Yuhei Shimizu

Luminescence and Valence of Tb Ions in Alkaline Earth Stannates and Zirconates Examined by X-ray Absorption Fine Structures

The difference in Tb3+ green luminescence intensities in doped perovskite(ABO3)-type alkaline earth stannates, AeSnO3 (Ae = Ca, Sr, Ba), and the Mg codoping effect on the luminescence intensities in doped CaMO3 (M = Sn, Zr) were investigated utilizing the X-ray absorption fine structures (XAFS) of the Tb LIII absorption edge. It is considered that the local symmetry at A sites is responsible for the different Tb3+ luminescence intensities in AeSnO3 (Ae = Ca, Sr, Ba) doped with Tb ions at A sites. However, it was found from the XAFS spectra that some Tb ions are unintentionally stabilized at B sites as Tb4+, especially in BaSnO3. Not only the central symmetry for Tb3+ at A sites but also the presence of Tb4+ at B sites were considered to bring about the absence of Tb3+ luminescence in doped cubic BaSnO3. No obvious changes in the Tb3+ local structure at A sites were detected between Tb single doped and Tb-Mg codoped CaMO3 (M = Sn, Zr) from the extended XAFS oscillation, but the trace of Tb4+ at B sites in the Tb single doped sample was observed in the X-ray absorption near edge structures. It is, therefore, considered that the Tb3+ luminescence enhancement by Mg codoping is primarily attributed to the charge compensation rather than the changes in the local structure around Tb3+ at A sites.

Growth and superconductivity of niobium titanium alloy thin films on strontium titanate (001) single-crystal substrates for superconducting joints

Aiming to introduce NbTi alloy superconducting joints for REBa2Cu3O7−δ (REBCO, RE: rare-earth element) superconducting wires, NbTi alloy thin films were deposited at room temperature on SrTiO3 (STO) (001) single-crystal substrates, which have a high lattice matching with REBCO (001). The strain, crystallinity, surface morphology, and superconducting property of the films with various thicknesses were investigated. The NbTi films grew in the orientation with (110)NbTi//(001)STO:[001]NbTi and [11–0] NbTi//[100]STO; that is, the NbTi lattices had two directions in the (110) of NbTi. The strain decreased and the crystallinity improved as the film thickness increased. The films were found to crystallize immediately at the interface between the films and substrates by cross-sectional scanning transmission electron microscopy. The flat surfaces of the films have mesh-like morphologies due to the growth of elongated NbTi grains along the [100] and [010] of the STO, reflecting the in-plane two directions of the NbTi lattices. The superconducting transition temperature of the films increased with improvement in the crystallinity of the films. The preparation of superconducting NbTi alloy thin films with sufficient crystallinity at room temperature suggested the possibility of forming the films on REBCO and the applicability of the films as superconducting joints.

Preparation of YBa2Cu3O7-δ and La1.85Sr0.15 CuO4 Bilayer Structure for Superconducting Connection

To connect superconducting wires through a different superconductor, it is essential to evaluate a range of characteristics of the various superconducting materials. In this study, a combination of YBa₂Cu₃ O_7-δ (YBCO) and La_1.85Sr_0.15CuO₄ (LSCO), which are typical high-temperature superconductors, was investigated. An LSCO/YBCO layered thin film (LSCO/YBCO bilayer) was prepared and its crystallinity, surface morphology, and electrical properties were evaluated. The LSCO thin film grew epitaxially, oriented to the (00<italic>l</italic>) plane on the (00<italic>l</italic>)-oriented YBCO thin film due to the small lattice mismatch between YBCO and LSCO. The surface flatness of the LSCO/YBCO bilayer was almost the same as that of the LSCO thin film, because the LSCO thin film grew without roughening the surface of the YBCO thin film. The electrical resistivity of the LSCO/YBCO bilayer decreased at the individual superconducting-transition temperatures of YBCO and LSCO, and eventually became superconducting at 20 K. The superconducting properties of YBCO and LSCO were almost preserved, with the result that a superconducting connection between YBCO and LSCO was achieved.