Aichi Yamashita

Excess iron deintercalation induced superconductivity in Fe(Te, Se) and Fe(Te, S) via sulfur annealing

We discovered a novel annealing method for Fe-chalcogenide superconductors. It was found that sulfur annealing deintercalated excess Fe via formation of FeS2. Due to its specifics, sulfur annealing is applicable when preparing Fe-chalcogenide-based wires or cables.We discovered a novel annealing method for Fe-chalcogenide superconductors. It was found that sulfur annealing deintercalated excess Fe via formation of FeS2. Due to its specifics, sulfur annealing is applicable when preparing Fe-chalcogenide-based wires or cables.

Electrochemical Deposition of FeSe on RABiTS Tapes

FeSe film is successfully fabricated onto Rolling Assisted Biaxially Textured Substrate (RABiTS) tapes by an electrochemical deposition technique. The deposited FeSe films tend to become high crystallinity with a decrease in the applied voltage to -1.0 V, and the compositional ratio of Fe to Se approaches 1:1. The sample deposited at -1.0 V shows a superconducting transition approximately 8.0 K in the magnetic susceptibility.

Fabrication and Characterization of Sintered Iron-Chalcogenide Superconductors

Iron-chalcogenide superconductors are appealing for high-magnetic-field applications due to their promising superconducting properties, i.e., extremely high upper critical and irreversibility field.In this paper, the first achievements obtained from sintering polycrystalline samples are presented. The sintering of FeSe polycrystalline powders and pellets was obtained in several steps starting from stoichiometric quantities of freshly polished powders shots; after grinding, powders were loaded into cleaned and dried silica tubes sealed under vacuum for subsequent heat treatments in order to obtain the superconducting phase. Samples were then characterized from the structural, transport, and magnetic points of view. From the structural characterization, it has been discovered that, in addition to the presence of the desired tetragonal FeSe phase, hexagonal phase and several impurities are present in the samples; the lack of homogeneity inside the samples is confirmed by the superconductive characterization: despite the small undeniable enhancement of the results with the application of subsequent HTs and, nearby, a resistive superconductive onset of 12 K, the transition is still broad after the third HT.

Superconductivity in FeTe1−xSx Induced by Electrochemical Reaction Using Ionic Liquid Solution

Superconductivity in FeTe0.8S0.2 is successfully induced by an electrochemical reaction using an ionic liquid solution. A clear correlation between the Fe concentration in the solution and superconductivity was confirmed, suggesting that superconductivity is induced by the deintercalation of excess Fe.

X-ray Fluorescence Holographic Study on High-Temperature Superconductor FeSe0.4Te0.6

Abstract To observe the difference of atomic heights between the Se and Te layers with respect to the Fe layer in FeSe0.4Te0.6 single crystal, a Fe Kα fluorescence X-ray holography (XFH) experiment was performed at room temperature. The crystal structure of superconductor FeSe0.4Te0.6 obtained by X-ray diffraction (XRD) at a low temperature has distinct z-coordinates of Se and Te, remarkably different from each other. The reconstructed atomic image around central Fe atoms by XFH, however, reveals the different and complex results.

Observation of zero resistance in as-electrodeposited FeSe

Abstract Superconducting FeSe films were electrochemically deposited on rolling-assisted biaxially textured substrate (RABiTS) tape. We observed zero resistivity in the as-electrodeposited FeSe film prepared on the RABiTS when the appropriate voltage was applied while it was dipped into the solution. When the RABiTS tape was dipped in the solution without applying voltage, a thin Se film was deposited on the substrate. The compositional ratio of the FeSe film got closer to the stoichiometric ratio with decreasing the dipping time before applying voltage.

Phase Diagram of FeSe Deposited by Electrochemical Technique with Different Temperature and Voltage

High-quality crystalline FeSe is successfully synthesized in 5 min by an electrochemical deposition technique via increasing the solution temperature. The synthesis of FeSe have been controlled by ...

Superconducting joints using Bi-added PbSn solders

Superconducting joints between NbTi and Bi2Sr2Ca2Cu3O10 superconducting wires were successfully fabricated using Bi-added PbSn solders with an in situ sheath-dissolution technique without a removal process for the sheath materials. A backscattered electron image and an energy-dispersive X-ray spectroscopy analysis of the cross-sectional plane of the joints revealed that the joints using Bi-added PbSn solder had a homogeneous morphology with tiny non-superconducting islands. The superconducting joints exhibited high critical currents above 200 A in self-field and 50 A under an applied magnetic field of 5 kOe.

Universal scaling behavior of the upper critical field in strained FeSe0.7Te0.3 thin films

Revealing the universal behaviors of iron-based superconductors (FBS) is important to elucidate the microscopic theory of superconductivity. In this work, we investigate the effect of in-plane strain on the slope of the upper critical field H c2 at the superconducting transition temperature T c (i.e. −dH c2/dT) for FeSe0.7Te0.3 thin films. The in-plane strain tunes T c in a broad range, while the composition and disorder are almost unchanged. We show that −dH c2/dT scales linearly with T c, indicating that FeSe0.7Te0.3 follows the same universal behavior as observed for pnictide FBS. The observed behavior is consistent with a multiband superconductivity paired by interband interaction such as sign change s ± superconductivity.

The influence of the in-plane lattice constant on the superconducting transition temperature of FeSe0.7Te0.3 thin films

Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.