Abstract: A superconducting device has a structure of superconductor - normal-conductor (semiconductor) - superconductor. The superconductors constituting the superconducting device are made of a super-conducting oxide material of K.sub.2 NiF.sub.4 type crystalline structure or perovskite type crystalline structure which contains at least one element selected from the group consisting of Ba, Sr, Ca, Mg and Ra; at least one element selected from the group consisting of La, Y, Ce, Sc, Sm, Eu, Er, Gd, Ho, Yb, Nd, Pr, Lu and Tb; Cu; and O.

Abstract: A process for controlling an oxygen content of a non-superconductive or superconductive oxide is provided, in which a beam of particles such as ions, electrons or neutrons or an electromagnetic radiation is applied to the non-superconductive or superconductive oxide of a perovskite type such as YBa.sub.2 Cu.sub.3 O.sub.7-x, thereby increasing or reducing the oxygen content of the oxide at the sites of oxygen in the crystal lattice of the oxide. Furthermore, a superconductive device such as a superconductive magnet, superconductive power transmission wire, superconductive transformer, superconductive shield, permanent current switch and electronic element is made by utilizing the process for controlling the oxygen concentration of the superconductive oxide.

Abstract: An oxide-superconductor improved so as to have a high critical temperature (T.sub.c) comprises an oxide having a K.sub.2 NiF.sub.4 crystalline structure similar to a perovskite crystalline structure and represented by the following formula:(Ba.sub.x Sr.sub.z La.sub.1-x-z).sub.2 Cu.sub.1-w Ag.sub.w O.sub.4(1-y)wherein 0.1<x+z<0.3, w=0, and y.gtoreq.0; or 0.ltoreq.x<1, 0.ltoreq.z<1, 0<w<1, and y.gtoreq.0. This invention also relates to an oxide-superconductor wherein an Na, K, Rb, Cs or F atom is contained in an oxide-superconductor of A.sub.3 Cu.sub.3 O.sub.7 type having at least one of a CuO.sub.6 octahedron and a CuO.sub.5 pentahedron.

Abstract: A SQUID for detecting a weak magnetic field is constructed of a sensor coil which detects a signal magnetic flux, and a superconducting loop which is magnetically coupled with the sensor coil to generate a periodic voltage corresponding to an interlinking magnetic flux from the sensor coil. The periodic voltage from the SQUID is amplified, and is taken out. The periodic voltage taken out is synchronously detected by a phase-sensitive detecting amplifier. The synchronously-detected signal is converted into a light signal, which is fed back to the SQUID. The light signal fed back is converted into an electric signal by a superconducting phototransistor which is arranged in a cryogenic atmosphere. A magnetic flux corresponding to the electric signal from the superconducting phototransistor is fed back to the superconducting loop by a feedback coil.