The particulars of dc current passage through a structure consisting of a doubly connected superconductor (DCS) with branches that are asymmetric with respect to length and critical current have been investigated experimentally. The short branch, which has the lowest critical current, was a clamping niobium-niobium point contact with length comparable to the coherence length of the superconductor. In contrast to a previously studied DCS with a short branch much longer than the coherence length, it was found that when the short-branch current reaches the critical value the currents in the branches of the DCS do not undergo self-excited oscillations; a current exceeding the critical value enters the long branch when this current is increased in portions (is quantized), and when it is subsequently decreased it freezes partially or completely in the DCS circuit.

This review article is a commemoration of the 30th anniversary of the discovery of YBa 2 Cu 3 O 7-d high-temperature superconductors (HTSCs). As a result of this discovery a family of (RE)Ba 2 Cu 3 O 7-d (RE stands for "rare earth") HTSCs has found great practical use. The review article consists of a brief history of how YBa 2 Cu 3 O 7-d was conceived and five sections describing the family of compounds: crystallography, phase diagrams, manufacturing techniques, main superconducting properties, and fields of application.

The distribution of the transport current in branches of a doubly connected superconductor (DCS) with two clamping point contacts is investigated experimentally when the contacts are in the different states: (a) one or both contacts are in the critical state, (b) both are in the supercritical (resistive) state. In the state (a) the transport current frozen in the DCS and injected through one of the contacts can be increased or decreased by tuning the value and polarity of the current passing through the other contact. In the state (b), the current self-oscillations the amplitude and the frequency of which depend on the value of the injected transport current appear in the branches of the DCS. A role of the parametric Josephson inductance and resistivity of the contact in formation of its critical state and in distribution of the current in the branches of the DCS is discussed.