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Register a new userImprovement of the superconducting properties of polycrystalline FeSe by silver addition
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We investigated the influence of different Ag additions (up to 10 wt %) on the superconducting properties of FeSe0.94. The structural investigations (XRD and SEM) indicated that Ag is present in three different forms. Ag at grain boundaries supports the excellent intergrain connections and reduces superconducting transition width to values smaller than 1K at B=0 and smaller than 2.74 K at B=14 T. Ag insertion in the crystal lattice unit cell provides additional carriers and changes the electron hole imbalance in FeSe0.94. This results in an increase in the magnetoresistive effect (MR) and critical temperature (Tc). Reacted Ag forms a small amount (~1%) of Ag2Se impurity phase, which may increase the pinning energy in comparison with that of the undoped sample. The enhanced upper critical field (Bc2) is also a result of the increased impurity scattering. Thus, unlike cuprates Ag addition enhances the Tc, Bc2, pinning energy and MR making the properties of polycrystalline FeSe0.94 similar to those of single crystals.
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The tetragonal FeSe phase is an intensively investigated iron based superconductor. In this study we examined the influence of Ag addition on the superconducting properties of selenium deficient polycrystalline FeSe0.94. The samples were obtained by solid state reaction and melting methods. XRD analysis shows the presence of tetragonal phase and EDX analysis establishes inhomogeneous Ag distribution in the grains. The superconducting properties were investigated by fundamental and third harmonic AC magnetic susceptibility. The intergranular critical current determined from AC magnetic susceptibility in the Ag doped sample is several times higher than that in the undoped one, obtained by melting at approximately the same temperatures. Intra-granular current is field independent up to almost 1000 Oe. Using the temperature dependence of third harmonic AC magnetic susceptibility at different DC magnetic fields, the irreversibility lines were obtained for all samples. It is found that Ag addition increased the irreversibility field in comparison with undoped melted and powder sintered samples. All results show that the Ag addition in selenium deficient (FeSe0.94) samples leads to improvement of inter- and intra- granular properties: screening ability, pinning, activation energy and critical current and improves the irreversibility line.
We report enhancement in the magnetic critical current density of indium added polycrystalline SmFeAsO1-xFx. The value of magnetic Jc is around 25 kA/cm2 at 4.2 K under self-magnetic field. Polycrystalline SmFeAsO1-xFx is mainly composed of the superconducting grains and a little of amorphous FeAs compounds. These areas randomly co-exist and amorphous areas are located between superconducting grains. Therefore, the superconducting current is prevented by the amorphous areas. In this study, it is found that indium addition to polycrystalline SmFeAsO1-xFx removes these amorphous areas and induces the bringing together the superconducting grains. It means the total contact surfaces of grains are increased. We suggest that the enhancement of the magnetic critical current density is a direct effect of the indium addition.
We have investigated the substitution effect of Eu on the superconductivity in La2-xEuxO2Bi3Ag0.6Sn0.4S6. Recently, we reported an observation of superconductivity at 0.5 K in a layered oxychalcogenide La2O2Bi3AgS6. The Sn doping at the Ag site was found to raise the superconducting transition temperature, Tc to 2.5 K in La2O2Bi3Ag0.6Sn0.4S6. To further improve the superconducting properties, we have partially substituted Eu for the La site to increase the chemical pressure in La2-xEuxO2Bi3Ag0.6Sn0.4S6 (x = 0.1 to 0.6). With the increase in Eu concentration, x, the lattice constant a was found to shrink, while the lattice constant c was marginally shortened, which suggests that the chemical pressure induced by the Eu doping is uniaxial along the a-axis. Tc was observed to increase with increasing x up to x = 0.4, further decreasing for higher Eu concentrations of x = 0.5 and 0.6. From the magnetic susceptibility and resistivity measurements, the bulk nature of superconductivity has been observed for x = 0.1 to 0.5 with Tc = 2.5 to 4.0 K, respectively. The upper critical field (Bc2) was noted to be 3.5 T for x = 0.4, which also has the highest Tc.
To investigate the interlayer interaction in the recently synthesized high-entropy-alloy-type (HEA-type) REO0.5F0.5BiS2 superconductors (RE: rare earth), we have systematically synthesized samples with close lattice parameters and different mixing entropy (DSmix) for the RE site. The crystal structure was investigated using synchrotron X-ray diffraction and Rietveld refinement. For the examined samples with different DSmix, the increase in DSmix does not largely affect the bond lengths and the bond angle of the BiS2 conducting layer but clearly suppresses the in-plane disorder at the in-plane S1 site, which is the parameter essential for the emergence of bulk superconductivity in the REO0.5F0.5BiS2 system. Bulk nature of superconductivity is enhanced by the increase in DSmix for the present samples. The results of this work clearly show that the increase in mixing entropy at the blocking layer can positively affect the emergence of bulk superconductivity in the conducting layer, which is the evidence of the interaction between the high entropy states of the blocking layers and the physical properties of the conducting layers.
Enhancements of superconducting properties were observed in FeSe wires using a quenching technique. Zero resistivity was achieved at about 10 K in quenched wires, which is about 2 K higher than that of polycrystalline FeSe bulk. Furthermore, transport Jc of quenched wires showed three times higher than that of furnace-cooled wires. In contrast, the quenched polycrystalline FeSe bulks did not show the enhancement of Tc. The quenching technique is a greatly promising for fabricating FeSe wires with high Tc and high Jc, and quenched FeSe wires have high potential for superconducting wire applications.
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