Qing-Ping Ding

Low-temperature synthesis of FeTe0.5Se0.5 polycrystals with a high transport critical current density

We have prepared high-quality polycrystalline FeTe0.5Se0.5 at temperature as low as 550 °C. The transport critical current density evaluated from the current–voltage characteristics is over 700 A cm − 2 at 4.2 K under zero field, which is several times larger than that for FeTe0.5Se0.5 superconducting wires. The critical current density estimated from magneto-optical images of flux penetration is also similar to this value. The upper critical field of the polycrystalline FeTe0.5Se0.5 at T = 0 K estimated using Werthamer–Helfand–Hohenberg theory is 585 kOe, which is comparable to that of single crystals. This study gives some insight into how to improve the performance of FeTe0.5Se0.5 superconducting wires.

Superconducting properties and magneto-optical imaging of Ba0.6K0.4Fe2As2 PIT wires with Ag addition

We have fabricated (Ba,K)Fe2As2 superconducting wires through an ex?situ powder-in-tube method. Silver was used as a chemical addition to improve the performance of these superconducting wires. The transport critical current densities (Jc) have reached 1.3???104?A?cm?2 and 1.0???104?A?cm?2 at 4.2?K under self-field in the wires with and without Ag addition. We used a magneto-optical (MO) imaging technique to investigate the properties of grain boundaries in the (Ba,K)Fe2As2 superconducting wire with Ag addition. MO images show the weak links in the Fe-based superconducting wires for the first time. An intragranular Jc of 6.0???104?A?cm?2 at 20?K is obtained from the MO image, which is consistent with the estimation from M?H measurement.

Magneto-optical imaging and transport properties of FeSe superconducting tapes prepared by the diffusion method

FeSe tapes have been prepared by using a diffusion method. By adjusting the reaction temperature and the thickness systematically, a transport critical current density as high as 600?A?cm?2 at 4.2?K under self-field is achieved. Magneto-optical images indicate considerable distribution of Tc and weak-link features in the FeSe tape. This study gives some insight into how to further improve the quality and performance of FeSe tapes and other superconducting tapes prepared through the diffusion method.

Large, Homogeneous, and Isotropic Critical Current Density in Oxygen-Annealed Fe1+yTe0.6Se0.4 Single Crystal

We reported a controllable way of removing excess Fe in Fe1+yTe0.6Se0.4 by annealing in a fixed amount of O2. Compared with the weak superconductivity induced by dilute acids, O2 annealing can successfully induce bulk superconductivity. Proper O2 annealing changes the temperature dependence of resistivity at low temperatures from semiconducting to metallic, which comes from the deintercalation of excess Fe. Critical current densities with field along the c-axis and a b-plane are found almost isotropic with large values of JcH?c~3×105 A/cm2 and JcH?a b~2.5×105 A/cm2 at 2 K. Furthermore, magneto-optical images reveal isotropic current flow within the a b-plane.

Magnetic and transport properties of iron-platinum arsenide Ca10(Pt4−δAs8)(Fe2−xPtxAs2)5single crystal

We report superconducting properties of single crystalline Ca10(Pt4-{\delta}As8)(Fe2-xPtxAs2)5 by X-ray diffraction, magnetization, resistivity, and magneto-optical imaging measurements. The magnetization measurements reveal fish-tail hysteresis loop and relatively high critical current density Jc ~ 0.8\times105 A/cm2 at low temperatures. The exponential temperature dependence of Jc, which arises from nonlinear effective flux-creep activation energy, has been observed. Upper critical field determined by resistive transition shows a relatively large anisotropy. The magneto-optical images reveal homogenous current flow within the crystal.

Magneto-optical imaging of polycrystalline FeTe1−xSex prepared at various conditions

FeTe0.5Se0.5 superconducting wires have been fabricated using ex-situ PIT method with an Fe sheath. Among the other FeAs-based superconductor, FeTe0.5Se0.5 has great advantage for application due to binary composition and less toxic. Surprisingly, superconducting current was observed in the as-fabricated wire without any heat treatments. Zero resistivity and transport critical current density (Jc) were 3.2 K and 2.8 A/cm2, respectively. In addition, by annealing at 200 degC for 2 h, critical temperature Tczero and Jc were enhanced up to 9.1 K, and 64.1 A/cm2, respectively. This suggests that the grain connectivity was improved by heat treatment, and superconducting property of FeTe0.5Se0.5 wire was enhanced.

Magneto-Optical Characterization of Iron-Based Superconducting Wires and Tapes

The critical current density, <i>J</i>_c, in high-quality single crystals of iron-based superconductors (IBS) has reached larger than 10⁵ A/cm² at 4.2 K. This large value of <i>J</i>_c urged the development of potential superconducting wires or tapes based on IBS. We have fabricated superconducting wires and tapes of IBS using several techniques and made an extensive characterization by structural, transport, and magnetic measurements. The powder-in-tube method is used for (Ba, K)Fe₂As₂ and Ba(Fe, Co)₂As₂ wires and the diffusion method is used for FeSe tape. Intra- and inter-granular <i>J</i>_cs have been evaluated by direct transport and indirect magnetization measurements as well as using magneto-optical (MO) imaging. The intergranular <i>J</i>_cs at 4.2 K under self-field for (Ba, K)Fe₂As₂ and Ba(Fe, Co)₂As₂ wires and FeSe tapes are 1.3 × 10⁴ A/cm², 3.8 × 10³ A/cm², and 600 A/cm², respectively. On the other hand, MO images have demonstrated that the intragranular <i>J</i>_c is at least one order of magnitude larger than the intergranular <i>J</i>_c, which is consistent with the magnetic evaluation.

FeSe superconducting tapes with a high critical current density fabricated by diffusion method

We have prepared high-quality FeSe tape through diffusion method. The transport critical current density evaluated by the current-voltage characteristics is 600 A/cm2 at 4.2 K under zero field. The upper critical field of the FeSe tape at T = 0 K estimated by Werthamer-Helfand-Hohenberg theory is 243 kOe, which is similar to that of single crystals.

Comparison of Jc characteristics in PIT wires based on BaFe2As2 with different substitutions

Three kinds of superconducting wires based on BaFe2As2 with different substitutions are fabricated using powder-in-tube method and characterized including magneto-optical imaging. In the case of (Ba,K)Fe2As2 wires processed by hot isostatic press, critical current density (Jc) of 32 kA/cm2 has been achieved at 4.2 K under self-field. Wires fabricated in a similar way with Ba(Fe,Co)2As2 resulted in much lower Jc of 7.8 kA/cm2 at 4.2 K under self-field. In the case of BaFe2(As,P)2, Jc of ambient-pressure processed wire has Jc of only 1.0 kA/cm2 at 4.2 K under self-field. Origins of these differences are discussed.