for a textured Bi2Sr2Ca1Cu2Ox tape was measured at various temperatures and magnetic fields. Anisotropy at 4.2 K was small enough for practical applications in fields up to 25 T. However, the anisotropy increases rapidly with increasing temperature. Angular dependence of Jc shows a history effect, which is characteristic of ceramic superconductors.

Large Transport Critical Current Densities of Ag Sheathed (Ba,K)Fe2As2+Ag Superconducting Wires Fabricated by an Ex-situ Powder-in-Tube Process

We report large transport critical current densities observed in Ag-added (Ba,K)Fe2As2 superconducting wires prepared by an ex-situ powder-in-tube (PIT) process. The wire has a simple composite structure sheathed only by Ag. A precursor bulk material prepared by a melting process was ground into powder and put into a Ag tube. The composite was then cold worked into a wire and heat treated at 850 °C for sintering. Transport critical current densities, Jc, at 4.2 K are 1.0×104 A/cm2 (Ic = 60.7 A) in self field and 1.1×103 A/cm2 (Ic = 6.6 A) in 10 T. These are the highest values reported so far for the Fe-based superconducting wires.

Approach for the fabrication of MgB2 superconducting tape with large in-field transport critical current density

The letter reports the fabrication of MgB2 superconducting tape using a flexible metallic substrate as well as its strong pinning force, which was verified by direct measurement of transport critical current density. The tape was prepared by depositing MgB2 film on a Hastelloy tape buffered with an yttria-stabilized-zirconia layer. The Jc of the tape exceeds 105 A/cm2 at 4.2 K and 10 T, which is considered as a common benchmark for magnet application. The Jc dependence on magnetic field remains surprisingly very small up to 10 T, suggesting that the tape has much better magnetic field characteristic than conventional Nb–Ti wires in liquid helium.