Tsutomu Nojima

Superconductivity in YBa2Cu3-yNiyO7-δ

Superconductivity in alloyed compounds YBa2Cu3-yNiyO7-δ has been studied in order to gain direct insights on the roles different Cu sites play in electrical conduction. Electron probe microanalysis as well as X-ray powder diffraction analysis shows that Ni is actually substituted for Cu in the oxygen-deficient perovskite compound for y at least as much as 0.27. Electrical resistivity measurements revealed that the substituted Ni does not substantially destroy the superconductivity.

Universality of magnetization curves in superconducting Bi-Sr-Ca- Cu-O and Y-Ba-Cu-O films

Abstract Magnetization measurements have been performed on c-axis oriented films of Bi2Sr2CaCu2O8 and YBa2Cu3O7−δ in wide ranges of the temperature T and magnetic field H. For fields along the c-axis, we found a universal relation between the reduced quantity of magnetization hysteresis m=ΔM/ΔM0 and the reduced field h=H/H ∗ , where ΔM0 is the hysteresis of magnetization ΔM at H=0 and H∗ is the irreversibility field. The relation is practically independent of temperature and materials investigated. The hysteresis m decreases exponentially with increasing h except h∼1, while the field dependence deviates from the exponential decay for h∼1 and m=0 at h=1. This dependence of m can be interpreted within the framework of the flux creep model in wh ich the logarithmic current dependence at high current density and the linear current dependence at low density are assumed for the effective pinning potential.

Superconductivity in modified systems based on YBa2Cu3O7−δ

Abstract In order to study the impurity effects and the different role each of the two distinctive Cu sites plays, superconductivity in YBa 2 (Cu 1− x M x ) 3 O 7−δ was investigated with various 3d and other elements M substituted for Cu. An orthorhombic to tetragonal phase transition is induced by M = Fe, Co and Ga. With M = Fe, it is induced at x c ≈ 0.02 at room temperature. The resistivity and magnetic susceptibility measurements support the bulk nature of superconductivity in the impurity-induced tetragonal phase.

A Universal Relation for Magnetization Curves in High-Tc Superconducting Films

Magnetization M was measured for c-axis oriented films of Bi2Sr2CaCu2O8 and YBa2Cu3O7−δ at various temperatures T and fields H. For H along the c-axis, we found a universal relation between ΔM/ΔMo and H/H*, where ΔMo is the hysteresis of magnetization ΔM at H = 0 and H* is the irreversibility field. The relation is almost independent of temperature and materials investigated. The result will be discussed within the framework of the flux creep model.

Magnetization curve and its relaxation of Bi-Sr-Ca-Cu-O films

We have measured magnetization and its relaxation of Bi-Sr-Ca-Cu-0 oriented films for wide ranges of temperature and field. The magnetization curves measured in the field along c-axis exhibit that the field of irreversibility limit, H 0 (T), deviates from a (1-T/T c ) 3/2 dependence below 40 K. The magnetization decays logarithmically with time in relatively weak field, whereas the simple logarithmic decay doesn’t hold in fields near H 0 .

Effects of plasma oxidation on transport properties of sputtered Y-Ba-Cu-O films

Effects of plasma oxidation on superconducting and normal properties were studied for rf-sputtered Y-Ba-Cu-O films. The plasma oxidation was performed in a mixture gas of Ar-50%O2 both before and after annealing. The oxidation enhances superconducting transition temperature, Tc, and improves metal-like behavior of normal resistance as well. Typical films obtained after the oxidation exhibited the transition with an onset at 94 K and with zero resistance below 82 K. The improvement was found to hold for more than 30 days after oxidation.