Low Critical Currents Research Articles

The Microstructure of High—Critical Current Superconducting Films

The microstructure of superconducting films that have shown high-critical current densities has been studied. The films are shown to be epitaxial and contain twins and precipitates. The main difference between these films and low current carrying samples is the absence of grain boundaries. These boundaries are therefore identified as the cause of the lower critical current in ceramic samples.

Microstructure and critical current of superconducting YBa2Cu3O7−x

In the few months that have passed since the discovery of high-transition-temperature superconducting oxides1–3, remarkable progress has been made in identifying the structure of superconducting phases, improving our understanding of factors that control their synthesis, and determining their basic properties. In one area, however, progress has been disappointingly slow; that is, in the control and optimization of the superconducting critical current density, a property vital for effective technological application of these materials. Bulk samples display a critical current that is generally very low and, furthermore, drops steeply in an applied magnetic field. There is persuasive evidence, however, that low critical currents are not an inherent characteristic of the material4,5, and recent work on thin epitaxial films6 has indicated that values as high as 106 A cm−2 can be achieved. It is also generally established that the critical current that can be carried by any superconductor depends predominantly on the local micro-structure, its degree of uniformity and the nature of its characteristic defect structures7. We present here the main results of a detailed electron microscopical study of sintered YBa2Cu3O7−x8, and discuss their relevance to observed critical current and its modification by increased silicon impurity levels9. In the light of these results we assess the prospects for controlling microstructure development so as to optimize the critical current for technological application.

Subharmonic Generation in an LC Resonant Josephson Tunnel Junction

We have measured up to 4 subharmonics at 10 GHz in an LC resonant Josephson tunnel junction by using the internal Josephson oscillator as a frequency tuned pump. The oscillator linewidth (6.5 MHz), output power (5 pW), and resonance Q-value (100) for the fundamental oscillation , n=1, agreed with the RSJ model for low critical currents. However for large critical currents and n=1 to 5, narrower linewidths (6.5 to 0.5 MHz), higher output power (5 to 60pW) and different Q's (40 to 530) were observed. These are interesting properties for oscillators where high power,narrow linewidths, and small Q's are required.

Power distribution in a Josephson package environment

An ac power supply designed to power 8K Josephson logic circuits has been tested in a package environment for the first time. The test was part of an experiment that utilized the essential package components for a Josephson computer. All accessible chip-level power signals had the predicted amplitude variation with frequency over the range 50-400 MHz when normal fabrication tolerances were taken into account. This power supply, with reduced loading to accommodate some lines with unexpectedly low critical currents, was used to power the logic circuits in a path representative of a critical path for a proposed prototype processor. A minimum cycle time of 3.7 ns was achieved.

Intrinsic noise characteristics of NbN SQUID’s

Thin-film NbN SQUID’s with granular weak links have been fabricated and their rf I-V characteristics measured at 23 MHz. Three different temperature variations of the critical currents were observed. A Josephson-like temperature dependence for those devices with strongly coupled grains and a bulk-mean-field-like dependence for those with the weakest coupled grains were found. An intermediate case where the critical current follows the bulk-mean-field theory at low critical currents and switches over to Josephson-like for larger currents was also observed. The noise properties of these devices were deduced from the measurement of the step rise parameter a. The data were in good agreement with the theory of Kurkijarvi over a wide range of temperature and critical currents for those devices with a bulk-mean-field-like critical current dependence and a low critical current density (∼104 A/cm2). Other devices were in agreement over a much smaller region of temperature and critical current. These devices are rugged, stable, and show very little change in their characteristics after extended storage at room temperature.

A paradox in the crossover of the mechanisms causing the hysteresis in long, thin-film superconductors

We point out an apparent paradox encountered in the region where the temperature of a voltage-sustaining film at the upper critical current crosses its T/sub c/ concerning the crossover of the two dominant heating mechanisms currently used to account for the lower critical currents observed on the hysteretic current-voltage characteristics of long, thin-film superconductors at varying high power levels. We explore a possible solution of this paradox by carefully examining the relevant heating models and available experimental observations.

Analysis of the ac SQUID with low inductance and low critical current

The properties of the ac SQUID magnetometer has been analyzed. The results are valid in the low-inductance low-critical-current regime, where the Lri0 producted is belowthe value at which the relation between the enclosed and externally applied magnetic dc flux becomes reentrant. The effects of the screening current circulating in the SQUID ring as well as of the SQUID-ring time constant, τ-Lr/R9 are taken into account. Here LR IS THE SQUID-ring inductance, and R is the shunt resistance in the shunted junction model assumed to describe the weak link. It is shown that for finite values of ωτ, where ω/2π is the pump frequency, the magnetometer response is due to both the flux dependence of the dispersion and f the dissipation in the tank circuit, and that, generally, the two mechanisms contribute simultaneously. Furthermore, for appropriately chosen ac bias conditions theyt may add constriuctively with the result that the optimal response occurs at a definite and finite value of ωτ. If ωτ is increased beyond this optimal value the weak link behavior is dominated by the Ohmic current channel implying that only if the shunt conductance contains a term depending on the superconducting phase-as the so-called cosine-phji term predicted by tunneling theory-the SQUID will stay sensitivity to magnetic fiels for large ωτ. The influence of the cosine-phi term has been included in the analysis, and as expected it is found to play a significant role particularly fior large ωτ. It is, however, also shown that it is nontrivial to ext ract its magnitude from experiments based on the behavior of the ac SQUID, and that the recently published observations using this technique must be reinterpreted.

Erratum: Calculation of radial magnetic fields for axisymmetric solenoids with rectangular cross section

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Tunneling-current-induced transitions of superconducting thin films

The resistive state of a superconducting film can be induced either by passing sufficient current along the film or by raising the temperature above T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> . We have been studying this transition in aluminum films when they are injected with "hot" (i.e. nonthermal) phonons and electrons. This injection efficiently breaks electron pairs in the region near the injection point. These "hot", electrons and phonons are produced by tunnel junctions which incorporate or are adjacent to the Al film under study. This method of current injection usually results in significantly lower critical currents than the critical current measured conventionally along the film. Well-defined and reproducible critical currents together with I-V traces for the Al film, which under certain conditions are characterized by a series of constant current voltage steps, have been obtained with a variety of geometrical configurations and different tunnel junction resistances.

A new superconducting switch

The sharp, reversible transition of a superconductor from the normal to the superconducting state gives it obvious switching properties. One use for such a switch would be the protection of power-system components, especially superconducting cables and generators, which are now being developed. The bulk of the paper is concerned with a preliminary design study of such a device.The required lengths of superconductor are such (similar 1 km) that burn-out at regions of low critical current (Ic) would be probable before the rising fault current in the whole superconductor reaches the average Ic and switching occurs. The proposed switch avoids this difficulty by backing the superconductor with a semiconductor which has a negative temperature coefficient of resistance in the relevant temperature range, thus preventing the catastrophic temperature rise that would occur in the normal superconductor alone because of its positive temperature coefficient. The crucial requirement of the semiconductor is that it should have, just below its melting point, the correct intrinsic resistivity, for which an expression is derived.An illustrative design for a high-power generator-protection switch is worked out, and problems and incentives are outlined.

Calculation of radial magnetic fields for axisymmetric solenoids with rectangular cross section

The ratio of maximum radial field to maximum axial field has been calculated numerically for axisymmetric solenoids of rectangular cross section and constant current density over a wide range of coil configurations. It is also shown that for a certain range of coil shapes the maximum modulus of the magnetic field occurs on the end faces of the windings. These considerations are pertinent in the design of superconducting magnets for two reasons: (i) Many superconductors have lower critical current in a field normal to the wide surface than in a field parallel to the surface, and (ii) large radial fields can cause catastrophic flux jumps.

MEASUREMENT OF TRANSITION SPEED OF JOSEPHSON JUNCTIONS

The speed of the transition between the pair tunneling state and the single-particle tunneling state of Josephson junctions is in the nanosecond region for sufficiently large junction areas and low critical currents. The transition time varies with junction capacitance and inversely with critical current. The delay is calculated to be small compared to the transition time.