S. I. Bondarenko

Freezing and quantization of current passing through a doubly connected superconductor with a point contact

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.

Study of locally frozen magnetic field in a high-Tc superconducting ceramic

The properties of a locally frozen (in a region of diameter 0.5 mm) magnetic field in a YBa2Cu3O7-x slab 0.5 mm thick are investigated as a function of the value of the excitation field, the regime of freezing, and the transport current through the sample. The first regime is cooling of the ceramic to 77 K in the excitation field with a subsequent turning off of the excitation field, and the second regime is cooling in the Earths magnetic field with a subsequent turning on and off of the excitation field. At an excitation field up to 2000 A/m in these regimes two different types of macroscopic current vortex structures, which generate the frozen field, are formed. The local critical field of excitation when the vortex structure is formed in the second regime exceeds the uniform perpendicular critical field of the slab by a factor of 10 and equals 1700 A/m. On the other hand, the vortex structure of the first type can be formed by practically any weak excitation field, including fields smaller than the critical field of the vortex structure of the second type. In a representation of the ceramic as a Josephson medium, physical models of the two types of vortex structures are proposed which correspond most fully to the results of experiments. The displacement of the vortex structure of the first type upon the passage of transport current through the slab, as a result of the action of the Lorentz force on that structure, is registered. This makes it possible to calculate the pinning force Fp and to estimate the value of the viscosity n for the motion of such a vortex structure in the ceramic: Fp=6x10-8 N, n =6x10-5 kg/s.

Measurement of energy gaps in superconductors by means of quantum interference devices

The effect of temperature on the form of discrete changes in the current in highly inductive (∼10−6 H) doubly-connected superconductors with niobium-niobium clamped point contacts is determined experimentally. The magnitude and duration of the voltage pulse on a doubly-connected superconductor is measured at the time of the discrete change in its current state. The pulse magnitude is close to the energy gap 2Δ/e of the superconductor and its duration (∼10−6 s) corresponds to the minimum possible time (∼10−12 s) for a change in the state of the contact when the depairing current through it is reached. The measurement data are discussed in terms of models of the quantum interference of currents in a doubly-connected superconductor with clamped point contacts in the form of a quantum interferometer.

Current states of a doubly connected superconductor with two point contacts

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.

Detection of self-oscillations of the transport current in a doubly connected superconductor

It has been found experimentally that when dc current is passed through a circuit consisting of two superconductors connected in parallel and reaches its critical value in one of the circuit branches the current in the branches undergoes quasi-harmonic undamped oscillations. The mechanism resulting in the appearance of the self-oscillations is discussed. The characteristics of magnetic field freezing in a circuit with self-oscillating current are examined.

Quantization of diamagnetic current in a superconducting ring with the Josephson point contact

It was established experimentally that a critical value of the diamagnetic current, excited by an external magnetic field in a superconducting ring (with an inductance of about ∼10−6 H) with a Nb-Nb clamping point contact having the Josephson contact properties is a strictly periodic function of field strength, despite the complex microstructure of the clamping contact. The reasons of the periodic dependence are discussed on a basis of the interference model of diamagnetic current and quantized values of the circulating current in the microinterferometer formed by the clamping contact.

The displacement and annihilation of macroscopic regions with hypervortices in ceramic YBa2Cu3O7−x

Features of the controllable displacement, annihilation, and transformation of macroscopic regions of a weak frozen magnetic field (FMF) have been experimentally studied in a plate of granulated ceramic YBa2Cu3O7−x under the action of the transport current flowing through it. It is shown that the displacement of regions with an FMF lower than the first critical field of the ceramic granules (Hc1g) is associated with a relatively weak pinning force of the hypervortices that is less than the maximum experimentally attainable Lorenz force. A discussion is presented of the role of the transport current that not only is one of the conditions for the appearance of the Lorenz force but also acts on the internal current structure of the local FMF, which in turn influences its mobility and magnitude. If a local field greater than Hc1g is frozen in, the experimentally produced Lorenz force is less than the pinning force of the Abrikosov vortices in the mixed state of the granules, and there is no displacement of th...

Thin film superconducting quantum interferometer with ultralow inductance

A simple method has been developed for manufacturing a thin film superconducting quantum interferometer (SQI) with ultralow inductance (~10^-13 H). Current-voltage and voltage-field characteristics of the SQI are presented. The basic design equations are obtained and confirmed experimentally. The SQI has been used for the first time to determine the penetration depth of a magnetic field into a film of 50% In-50% Sn alloy.

Interference method of definition of energy gap and relaxation time of superconducting state

A method which allows measuring simultaneously an energy gap and a relaxation time of a superconducting state is proposed.

Current states of a doubly connected superconductor with film bridges

Transport current distribution in the branches of a doubly connected superconductor in the form of a thin-film high-inductance circuit with two bridges of different width in the branches was measured. For the transport current lower than the sum of critical currents of the bridges, its distribution was found to exhibit an anomalous behavior upon reaching the critical current of one of the bridges. For a fixed value of the transport current through the circuit higher than the sum of critical currents of the bridges, low-frequency continuous harmonic voltage self-oscillations together with synchronous current self-oscillations appeared in the circuit branches. The mechanism responsible for the onset of the self-oscillations is discussed.