Atsuko Soeta

Introduction of pinning centers into Tl‐(1223) phase of Tl–Sr–Ca–Cu–O systems

Pinning centers were successfully introduced into the (1223) phase of Tl–Sr–Ca–Cu–O system using its derivative (Tl/Pb)–(Sr/Ba)–Ca–Cu–O with the nominal composition of (0.5/0.5)−(1.6/0.4)–2–3. The bulk sample showed a strong flux pinning characteristics. The critical current density, Jc, was obtained from the magnetization measurement. The intragrain Jc was ∼43,000 A/cm2 at 77 K and 1 T. The transport Jc of a tape with the (1223) material in core was ∼500 A/cm2 at 77 K and 5 T.

Flux pinning in single Tl-layer 1223 superconductors

Abstract The presence of pinning centers in high temperature superconductors (HTSCs) is a prerequisite for obtaining a high critical current density ( J c ) in a magnetic field. We have discovered that, perhaps more generally, HTSCs having only a single layer between the superconducting copper layers (for example the YBaCuO or TlSrCaCuO systems) can possess effective pinning centers that are absent in the double layer cases. Several samples prepared by a partial melt process, with the nominal composition (Tl 0.5 Pb 0.5 ) 1 (Sr 0.8 Ba 0.2 ) 2 Ca 2 Cu 3 O 9 , showed the high intragrain J c , by the magnetization method, greater than 2 × 10 4 A/cm 2 at 77 K and 1 T. This result suggests that more effective pinning centers exist in the (1223) phase of the modified TlSrCaCuO system (ref. [1]). We propose an explanation why both YBa 2 Cu 3 O 7 and (1223) of TSCCO contain useful pinning centers, whereas both Bi- and Tl-double-layer superconductors do not. Effective pinning, in this case greater for single layers rather than double layers, is related to the strength of the superconductive order-parameter in these layers intervening the CuO layers.

Superconductivity of Tl-Sr-Ca-Cu-O System in Relation to Tl-Ba-Ca-Cu-O and Bi-Sr-Ca-Cu-O Systems

It was found that Tl-Sr-Ca-Cu-O system gives rise to at least two superconducting phases, a high-Tc phase at 100 K and a low-Tc phase at about 75 K. The two phases are isostructural to those of Bi-Sr-Ca-Cu-O system. Comparing the Tcs and crystal structures of the Tl and Bi systems, it was suggested that Tc is mainly determined by the combination of two alkaline earth elements, i.e., Sr/Ca or Ba/Ca, when the repetition unit in the layered perovskites is equal.

Crystalline structures and superconducting properties of rapidly quenched BiSrCaCu2Ox ceramics

Phase changes and superconducting properties of rapidly quenched BiSrCaCu2Ox ceramics have been studied. The rapidly quenched sample consists of an amorphous state which begins to crystallize around 510°C. The crystal system was found to be tetragonal with a=5.39 A and c=24.5 A. By annealing above 700°C, a tetragonal system with a=5.41 A and c=30.8 A was produced. The former crystal was a semiconductor and the latter was a superconductor with Tc=75 K. A superconducting phase with Tc=105 K was observed in a sample annealed at 840°C. The superconducting volume ratio in the samples had a maximum at 803–840°C. The decrease of the volume ratio at the higher temperatures is considered to result from a decomposition of the superconducting phase (75 K) into a nonsuperconducting phase.

Relationship between crystal structures and solid solution of Tl-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O superconductors

We report on the superconductivity and crystal structures in Tl-(Ba, Sr)-Ca-Cu-O (TBSCC) system. In the case of TBSCC system, the 2223 phase was obtained as a single phase more easily compared with the Tl-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems. As a result, we achieved a high superconducting volume fraction, 95% above 100 K. The crystallographic transformation was caused by changing the ratio of Ba to Sr. By the X-ray diffraction and TEM analysis, the crystal structure was determined as a 2223 structure consisting of Tl-O bilayers in the Ba-rich side, and a 1223 structure consisting of Tl-O monolayer in the Sr-rich side.

Flux pinning in TI-(1223) superconductor

Abstract A strongly pinned (TI/Pb)-(Sr/Ba)-CaCuO bulk superconductor of TI-(1223) phase with a single TI-O plane has been developed with a magnetic J c over 43 000 A cm −2 at 77 K and about 600 000 A cm −2 at 20 K and 1 T. Single crystals of TI-(1223) have been prepared and used for investigations of magnetization hysteresis anisotropy using SQUID. When the field was applied parallel to the c axis, a high field peak related to inhomogeneous stoichiometry and properties across the specimen was observed in the magnetization hysteresis loop around 1 T at 77 K. Ag-sheathed tapes with TI-(1223) core were fabricated by the drawing-rolling method. The transport J c was 25 000 A cm −2 in the absence of a magnetic field, and stayed - 500 A cm −2 up to 8 T at 77 K.

Magnetization and anisotropy in single crystals of Tl-(1223) phase of Tl-Sr-Ca-Cu-O system

The magnetization and intra-grain critical current density of Tl-(1223) phase of Tl-Sr-Ca-Cu-O system were investigated using single crystals. The intragrain Jc deduced from the magnetization was found to be 2-8×104 A/cm2 at 77 K and 1 T. The irreversibility field H* was estimated to be ~2 T for H//c-axis and >5 T for H⊥c-axis at 77 K.

Existence of Superconducting States Above 30 K in Sr-V-O Systems Doped with Various Elements

Electrical properties of the Sr-V-O systems doped with Tl, La, and Ce were extensively studied. The Sr-V-O system was prepared in the reduced form, for example, SrVO3-δ, and was doped with Tl. In the Tl-Sr-V-O system a sharp drop in the resistivity was observed at around 30 and 130 K in several samples. A diamagnetic signal was clearly observed below 30 K. The superconducting states of the Tl-Sr-V-O system are found to be metastable in the ambient atmosphere. It is suggested that there are at least two superconducting phases or crystal structures giving rise to the Tcs. The highest onset temperature observed for the sharp resistivity drop was 160 K.

Surface atomic images of superconducting (Tl, Pb)Sr2Ca2Cu3O9

Abstract The surface structural chemistry of the high- T c superconductor, (Tl, Pb)Sr 2 Ca 2 Cu 3 O 9 (Tl-1223), has been investigated by high resolution electron microscopy. Clean and flat (114) and (001) surfaces were observed using a profile imaging technique after electron beam irradiation. The (001) surface was found to be terminated with a (Tl, Pb)O layer and to be stepped, consisting of different members of the (Tl, Pb)Sr 2 Ca n −1 Cu n O 2 n +3 family with n =1, 2, 3 and 4. The differences between the surface structures of Tl-1223 and Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223) are discussed.

Introduction of pinning centers into TlSrCaCuO systems

Abstract Pinning centers were successfully introduced into the (1223) phase of TlSrCaCuO system using its derivative (Tl/Pb)(Sr/Ba)CaCuO with a nominal composition of (.5/.5)−(1.6/.4)−2−3. The bulk sample showed a strong flux pinning characteristics. The critical current density, Jc, obtained from the magnetization measurement was > 18,000 A/cm 2 at 77K and 1 T. A tape with the Tl-(1223) material in core had an inter-grain (transport) Jc ∼ 500 A/cm 2 at 77K and 5 T.