Small Critical Currents Research Articles

EFFECT OF JOSEPHSON MEDIUM UNDER TUNNELING IN BI-SR-CA-CU-O

Tunneling characteristics of high-T c oxide superconductor Bi-Sr-Ca-Cu-O have been measured in the wide bias voltage and temperature range. Near zero bias voltage tunneling conductivity σ (U) is expressed by the U1/2 at low temperature, which goes to the linear one at U > 100 mV . Background conductivity metaloxide is determined by Shottky barrier on surface high-T c . It has been found that the peaks of σ (U) curves for low-ohmic contacts are well described theoretical model of destruction of superconductivity of the intergranular weak links. Parameters of these links are estimated. Zero bias anomaly conductivity peaks are explained by breakdown weak links with small critical current.

Josephson edge-triggered gates for sequential circuits

A Josephson sequential logic family with a very wide operating margin (+or-67%) and insensitivity to global parameter variations is proposed. Derived from the original idea of the edge-triggered latching comparator by C. Hamilton et al. (see IEEE Trans Magnetics, vol.MAG-21, p.197-9, 1985), this logic gate consists of a pair of conventional gates in series biased by a delay clock. In normal operation, switching occurs in one and only one of the gates, depending on which one has the smaller critical current. The authors have built and tested a few circuits to illustrate this logic gate design: a 32-b shift register designed by OR gates with +or-42% bias margin and +or-89% input margin, a 4-b pseudorandom sequence generator designed by exclusive-OR gates with +or-27% bias margin and +or-78% input margin, and cross section of a 6-b NOR gate decoder with +or-33% bias margin. >

Fluxon pinning on a periodical structure of magnetic inclusions in a Josephson contact

The low speed motion of a fluxon in a Josephson contact containing a periodical system of magnetic inclusions is considered. The existence of fluxon pinning at some distance from the magnetic inclusion is shown. This distance depends both on the structure period and on the parameters of the magnetic inclusions. The critical current of fluxon depinning is obtained. The existence of two different types of critical current is shown: the main critical current of the fluxon leaving the inclusion and a small critical current of the fluxon jumping between neighbouring potential minima.

White-noise measurements and Joule heating in superconducting point contacts

We report measurements of the white noise in superconducting niobium point contacts. These measurements, the junction parameters, and the method of fabrication of the point contacts are compared with those of other experiments. The power spectral density was found to have an approximately parabolic dependence on the average voltage V rather than the linear dependence expected in the shot-noise limit. We also found that the magnitude of the excess noise was larger than has been previously reported. Because of these differences, the noise data presented add to the existing body of data. The V2 dependence of the power spectral density suggests that Joule heating may have been responsible for the excess noise. Joule heating was determined to be present in the junctions from the relatively small critical currents I0 ≲2 μA and the effect that a resonant cavity had on the Josephson oscillation. However, we show that Joule heating cannot account for the level of the excess noise observed in present or previous data.

Flux creep in a polycrystalline Ba0.6K0.4BiO3 superconductor.

Magnetization curves have been measured as a function of temperature for a polycrystalline sample of ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$. These measurements reveal significant reversibility at modest fields for all temperatures studied. The irreversibility line (${H}^{\mathrm{*}}$,${T}^{\mathrm{*}}$) is determined for this material and shows that ${H}^{\mathrm{*}}$ is less than 0.8 T at all temperatures. The logarithmic magnetic relaxation rate A=dM/d ln(t) has been studied for this material, as a function of both field and temperature. Analysis using the flux-creep model reveals the average pinning energy ${U}_{0}$ to be 0.08\ifmmode\pm\else\textpm\fi{}0.02 eV. This extremely small pinning energy is used to explain the small critical current, its strong dependence on field, and the significantly reversible behavior in this material.

Superconducting properties of rapidly quenched and heat treated Zr-V and Hf-V foils

The results of rapid quenching and subsequent heat treatments on the superconducting properties of the Zr-V and Hf-V binary systems are presented. Metallic glasses can be prepared in these systems which have very attractive properties for practical application. The existence of a flexible glassy phase raises the prospect of alternative methods for the construction of superconducting devices. As-quenched, partially crystalline material is characterized by small critical currents, yet relatively large upper critical fields and transition temperatures. Heat treatments considerably enhance the current carrying capability.

On the hysteretic behaviour of a superconducting ring, containing two weak links in an applied magnetic field

A calculation is presented which describes the behaviour of a superconducting ring, containing one, respectively two weak links, with respect to the entry of applied magnetic flux. The case that no bias current is applied is considered, specially in the region of small critical current. Both situations are compared with each other and it will be shown that some important differences exist. In contrast to a single weak-link device, a symmetric double weak-link device exhibits hysteresis for all values of its critical current. The given calculations show that the double weak link system jumps from one flux state to another when the total embraced magnetic flux is equal to ( n + 1 2 .Φ 0( n = 0, 1, 2,....).

Observation of a half-quantum-flux periodicity pattern in an rf SQUID with small critical currents

We have found experimentally that rf SQUID's working in a nonhysteretic mode with small critical currents give exactly at resonance and at very low rf bias a “triangular pattern” with a period equal to half of one elementary quantum of magnetic flux. We discuss a possible explanation of this peculiar result.

Superconducting single-junction interferometers with small critical currents

A simple theory of the superconducting single-junction interferometer (ac Squid) becomes possible if the critical current of the single junction is small relative to Φ0/LR, where Φ0=h/2e is one flux quantum and LR is the inductance of the interferometer loop. This theory predicts a magnetic flux sensitivity which has the same dependence on interferometer parameters as the existing theories for higher critical currents. In contrast to those theories, however, the present theory predicts that the maximum flux sensitivity will be achieved off resonance. Theoretical predictions for comparison with experiment are displayed as a function of the normalized difference between the operating frequency ω and the resonant frequency ω0.

On the d.c. voltage across a double point contact between two superconductors at zero applied d.c. current in situations in which the junction is in the resistive region due to the circulating current of flux quantization

The relationship between current, voltage and magnetic field as a function of temperature for a double point contact between two superconductors has been investigated in more detail. Special attention has been given to its behaviour in the critical temperature region, in which the double junction is in the resistive-superconductive region due to the net circulating current (around the area between the contacts) which screens the magnetic field partially due to flux quantization. A d.c. voltage has been observed when no d.c. contact currents are applied. This voltage changes periodically as a function of an applied magnetic field of moderate strength. Double point contacts have been investigated with relatively small self-inductances and small critical currents in order to obtain a large critical temperature region in which the voltage oscillations at zero applied d.c. current can be observed. It appears that the presence of a small electromagnetic field in the radio frequency range is responsible for the observed energy dissipation, as a rectification process takes place in the double junction.