Yusuke Ichino

The influence of the geometric characteristics of nanorods on the flux pinning in high-performance BaMO3-doped SmBa2Cu3Oy films (M = Hf, Sn)

Recently, the BaHfO3 (BHO) nanorod has attracted attention as a new c-axis-correlated pinning center in REBa2Cu3Oy films. We fabricated SmBa2Cu3Oy (SmBCO) films with BHO nanorods using an alternating-targets technique with pulsed laser deposition on single-crystal LaAlO3(100) substrates, and then compared the microstructure and flux pinning properties with those of BaSnO3 (BSO)-doped SmBCO films. Transmission electron microscopy observations indicated that the BHO and BSO nanorods both grew straight, but the inclination of the BHO nanorods from the c-axis of the SmBCO was less than that of the BSO nanorods. The inclination had a strong influence on the flux pinning property. The flux pinning force of the BHO-doped SmBCO film ( with Jc = 2.0 MA cm−2 at 77 K under 1.4 T) became stronger than that of the BSO-doped SmBCO film ( with Jc = 1.4 MA cm−2 at 77 K under 1.8 T) due to the shape of the BHO nanorods without inclining.

High-Critical-Current-Density Epitaxial Films of SmBa2Cu3O7-x in High Fields

The critical current density (Jc) and irreversibility field (Birr) of epitaxial SmBa2Cu3O7-x (SmBCO) films are reported. The Birr and Jc of these films deposited on MgO(100) substrates by pulsed-laser deposition depend critically on the substrate temperature. The use of a thin SmBCO seed layer grown at a high substrate temperature enabled us to obtain fully c-axis-oriented SmBCO films at relatively low temperatures, resulting in a high Jc of 1.7 ×105 A/cm2 of B ∥c at 5 T and 77 K. This value is as high as that of the optimized NbTi superconducting wires achieved at 5 T and 4.2 K.

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...

High-Critical-Current-Density SmBa2Cu3O7-x Films Induced by Surface Nanoparticle

In oxide-based superconducting thin films, conceivable factors that give rise to a pinning center include dislocations, precipitates, twin boundaries, and a-axis-oriented phases. In this study, crystalline defects were created perpendicular to a high-Jc Sm1+xBa2-xCu3Oy film surface during a film deposition process. We obtained extremely high Jc values, Jc=0.37 MA/cm2 (77 K, B∥c, B=5 T) and Jc=0.1 MA/cm2 (77 K, B∥c, B=8 T). These values are equal to or exceed that of the previously reported NbTi wire material measured in a high magnetic field at 4.2 K.

Effect of Sm/Ba substitution on the J/sub c/ in magnetic field of SmBCO thin films by low temperature growth technique

We have reported superconducting properties of Sm/sub 1+x/Ba/sub 2-x/Cu/sub 3/O/sub y/ (SmBCO; x=0.08) thin films prepared by the low temperature growth technique (LTG). LTG process is consisted of two steps as follows; a seed layer is deposited at 830/spl deg/C on MgO(100), and then upper layer is deposited at 740-780/spl deg/C on the seed layer. In order to investigate the effect of the fluctuations Sm/Ba composition ratio in the SmBCO matrix, we deposited upper layer with the Sm/Ba composition of x=0-0.12. The SmBCO films with x=0.04 and 0.08 show critical current density (J/sub c/) at 77 K of up to 5/spl times/10/sup 6/ A/cm/sup 2/ under 0 T and 1/spl times/10/sup 5/ A/cm/sup 2/ under 5 T for B//c at 77 K, respectively. The J/sub c/ of x=0.12 film dropped more sharply compared with the cases of x=0.04 and 0.08 films. From the microstructure analysis, it was found that the main phase of x=0.12 film was Sm-rich (Sm/sub 1.1/Ba/sub 1.9/Cu/sub 3/O/sub y/) phase, while that of x=0.04 and 0.08 films was stoichiometry. We speculated that the low-J/sub c/ of x=0.12 film in the magnetic fields was ascribed to the low physical meters of T/sub c/ and irreversibility field of the Sm-rich SmBCO main phase.

High critical current density in high field in Sm/sub 1+x/Ba/sub 2-x/Cu/sub 3/O/sub 6+y/ thin films

Critical current density (J/sub c/), irreversibility field (B/sub irr/) and microstructure in epitaxial SmBa/sub 2/Cu/sub 3/O/sub 7-x/ (SmBCO) films are reported. We have developed a novel approach to deposit high performance Sm/sub 1+x/Ba/sub 2-x/Cu/sub 3/O/sub 6+y/ (Sm-123) at relatively low substrate temperature on MgO. The use of thin SmBCO seed layer grown at a high substrate temperature enabled us to obtain fully c-axis oriented SmBCO films with high critical current density (J/sub c/) of 2.8/spl times/10/sup 5/ A/cm/sup 2/ under 5 T for B//c at 77 K. This value is as high as in the optimized NbTi superconducting wires achieved for 5 T at 4.2 K. Compared with YBa/sub 2/Cu/sub 3/O/sub 7-y/ films, Sm-123 films showed higher J/sub c/ in high fields at 77 K. Transmission electron microscopy TEM analyses clarified the Sm/Ba composition ratio fluctuations in the Sm-123 matrix composition in the range of x=0/spl sim/0.14 with a wavelength of 50/spl sim/100 nm.

c-axis correlated pinning behavior near the irreversibility fields

The authors found that the peak at the parallel external field (B‖c) to the c axis in the angular dependence of Jc shrinks and almost disappears with increasing a magnetic field but it grows again with further increasing of a magnetic field at various temperatures for high-Jc Sm1+xBa2−xCu3Oy films. These behaviors can be explained by the flux pinning properties of the interstitial vortices, which locate in between the vortices pinned by the c-axis correlated disorders. From the obtained experimental results, the collective correlated pinned glass state is proposed in a high field region in the vortex glass states.

In-field characterization of FeTe0.8S0.2 epitaxial thin films with enhanced superconducting properties

Epitaxial Fe?Te?S thin films (thickness range 100?200?nm) were deposited by pulsed laser deposition at 400??C on SrTiO3(100) and MgO(100) single-crystal substrates. The films reached the zero-resistance condition: Tc0 = 3.54?K for on SrTiO3 and 5.37?K for on MgO, and presented a large upper critical field: ?T for on SrTiO3 and 74.2?T for on MgO, using 90% of the normal state resistance and the Werthamer?Helfand?Hohenberg model. The coherence length ?0 is very short: 2.30?nm for on SrTiO3 and 2.10?nm for on MgO. The film grown on MgO showed ?T; then the isotropic parameter ? is 1.09. The best Jc observed is 104?A?cm ? 2 (at 2?K, self-field) for the sample deposited on MgO.

Dislocation density and critical current density of Sm1+xBa2-xCu3Oy films prepared by various fabrication processes

Dislocations are effective flux-pinning centers in REBa2Cu3Oy films at a magnetic field. To investigate the relationship between critical current density (Jc) and dislocation density, we discussed the dislocations in conventional pulsed-laser-deposition (PLD)-SmBCO films, vapor–liquid–solid (VLS)-SmBCO films, and low-temperature growth (LTG)-SmBCO films. The LTG-SmBCO and VLS-SmBCO films showed high-Jc at low magnetic fields. From the observation of etch pits, we found that the LTG-SmBCO and VLS-SmBCO films had high-dislocation densities. We speculate that Jc at low-magnetic fields is affected by dislocation density. LTG is effective for increasing dislocation density without deteriorating crystallinity or superconducting properties.

Characteristics of high-performance BaHfO3-doped SmBa2Cu3Oy superconducting films fabricated with a seed layer and low-temperature growth

BaHfO3(BHO)-doped SmBa2Cu3Oy (SmBCO) superconducting films were fabricated with a seed layer and low-temperature growth (LTG technique) via pulsed laser deposition. The LTG technique enables the fabrication of high-quality SmBCO films even at a low substrate temperature during the deposition, and also greatly increases the number density of BHO nano-rods. Transmission electron microscopy observations showed that the 5.6 vol% BHO-doped sample had a high number density [(4.8 ± 0.3) × 103 μm−2] of small (5.4 ± 0.7 nm) BHO nano-rods. This sample exhibits maximum flux pinning forces (Fp) of 405 GN m−3 in 9.0 T at 40 K and 105 GN m−3 in 9.0 T at 65 K. The high density and small size of the nano-rods in a REBa2Cu3Oy matrix (where RE is a rare earth element) is considered to be very effective for enhancing Fp at low temperatures under a high field.