Shigeru Horii

Superconductivity at 17 K in (Fe2P2)(Sr4Sc2O6): a new superconducting layered pnictide oxide with a thick perovskite oxide layer

A new layered pnictide oxide (Fe2P2)(Sr4Sc2O6) has been synthesized by solid-state reaction. This material has an alternating layer stacking structure of anti-fluorite Fe2P2 and perovskite-based Sr4Sc2O6 oxide layers. The space group of the material is P4/nmm and the lattice constants a and c are 4.016 and 15.543xa0A, respectively. The interlayer Fe–Fe distance corresponding to the c-axis length is the longest ever reported in the iron-based pnictides. In both magnetization and resistivity measurements, the present compound exhibited superconductivity below 17xa0K; this temperature is much higher than that for LaFePO and the highest in arsenic-free iron-based pnictide systems under ambient pressure.

New iron-based arsenide oxides (Fe2As2) (Sr4M2O6)(M = Sc, Cr)

We have discovered new layered oxyarsenides (Fe2As2)(Sr4M2O6) (M = Sc, Cr: M-22426). These materials are isostructural with (Fe2P2)(Sr4Sc2O6), which was found in our previous study. The new compounds are tetragonal with a space group of P4/nmm and consist of the anti-fluorite type FeAs layer and perovskite-type blocking layer. The lattice constants are a = 4.050 A, c = 15.809 A for M = Sc and a = 3.918 A, c = 15.683 A for M = Cr. These compounds have long interlayer Fe-Fe distances corresponding to the c-axis length, the 15.8 A in Sc-22426 is the longest in the iron-based oxypnictide systems. Chemical flexibility of the perovskite block in this system was probed by chromium containing (Fe2As2)(Sr4Cr2O6). Different trends were found in bond angle and bond length of the new oxypnictides compared to the reported systems, such as REFePnO. Absence of superconductivity in these compounds is considered to be due to insufficient carrier concentration as in the case of undoped REFeAsO.

Flux pinning properties and microstructure of SmBa2Cu3Oy thin films with systematically controlled BaZrO3 nanorods

We report a way of tuning the flux pinning properties by controlling the size and number density of BaZrO3 (BZO) nanorods without much degradation of the superconducting properties. The BZO nanorods in REBa2Cu3Oy superconducting films are known as promising c-axis-correlated pinning centers. We fabricated SmBa2Cu3Oy (SmBCO) films with BZO nanorods by a low-temperature growth technique (LTG-SmBCO+BZO films). With decreasing substrate temperature of the upper layer Tsupper in LTG-SmBCO+BZO films, the diameter of BZO nanorods decreased and their number density increased, leading to a high matching field Bϕ. Also, the considerable upturn shifts in the irreversibility field line and plateau regions in the magnetic field dependence of critical current density Jc were observed in the range from Bϕ/3 to Bϕ. These results indicate that a Bose-glass-like state of vortices localized on BZO nanorods emerges, after overcoming the vortex glass state of vortices, which are frozen on inherent pointlike disorders within t...

The effects of growth temperature on c-axis-correlated pinning centers in PLD-ErBa2Cu3O7-δ films with Ba(Er 0.5Nb0.5)O3

Effects of growth temperatures in epitaxial ErBa2Cu3O7−δ films with one-dimensional nanorods were discussed from their microstructures and superconducting properties. c-axis-oriented BaNb2O6-doped ErBa2Cu3O7−δ films were grown on (100)-oriented SrTiO3 substrates grown at 710 and 760u2009°C by pulsed-laser deposition. Atomic force microscopy analysis showed that film surface steps on the three-dimensional islands increased with decreasing growth temperature (TS). Then, nanorod morphologies were also changed by TS. Ba(Er0.5Nb0.5)O3 nanorods were observed clearly in transmission electron microscopy images of the films. The nanorods were thinner and denser at a lower TS under a nominal BaNb2O6 (BNO) doping concentration. In the case of TS = 710u2009°C, the diameter and number density of nanorods with 1.5xa0wt%BNO doping were 4–6xa0nm and 2.6 × 1011xa0cm−2, respectively. Furthermore, the nanorods could effectively act as c-axis-correlated pinning centers in the film grown at a lower TS. Consequently, the pinning effects were controlled by nanorod morphologies through varying the TS.

New Series of Nickel-Based Pnictide Oxide Superconductors (Ni2Pn2)(Sr4Sc2O6) (Pn = P, As)

We have discovered new nickel-based pnictide oxide superconductors, (Ni2Pn2)(Sr4Sc2O6) (Pn = P, As). These compounds have a tetragonal unit cell with a space group of P4/nmm and they consist of alternate stacking of anti-fluorite Ni2Pn2 layers and K2NiF4-type Sr4Sc2O6 blocking layers. Lattice parameters were a = 4.044 A and c = 15.23 A for (Ni2P2)(Sr4Sc2O6) and a = 4.078 A and c = 15.41 A for (Ni2As2)(Sr4Sc2O6), indicating their thicker blocking layers than that of LaNiPO (c ~ 8.1 A). Both (Ni2P2)(Sr4Sc2O6) and (Ni2As2)(Sr4Sc2O6) exhibited superconductivity with zero resistivity at 3.3 K and 2.7 K, respectively. The perfect diamagnetism observed in both compounds guaranteed their bulk superconductivity.

Structural features of layered iron pnictide oxides (Fe2As2)(Sr4M2O6)

Abstract Structural features of newly found perovskite-based iron pnictide oxide system have been studied. Compared to RE Fe Pn O system, perovskite-based system tend to have smaller Pn –Fe– Pn angle and higher pnictogen height owing to low electronegativity of alkaline earth metal and small repulsive force between pnictogen and oxigen atoms. As–Fe–As angles of (Fe 2 As 2 )(Sr 4 Cr 2 O 6 ), (Fe 2 As 2 )(Sr 4 V 2 O 6 ) and (Fe 2 Pn 2 )(Sr 4 MgTiO 6 ) are close to ideal tetrahedron and those pnictogen heights of about 1.40xa0A are close to NdFeAsO with optimized carrier concentration. These structural features of this system may lead to realization of high- T c superconductivity in this system.

Microstructures and improved J c–H characteristics of Cl-containing YBCO thin films prepared by the fluorine-free MOD method

Undoped, Cl-doped, (Cl, Hf) co-doped and (Cl, Sn) co-doped YBa2Cu3O y (YBCO) thin films have been prepared by the fluorine-free metal–organic decomposition (FF-MOD) method on SrTiO3(100) single-crystalline substrates. Cross-sectional microstructures of these films were investigated in detail using scanning transmission electron microscopy (STEM). Rectangular-shaped oxychloride precipitates (Ba2Cu3O4Cl2) and fine particles (BaSnO3) were clearly observed in the (Cl, Sn) co-doped films. The magnetic angular dependence of the critical current density (J c–H–θ) of these films was evaluated. The existence of c-axis-correlated type pinning centers was suggested in Cl-containing YBCO films, whereas this type of pinning is not common in MOD-processed films. J c values were enhanced by Cl doping and further by (Cl, Sn) co-doping in all magnetic field directions at 77 K. This improved J c–H–θ property with c-axis-correlated pinning sites is the first report in FF-MOD-processed YBCO films.

Matching field effects in c-axis in-plane aligned a-axis-oriented YBa2Cu3Oy films with two-dimensional artificial pinning centers induced by multilayered nano-structures

A relationship between microstructures of YBa2Cu3Oy (Y123) films with two-dimensional artificial pinning centers (2D APCs) and their critical current density (Jc), especially under matching fields (B), is discussed. Using a pulsed laser depositionxa0(PLD) method, c-axis in-plane aligned a-axis-oriented Y123/Pr123 multilayer films were grown on substrates. The Pr123 interlayer distance was changed by controlling a target shape and its rotation speed in the PLD process. The multilayer structure was observed in cross-sectional transmission electron microscopy images. Critical temperatures of the films decrease as the Y123 layer becomes thinner. The magnetic field (B) dependence of Jc showed that the interlayers act as 2D APCs in multilayer films. The matching effects were observed in two ways: curves and irreversibility lines.

Possibility of material cost reduction toward development of low-cost second-generation superconducting wires

Two approaches to reducing the material cost of second-generation superconducting wires are proposed in this paper: (1) instead of the electrical stabilizing layers of silver and copper presently used on the superconducting layer, a Nb-doped SrTiO3 conductive buffer layer and cube-textured Cu are proposed as an advanced architecture, and (2) the use of an electromagnetic (EM) steel tape as a metal substrate of coated conductors in a conventional architecture. In structures fabricated without using electrical stabilizing layers on the superconducting layer, the critical current density achieved at 77 K in a self-field was approximately 2.6 MA/cm2. On the other hand, in the case of using EM steel tapes, although the critical current density was far from practical at the current stage, the biaxial alignment of YBa2Cu3O y (YBCO) and buffer layers was realized without oxidation on the metal surface. In this study, the possibility of material cost reduction has been strongly indicated toward the development of low-cost second-generation superconducting wires in the near future.

Growth of (Y[1−x] Ca[x])Ba[2]Cu[4]O[8] in ambient pressure and its tri-axial magnetic alignment

We report the growth of single crystals in ambient pressure and tri-axial orientation under modulated rotation magnetic fields (MRFs) for (Y1−x Ca x )Ba2Cu4O8 [(Y1−x Ca x )124] with x ≤ 0.1. Rectangular (Y1−x Ca x )124 crystals approximately 50 μm in size have been successfully grown for x ≤ 0.1 in a growth temperature region from 650 °C to 750 °C. Their critical temperatures increased with x and exhibited approximately 91 K for x = 0.1. By applying an MRF of 10 T, pulverised powders of (Y1−x Ca x )124 were tri-axially aligned in epoxy resin at room temperature in a whole x region below x = 0.1. The magnitude relationship of the magnetic susceptibilities (χ) along crystallographic directions for (Y1−x Ca x )124 was χ c > χ a > χ b at room temperature and was unchanged with a change in x. From changes in the degrees of the c-axis and the in-plane orientation (Δω) for the (Y1−x Ca x )124 powder samples aligned under three different MRF conditions, it was found that MRFs above at least 1 T were required to achieve almost complete tri-axial alignment with Δω < 5°. Irreversibility lines for H//c were successfully determined even from the powder samples by the introduction of magnetic alignment without using single crystalline samples. The present study indicates that magnetic alignment is a useful process for the fabrication of quasi-single-crystals from the perspective of solid-state physics and the production of cuprate superconducting materials.