Yoshihiro Sugawara

Critical currents of YBa2Cu3Oy thick films prepared by liquid phase epitaxial growth

We have succeeded in preparing c‐axis oriented high‐Jc YBa2Cu3Oy films on MgO(100) substrate by the liquid phase epitaxy (LPE) technique. The growth rate was typically about 2 μm/min, which was 10–102 times larger than that by ordinary vapor growth techniques. The film thickness ranged 10–50 μm by choosing the dipping time. The Tc of the best film exceeded 88 K after oxygen annealing, and the transport Jc was 1.1×105 A/cm2 at 77 K and 0 T. In‐field Jc’s at 77 K and 1.5 T were 2.8×104 A/cm2 and 2.0×104 A/cm2 for the B⊥ab plane and B∥ab plane, respectively. In addition, the peak effect of Jc was observed at several tesla for B∥ab plane geometry. Based on the microstructure observed by high resolution transmission electron microscopy, the relevant peak effect is considered to be caused by stacking faults which act as a field induced pinning center.

Interface structure of face-centered-cubic-Ti thin film grown on 6H-SiC substrate

A titanium thin film was deposited on the flat (0001) face of a 6H–SiC by electron beam evaporation at room temperature in a vacuum of 5.1 × 10 −8 Pa. The Ti film was epitaxially grown on the surface, and the interface between Ti and SiC was characterized by high-resolution electron microscopy. It was found that the structure of the deposited titanium is face-centered cubic (fcc), although bulk titanium metal usually has a hexagonal close-packed or body-centered cubic crystal structure. We believe that the unusual fcc structure of Ti thin film is due to the high adhesion of the film to the substrate and the high degree of coherency between them. The orientation relationship of the fcc-Ti/6H–SiC interface was (111) fcc-Ti //(0001) 6H–SiC and [ 1 10] fcc-Ti //[11 2 0] 6H−SiC . Preliminary calculations indicate that this orientation relationship maximizes the lattice coherency across the interface.

Atomic and Electronic Structure of V/MgO Interface

Thin films of vanadium were deposited on the (001) surface of a MgOsubstrate by molecular beam epitaxy (MBE) and the V/MgO interface wasinvestigated by cross-sectional high resolution electron microscopy(HREM) and electron energy loss spectroscopy (EELS). In order todetermine the location of atoms at the interface, computersimulations were performed for four possible models, and bestmatching between the experimental and simulated images was obtainedfor the model where the V atoms are located directly on top of the Mgatoms at the interface. Interface bonding mechanism was investigatedby a first principles molecular-orbital (MO) calculation using thediscrete-variational (DV)-Xα method for a modelcluster of the interface, i.e., (Mg9O9V5).The V-3d band was located in between the band-gap of MgO, and nearly empty Mg-3sp orbitals werefound to overlap with the V-3d band. The Mg-3sp and V-3d hybridizedin a bonding manner, thereby generates strong covalent bondingbetween V and Mg. Nearly filled O-2p orbitals were also found tohybridize with the V-3d orbitals in an antibonding manner. The bondoverlap population of the V–O bond was approximately four timessmaller than that of the V–Mg bond when the bond-length was thesame. The near edge structure of EELS specific to the interface wasobtained using a V/MgO multilayer specimen at both Mg-K and O-Kedges. Comparison between the experimental and theoretical spectra bythe present MO calculation clearly found the presence of hybridizedorbitals of V-3d with Mg-3p.

Transmission Electron Microscopy Analysis of a Threading Dislocation with c+a Burgers Vector in 4H-SiC

A threading dislocation (TD) in 4H-SiC, which was interpreted as a right-handed threading screw dislocation (TSD) by synchrotron monochromatic-beam X-ray topography (SMBXT) and molten KOH etching with Na2O2 additive (KN etching), was characterized by large-angle convergent-beam electron diffraction (LACBED) and weak-beam dark-field methods. It was found that this TD was a so-called c+a dislocation with Burgers vector of b=[0001]+(1/3)[2110], which is often misinterpreted as TSD (c-dislocation) by SMBXT and KN etching. The rotation direction of the screw component within the c+a TD determined by LACBED agreed with the SMBXT observation.

Initial growth mechanism of YBCO films in liquid phase epitaxy process

Abstract The initial stage of growth for YBa 2 Cu 3 O x (Y123) films on (100) MgO substrates by liquid phase epitaxy (LPE) was investigated. In the initial stage, Y123 seed films partially dissolved into BaO–CuO solution, and Y123 islands were epitaxially grown from the undissolved seed grains on an MgO substrate within 1 s. It was found by XRD analysis that the LPE films with grain orientation of fourfold symmetry on the surface of MgO substrate were grown from not only a fourfold symmetrical seed film but also an eightfold symmetrical one. This phenomenon cannot be fully explained by the coarsening mechanism due to the curvature difference. The difference in the interface energies between Y123 and MgO should be considered to understand the preferential dissolution and growth.

Improvement of superconducting properties of SmBa2Cu3Oy films on MgO substrate by using BaZrO3 buffer layer

Superconducting transport-properties of SmBa 3 Cu 3 O y (Sm123) films on MgO single crystalline substrates prepared by pulsed laser deposition were investigated. It was found that not only T c,zero values but also J c -B properties of the Sm123 films were improved by the BaZrO 3 (BZO) buffer layer on MgO substrates. T c,zero = 93.8K, J c (77K,1T, B||c) = 7.5 × 10 5 A/cm 2 were obtained for Sm123 films with 0.93 μm thickness. Cross-sectional TEM observations revealed that the Sm123 film grown on the BZO buffer layer has higher crystallinity compared to the Sm123 film without BZO. Thus, improvement of the microstructure for the Sm123 films led to the higher superconducting properties of the Sm123 films. In addition, the effects of the BZO buffer layer on the microstructure and correlation to the superconducting properties of the Sm123 films were explained in view of interfacial energies on the hetero-epitaxial growth.

Fabrication and characterization of Nd1+xBa2−xCu3O7−y thin films deposited by metal–organic chemical vapor deposition using liquid state sources

Abstract We have fabricated Nd 1+ x Ba 2− x Cu 3 O 7− y (NBCO) superconducting thin films by metal–organic chemical vapor deposition (MOCVD) using non-fluorinated sources. At oxygen partial pressures P O 2 =0.85 Torr and P O 2 =0.65 Torr, the films with the composition deviated from the stoichiometry (Nd:Ba:Cu=1:2:3) have better crystallinity than those with the stoichiometric composition. However, at P O 2 =0.01 Torr, high quality c -axis oriented films with the stoichiometric composition are obtained. This is closely related to the Nd–Ba substitution. The films deposited at P O 2 =0.65 Torr show semiconducting properties by DC four prove method and superconducting properties by SQUID measurement. On the other hand, the films deposited at P O 2 =0.01 Torr show superconductivity by both electric and magnetic measurements. TEM-EDX observation on the films deposited at P O 2 =0.65 Torr revealed that the phase separation according to the Nd–Ba substitution was the cause of the different behaviors of electrical and magnetic properties.

FIELD-INDUCED PINNING CENTERS OF YBA2CU3O7-Y SUPERCONDUCTING THICK FILM PREPARED BY LIQUID PHASE EPITAXY

Abstract Superconducting YBa 2 Cu 3 O 7− y films were fabricated by liquid phase epitaxy (LPE) on MgO (100) and NdGaO 3 (110) single-crystalline substrates. The film of 10 μm thickness on the MgO substrate shows a high critical current density over 10 5 A/cm 2 at 77.3 K. This film includes the stacking faults that were introduced due to the large lattice mismatch between the YBCO film and the MgO substrate. We measured J c − B characteristics of this high J c and found the peak effect: enhancing J c with increasing field. On the other hand, J c of the films on the NdGaO 3 substrate is comparably low, and we have a microstructure without stacking faults. These results suggest that the stacking faults in the films on MgO substrates behave as field-induced pinning centers at 77.3 K.

Growth mechanism of thick c-axis oriented YBa2Cu3O7 − y films prepared by liquid phase epitaxy

Abstract c -axis oriented YBa 2 Cu 3 O 7 − y (Y123) thick films were grown on NdGaO 3 (110) single crystalline substrates by liquid phase epitaxy (LPE). The growth mode of the film changes from a multi-nucleation growth to a spiral-step growth. The growth of the initial stage occurs due to the local supersaturation at the growth interface by the temperature difference between the substrate and the surface of the melt and the later stage growth is driven by the supersaturation from the transport of the solute through convection of the melt. The change of growth mode is also described as the change from the transient mode LPE (TMLE) growth to the steady state growth.

Flower-like surface modification of titania materials by lithium hydroxide solution.

Surface modification of titania materials to give flower-like structures has been achieved simply by the treatment in lithium hydroxide aqueous solution under mild conditions. The flower-like structured materials were characterized by X-ray diffraction, thermogravimetric analysis, and Raman scattering. The analyses indicate that the flower-like materials are composed of layered hydrous lithium titanate. It is suggested that the unique intercalation behavior of lithium ions into titania allows dissolution and re-precipitation of titania to form the flower-like structure. The obtained flower-like structure can be retained up to 700 °C, while the crystal phase transforms into Li(4)Ti(5)O(12).