After obtaining my doctorate in engineering from Keio University, I spent five years as a researcher at the Tokyo Institute of Technology, Suzukakedai Campus in Yokohama. During that time, I made the first report on a material system called iron-based high-temperature superconductors (Fe-SCs). Research on Fe-SCs is still hot topic in condensed matter physics. Now, I am continuing my research mainly on the fabrication of superconducting wire using Fe-SCs, while being distracted by some traditional affairs as a member of the faculty. Research on Fe-SCs has encouraged young researchers towards the further discovery of novel layered superconductors. This article was prepared by updating the content of “A Private Story, Discovery of Iron-based High Tc Superconductors” previously published in the membership journal of the Physical Society of Japan (Nihon Butsuri Gakkaishi).
We report discoveries and properties of new 112-type (Ca,Ln)FeAs2 (Ln = La ~ Gd) and 1144-type AeAFe4As4(Ae = Ca, Sr, Ba, Eu, A = K, Rb, Cs) Fe-based superconductor structures. (Ca,Ln)FeAs2 has a monoclinic system with a space group P21/m composed of two Ca(Pr) planes, Fe2As2 layers, and As2 zigzag chain layers. (Ca,Ln)FeAs2 shows superconductivity with a transition temperature (Tc) at 20 ~ 40 K. Although (Ca,Ln)FeAs2 has thick blocking layers, it has relatively high critical current density over 106 A/cm2 at 5 K (self-field). AeAFe4As4 has a tetragonal system with a space group, P4/mmm, Ae and A layers are inserted alternately between the Fe2As2 layers in the c-axis direction. AeAFe4As4 is a stoichiometric compound having Tc = 31 ~ 36 K, depending only on the combination of Ae and A. Both new superconductors are promising materials for application due to their characteristic properties.
This article reviews the recent development of bulk trapped field magnet made with iron-based superconductor (IBSC). A trapped field of over 1 T at 5 K and 0.5 T at 20 K has been measured between a stack of magnetized cylinders of bulk polycrystalline Ba0.6K0.4Fe2As2 superconductors 10 mm in diameter. Magneto-optical imaging revealed a trapped field distribution corresponding to uniform macroscopic current loops circulating through the sample. A standard numerical modelling technique using the measured Jc(B, T) characteristics of a small specimen reproduced the experimentally measured trapped fields well, again indicating the homogeneous current loops in the polycrystalline bulk. Given the untextured polycrystalline nature of the cylinders and their large irreversibility field, it is expected that larger IBSC bulks could trap much higher field.
The iron-based superconductors of K-doped Ba(Sr)Fe2As2(Ba(Sr)-122) are most potentially useful for high-field applications as a result of their high upper critical fields of over 50T and their small anisotropies. For Ba(Sr)-122, a powder-intube(PIT) method using an Ag sheath has been developed for tape and wire fabrications. Two important factors that greatly influence the transport Jc of PIT Ba(Sr)-122 tape are the density and c-axis grain orientation of the Ba(Sr)-122 core. The Jc of our PIT Ba-122/Ag tapes approach 4×104 A/cm2 at 4.2 K and 10 T for flat-rolled thin tapes. Alloying Ag with Sn is effective for improving the density and grain orientation of the Ba-122 core, and hence the Jc values. The application of uniaxial pressing in the final stage of deformation significantly enhanced Jc values up to ~7×104 A/cm2. This is also due to improving the density and grain orientation of the Ba-122 core. Another effective method to increase the density and grain orientation is double sheathing with an Ag or Ag-Sn inner sheath and stainless steel (SUS) outer sheath. This double-sheath architecture brings about much improved packing density and c-axis grain orientation in the Ba-122 core. The flat-rolled SUS/(Ag-Sn) sheathed Ba-122 tapes show Jc values of 1×105 A/cm2 at 4.2 K and 10 T with high homogeneity. The application of uniaxial pressing to this doublesheathed tape further increased Jc up to 1.4×105 A/cm2. The Jc values of both rolled and pressed Ba-122 tapes show extremely small magnetic field dependence up to 28 T, indicating that Ba-122 is a promising candidate as a conductor for high-field magnets.
In a history of superconductors 1911-2017, Fe-based superconductor ReFePnO1-xFx (Re: rare earth, Pn: Pnictogen) exhibits relative high superconducting transition temperatures (Tc) ≤ 58 K under ambient pressure. Electron doping by substitution of O2- with F- was required for appearance of a superconducting phase with high Tc. The Tc and upper critical field of ReFePnO1-xFx are very attractive for applications as superconducting wires and tapes under high magnetic fields, although demonstration of practical superconducting wires is a tough challenge due to the difficulty in controlling the F contents during a processing for ex-situ powder in tube method. In 2017, Zhang et al. reported that iron-based superconducting tapes with Cu sheath showed superconducting electrical current (Jc) ~ 18 kAcm−2. It is noteworthy that the tapes are sintered at 300°C.
This article reviews the developments over the past 10 years in the thin film growth of Fe-based superconductors, including recent topics of interest in this field. At the beginning of the research, there was pessimism that thin films of the highest-Tc compound, LnFeAs(O,F), would not be easy to grow since it is composed of five elements, namely two cations and three anions. The situation is different from cuprates such as Bi2Sr2CaCuO8, which also consists of five elements, but four cations and one anion. Indeed, there were enormous struggles in the initial attempts to grow LnFeAs(O,F) using pulsed laser deposition. But a major breakthrough was achieved using molecular beam epitaxy, and now LnFeAs(O,F) films with Tcend over 55 K are routinely grown, at least in Japan. The current best films of many of Fe-based superconductors show superconducting properties substantially superior to those of bulk single crystals. Furthermore the simplest material among Fe-based superconductors, FeSe, whose bulk Tc is as low as 8 K, is opening up a new frontier. The Tc of ultra-thin FeSe films are as high as 40 K or possibly it may even reach >100 K in the form of single atomic-layer films.
Iron-based superconductors have been receiving much attention as a new family of high-critical temperature superconductors since 2008 owing to their unique properties and distinct difference from cuprates and conventional superconductors. This paper reviews our research on iron-based superconductors' thin-film with an emphasis on growth, critical current properties, and device fabrication.
Since the discovery of Fe-based superconductors (FBS), great progress in the growth of single crystals and thin films of these materials have been made. Now high-quality FBS thin films are available, which spurs progress in fundamental and application research. In this paper, we report on the transport properties of epitaxial SmFeAs(O,F) and Fe(Se,Te) thin films. We found vortex trapping and lock-in caused by the Sm(O,F) blocking layer in SmFeAs(O,F) when the magnetic field is close to the ab-plane. The vortex trapping is hallmarked by a drop of the exponent n in E~J n (E and J are electric field and current density), whereas a local peak within the dip of n is a sign of the vortex lock-in. Interestingly, vortex trapping and lock-in were also observed in Fe(Se,Te), albeit Fe(Se,Te) shows a low superconducting transition temperature and low anisotropy.
This paper reviews our study on in-field superconducting properties and thermally activated vortex motion (creep) in BaFe2(As1-xPx)2 (Ba122:P) thin films with BaZrO3 nanoparticles fabricated on a MgO single crystal using pulsed laser deposition (PLD). We demonstrated that introducing controlled uniformly dispersed BaZrO3 nanoparticles into Ba122:P films significantly improves its in-field superconducting properties without degrading crystallinity, critical temperature and self-field critical current density (Jc). The nanoparticles doped Ba122:P films show an increase in Jc at all magnetic-field orientations and significantly reduced vortex creep, indicating that nanocomposite films could be a promising candidate for in-field applications.
The critical temperature Tc and critical current density Jc determine the limits to practical application of superconductors. The superconducting state emerges at Tc from the formation of the Cooper pairs and macroscopic spatial coherence. The current-carrying capability, measured in Jc, is the ability of defects in superconducting materials to pin the magnetic vortices by suppressing the pair potential locally, and this may drive down the Tc of superconducting materials with short coherence length. It is a very difficult and challenging task to raise Tc and Jc simultaneously by introducing defects in the cuprate and iron-based superconducting materials. Here, we review our contribution to understanding the irradiation defect structures in iron-chalcogenide FeSe0.5Te0.5 (FST) superconducting thin films and their correlation to superconducting properties. Using low-energy proton irradiation, we create cascade defects in FST thin films. Tc is enhanced due to the nanoscale compressive strain and proximity effect, while Jc is nearly doubled under a zero field at 4.2 K through strong vortex-pinning by the cascade defects and surrounding nanoscale strain. This route opens up the possibility of engineering the pinning landscape for all superconductors.