Review of stability in high temperature superconductors with emphasis on flux jumping

We have experimentally studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magnetic response has been investigated as a function of the dimensions of the tube, the magnitude of the applied field and the frequency. These results are explained quantitatively by employing the method of Brandt (1998 Phys. Rev. B 58 6506) with a Jc(B) law appropriate for a polycrystalline material. Specifically, we observe that the applied field can sweep into the central region either through the thickness of the shield or through the opening ends, the latter mechanism being suppressed for long tubes. For the first time, we systematically detail the spatial variation of the shielding factor (the ratio of the applied field over the internal magnetic field) along the axis of a high-temperature superconducting tube. The shielding factor is shown to be constant in a region around the centre of the tube, and to decrease as an exponential in the vicinity of the ends. This spatial dependence comes from the competition between two mechanisms of field penetration. The frequency dependence of the shielding factor is also discussed and shown to follow a power law arising from the finite creep exponent n.

Magnetic shielding properties of high-temperature superconducting tubes subjected to axial fields

We have studied mesoscopic Josephson junctions formed by highly n-doped InAs nanowires and superconducting Ti/Pb source and drain leads. The current–voltage properties of the system are investigated by varying temperature and external out-of-plane magnetic field. Superconductivity in the Pb electrodes persists up to ∼7 K and with magnetic field values up to 0.4 T. Josephson coupling at zero backgate voltage is observed up to 4.5 K and the critical current is measured to be as high as 615 nA. The supercurrent suppression as a function of the magnetic field reveals a diffraction pattern that is explained by a strong magnetic flux focusing provided by the superconducting electrodes forming the junction.

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Pb/InAs nanowire josephson junction with high critical current and magnetic flux focusing.

The magnetic shielding properties of YBa 2 Cu 3 O 7− x coatings, sprayed on silver cylinders with the continuous detonation spray (CDS) technique, have been investigated in weak magnetic fields. Extended films 40–80 μm thick showed flat-sided sharp-cornered hysteresis loops. Critical penetration field values, when scaled for thickness according to Bean's model were 1.6–3.7 mTmm -1 at 77.3 K and 21–35 mTmm -1 at 4.2 K. In the actual samples, J c was calculated to be 0.95−1.4×10 3 A cm -2 at 77.3 K and 10−12.5×10 3 A cm -2 at 4.2 K. Relaxation rates S = 11−23 ×10 -3 were measured for the flux creep at 77.3 K, and were found to be much lower at 4.2 K. The CDS technique allowed one to have precise control of the film thickness, a high coating productivity and a relatively low (∼ 1000°C) processing temperature.

Magnetic shielding properties of YBa2Cu3O7-x thick films deposited on silver cylinders with the continuous detonation spray technique

Experiments with fluxons moving in long Josephson junctions of different geometries are reviewed. A controllable way of introducing fluxons one by one into annular Josephson junctions has been realized. This allows a comparison with existing fluxon chain perturbation theory. Several new millimeter-wave stimulated regimes found in linear junctions are discussed. Large asymmetric zero-crossing steps of constant voltage have been observed in the I-V curves. These steps may have a potential application in Josephson voltage standards. Large narrowband radiation emitted from the flux-flow oscillator in the 62-77-GHz frequency range has been detected. >

Multi-fluxon effects in long Josephson junctions

High‐Tc superconducting tubes have been developed for magnetic shielding of SQUIDs at 77 K. The characteristics of such tubes show that adequate shielding exists for magnetic fields up to a critical field determined by the current density of the tube. Relaxation phenomena with two different time dependencies are observed when the external magnetic field enters the tubes.

Magnetic shielding and relaxation characteristics of superconducting YBa2Cu3O7 tubes

Magnetic shielding properties of YBaCuO tubes have been studied at temperatures of 77 K and 4 K. Shielding effectiveness was determined by measuring the magnetic field inside the tube in the presence of external magnetic fields. A high degree of shielding was achieved up to a critical external field determined by the critical current density of the material. Typically, this critical field is 23 Oe at 77 K extending up to 105 Oe at 4 K. Tubes made with material containing 10% silver exhibited much stronger flux pinning behavior when the external magnetic field started to penetrate the tubes. The tubes studied were effective in shielding a RF-biased SQUID (superconducting quantum interference device) for operation at 77 K.

Characteristics of YBaCuO magnetic shields

The noise characteristics and chaotic behavior of long Josephson junctions in a magnetic field are investigated. In a magnetic field, current-biased junctions exhibit the dynamics of fluxon motion affected by fluctuations. These consist of telegraph noise at voltage steps and instabilities due to chaotic behavior. Results on long junctions with McCumber number ranging from 10 to 100 show such behavior. The telegraph noise is driven by thermal fluctuations. Modeling of the junctions using a perturbed sine-Gordon equation shows chaotic and periodic regions. Some of the chaotic regions are between Fiske steps, while others are at the edge of the Fiske steps. These observations are confirmed by numerical calculations. >

Noise characteristics and instabilities of long Josephson junctions

Studies of long NbN Josephson junctions of the overlap type in the presence of an external magnetic field show constant voltage steps of the Fiske type, and large voltage steps. The latter are attributed to multifluxon resonances within the cavity of the junction. The multifluxon steps correspond to bunches of fluxons introduced in the junction because the junction electrode thicknesses are comparable to the penetration depth. These steps are very sharp, and they have large current amplitudes. When biased at such constant voltage steps, the device can be a resonant mode oscillator. The power and frequency are determined by the mode of fluxon motion inside the junction.

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Multi-fluxon steps in long Josephson junctions and their application to oscillators

Results are presented on the behaviour of a simple RF-biased SQUID (superconducting quantum interference device) made out of YBa/sub 2/Cu/sub 3/O/sub 7/. It consists of a fractured ceramic pellet stuck together at room temperature. This simple device is shielded by a high-T/sub c/ tube and shows the characteristic flux quantization behaviour up to 77 K. At 4.2 K its magnetic flux resolution is less than 2*10/sup -4/ phi /sub 0// square root Hz, and it shows sensitivity degradation as the temperature is raised. >

W.J. Yeh

Papers

Magnetic shielding and relaxation characteristics of superconducting YBa2Cu3O7 tubes

Characteristics of YBaCuO magnetic shields

Noise characteristics and instabilities of long Josephson junctions

Multi-fluxon steps in long Josephson junctions and their application to oscillators

High T/sub c/ RF-biased SQUID