Katsuzou Aihara

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.

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.

Upper critical field measurements of TlBaCaCuO and (Tl/Pb)(Sr/Ba)CaCuO thin films fabricated by excimer laser ablation

We studied characteristics of the upper critical field (Hc2) of the Tl2Ba2Ca2Cu3Ox (Tl-2223) system with a Tl-O bilayer and the (Tl0.5/Pb0.5)(Sr1.6/Ba0.4)Ca2Cu3Ox (Tl-1223) system with a Tl-O monolayer. Resistivity-versus-temperature curves were measured in applied magnetic fields up to 7 T. The Hc2 values of Tl-2223, Tl-1223 at 77 K were 0.75 and 4.8 T, respectively. The Hc2 perpendicular to the a–b plane of the Tl-1223 thin film was much higher than that of the Tl-2223 thin film. It was suggested that the Hc2 decreased in proportion to increasing the thickness of the Tl O layers, which were insulators.

Magnetic Properties of the (1223) Phase of Tl-Sr-Ca-Cu-O Superconductors with Strong Pinning in Liquid Nitrogen

Pinning centers were successfully introduced into the Tl-(1223) phase of the Tl-Sr-Ca-Cu-O system using a partial melt method. The magnetization hysteresis and relaxation were studied between 10-100 K and 0-5.5 T by VSM (Vibrating Sample Magnetometer) and SQUID (Superconducting QUantum Interference Device) magnetometry. The magnetization hysteresis ΔM was 7-25 emu/cm3 at 77 K and 1 T in Tl-(1223) bulk samples. The intragrain Jc at 1 T calculated by Beans model was 43000 and 230000 A/cm2 at 77 K and 40 K, respectively. From the zero-field-cooled and field-cooled measurements, the irreversible field at 77 K was 6.8 T. The thermal activation energy was estimated to be 250 meV at 77 K and 1 T from the measurement of the magnetization relaxation.