Tomoya Horide

Critical current control in YBa2Cu3O7-δ films using artificial pinning centers

High-temperature superconductors can carry high non dissipative currents at low temperatures as long as quantized vortices are pinned. However, the currents in a magnetic field at liquid nitrogen temperature (77 K) are not sufficiently high for practical use due to the weak pinning. For the improvement of nondissipative currents in YBa2Cu3O7-δ films, linear defects that extended to the c-axis of the film were introduced using a novel nanostructure technology. We observed a marked enhancement of the nondissipative currents even at 77 K and high fields when the fields were applied close to the c-axis. This is a clear demonstration of enhanced pinning using artificially incorporated pinning centers.

Microstructures and critical current densities of YBCO films containing structure-controlled BaZrO3 nanorods

BaZrO3 nanorods are known to be effective pinning centers as c-axis-correlated pinning centers. Furthermore, BaZrO3 nanorods in REBa2Cu3Oy (RE: rare-earth element) films are formed by self-assembled stacking of BaZrO3 using a target mixture of a superconductor and BaZrO3 for pulsed-laser deposition, which is a very easy fabrication technique. The density of BaZrO3 nanorods in YBa2Cu3Oy (YBCO) films can be controlled by varying the BaZrO3 content in a target. The BaZrO3 addition has two functions for superconductivity; one is the improvement of pinning forces due to the addition of pinning centers and the other is Tc degradation. The optimum BaZrO3 addition for Jc improvement in magnetic fields is found to be around 3 wt% because of a trade-off between the two functions described above. Furthermore, the length of BaZrO3 nanorods is found to be controlled using two types of target: pure YBCO and a mixture of YBCO and BaZrO3. Varying the BaZrO3 nanorod length has an effect on the pinning mechanism. In particular, magnetic field angle dependences of Jc are varied from c-axis-correlated pinning to nearly random pinning by changing the nanorod length. The magnetic field at the crossover of the pinning mechanism seems to be adjusted by the BaZrO3 nanorod length.

Critical Current Density Enhancement around a Matching Field in ErBa2Cu3O7-δ Films with BaZrO3 Nano-Rods

Field angular dependences of critical current density are measured for as-grown high crystalline quality ErBa2Cu3O7-δ films with BaZrO3 nano-rods. Large critical current density enhancement is observed. The magnetic field where the enhancement is observed coincides with the matching field calculated from the density of BaZrO3 nano-rods. The largest enhancement is observed in B ∥c-axis of ErBa2Cu3O7-δ films. This anisotropic critical current density enhancement is thought to be caused by the direction of BaZrO3 nano-rods which grow along the c-axis of the films. BaZrO3 nano-rods in the films will promise the application of coated conductors in a magnetic field.

Enhanced high-field performance in PLD films fabricated by ablation of YSZ-added YBa2Cu3O7?x target

YBa2Cu3O7?x+Y2O3-stabilized ZrO2 (YBCO+YSZ) mixed films were prepared on SrTiO3/MgO substrates by pulsed-laser deposition from a YBCO target with a thin YSZ sector on top. The Jc at 77?K for the mixed YBCO+YSZ thin film was enhanced. The Jc at 77?K for the pure film and the mixed YBCO+YSZ thin film were 1.46 and 1.95?MA?cm?2 (self field) (and 0.09 and 0.28?MA?cm?2 (, B = 5?T)), respectively, even when both films were prepared using the same deposition conditions. The maximum pinning force of a doped film (77?K, ) was 15.9?GN?m?3, which is comparable to the value reached by NbTi at 4.2?K. In the YSZ-added sample, the angular dependence of Jc showed a broad peak at due to the flux pinning by the c-axis correlated defects. Elongated nanorods grown parallel to the c axis were consistently observed in the cross-sectional TEM images.

Insertion of nanoparticulate artificial pinning centres in YBa2Cu3O7- x films by laser ablation of a Y2O3-surface modified target

YBa2Cu3O7?x+Y2O3 (YBCO+Y2O3) mixed films were prepared on SrTiO3/MgO substrates by pulsed-laser deposition from a YBCO target with a thin Y2O3 sector on the top. The pinning properties for the mixed YBCO+Y2O3 thin film were strongly enhanced, especially at the temperatures lower than 77?K: the maximum global pinning forces FP for the Y2O3 doped sample are 7.8?GN?m?3 near 2?T at 77?K, 54.5?GN?m?3 near 4?T at 65?K, and 189?GN?m?3 near 9?T at 40?K. In the Y2O3-added samples, except for the plane configuration, the angular dependence of Jc showed a plateau, due to the flux pinning by isotropic pinning centres. Y2O3 nanoparticles randomly dispersed inside the YBCO matrix were consistently observed in the cross-sectional TEM images.

Jc improvement by double artificial pinning centers of BaSnO3 nanorods and Y2O3 nanoparticles in YBa2Cu3O7 coated conductors

Double artificial pinning centers of BaSnO3 nanorods and Y2O3 nanoparticles were investigated in YBa2Cu3O7 films on textured substrates prepared using ion beam assisted deposition. The BaSnO3 and Y2O3 content was varied in pulsed laser deposition. Transmission electron microscopy observation revealed the systematic change in density of the BaSnO3 nanorods and the Y2O3 nanoparticles in the films. In YBa2Cu3O7 + BaSnO3 films, maximum global pinning force (Fp,max) was improved at high magnetic fields, and Fp,max was shifted to high magnetic field by the Y2O3 incorporation due to an increase in density of the pinning centers. The angular dependences of critical current density (Jc) in the YBCO + BaSnO3 films were tuned by introducing Y2O3, and some of the YBa2Cu3O7 + BaSnO3 + Y2O3 films exhibited isotropic Jc behavior at low magnetic field. The double artificial pinning centers of BaSnO3 nanorods and Y2O3 nanoparticles are effective in improving Jc angular dependences, Jc values at high magnetic fields, and Fp,max.

Incorporation of double artificial pinning centers in YBa2Cu3O7?d films

YBa2Cu3O7?x+BaZrO3+Y2O3 (YBCO+BaZrO3+Y2O3) mixed films were prepared on SrTiO3/MgO substrates by pulsed-laser deposition, with an YBCO+BaZrO3 target with a thin Y2O3 sector stuck on the top. The pinning properties for the double-doped YBCO+BaZrO3+Y2O3 thin film were enhanced in comparison with those of a pure YBCO sample prepared in the same experimental conditions. The maximum global pinning forces Fp for the double-doped sample are 12.8?GN?m?3 near 3?T at 77?K, 46.3?GN?m?3 near 4?T at 65?K, and 122?GN?m?3 near 9 T at 40?K. In the double-doped sample the angular dependence of Jc showed a broad peak at , due to the flux pinning by c-axis correlated defects, combined with a plateau, due to the flux pinning by isotropic pinning centers. Both BaZrO3 nanorods parallel to the c-axis and Y2O3 nanoparticles randomly dispersed inside the YBCO matrix were consistently observed in the cross sectional transmission electron microscopy images.

High-J/sub c/ Gd-Ba-Cu-O epitaxial films prepared by pulsed laser deposition

For applications of high-temperature cuprate superconductors in magnetic fields at 77 K, a RBa/sub 2/Cu/sub 3/O/sub x/ (R123; R=Nd, Sm, Gd, etc.) film is one of the promising materials due to its high critical temperature T/sub c/ and high irreversibility field B/sub irr/. Especially, a substitution between R and Ba sites that results in a T/sub c/ degradation is not likely to occur in GdBa/sub 2/Cu/sub 3/O/sub x/ (Gd123) films, compared to the cases of Nd and Sm ions. This is an advantage for an application of Gd123 film. We studied the c-axis oriented Gd123 film for obtaining a high critical current density J/sub c/ in a magnetic field at 77 K. The substrate temperature to deposit the epitaxial films on MgO(100) was varied between 700-800/spl deg/C under an oxygen partial pressure of 200 mTorr. T/sub c/ values of the films were within 89.8-92.7 K. Interestingly, the J/sub c/ at 77 K for 5 T (B//c) reached 0.2 MA/cm/sup 2/. The values are three times higher than those of Y123 films, which indicate that Gd123 films have the superior potentiality in magnetic fields at 77 K.

The crossover from the vortex glass to the Bose glass in nanostructured YBa2Cu3O7−x films

The glass-liquid transitions of quantized vortices were studied to understand the vortex behavior in YBa2Cu3O7−x films. We systematically controlled the pinning strength, spatial distribution, and shape of disorders by engineering of nanoinclusions in YBa2Cu3O7−x films. In these disorder systems, we observed the crossover between the vortex glass (VG) and the Bose glass (BG). The increase in pinning strength and ordering of isotropic disorders do not change the critical exponents and the conversion between the BG and the VG is triggered only by the anisotropy of pinning centers.

Tailoring the vortex pinning strength of YBCO thin films by systematic incorporation of hybrid artificial pinning centers

The effect of hybrid (columnar and spherical together) artificial pinning centers (APCs) on the vortex pinning properties of YBa2Cu3O7−δ (YBCO) thin films is investigated in detail on the basis of variation of critical current density (J C ) with applied magnetic field and also with the orientation of the applied magnetic field at 65 K and 77 K. Premixed YBCO + BaSnO3 composite targets are used for the deposition of the YBCO films which consist of self-assembled BaSnO3 nanocolumns (1D APCs); on the other hand, for the deposition of the YBCO films with hybrid APCs (BaSnO3 nanocolumns together with Y2O3 nanoparticles), the surface of the premixed YBCO + BaSnO3 composite targets are modified by putting a thin Y2O3 sectored piece on the premixed YBCO + BaSnO3 composite targets by means of silver paste. F pmax value increases systematically with incorporation of 1D and 1D and 3D APCs and it also shifts towards higher applied magnetic fields. Films with 1D APCs exhibit a strong J C peak at Θ = 0° (H//c-axis) whereas films consisting of hybrid APCs exhibit enhanced J C at all the investigated angular regimes. A possible mechanism of vortex pinning in samples with hybrid APCs is also discussed suggesting the role of 1D and 3D APCs.