Granular High-temperature Superconductors Research Articles

The equilibrium magnetization of granular high temperature superconductors in low fields: II. The model

In the spontaneous flux expulsion of granular high temperature superconductors, only 1 3 of the full Meissner effect is observed below H c1, followed by full expulsion at H< H m⪡ H c1. These deviations from the standard behavior of type-II superconductors are shown to be equilibrium effects caused by intrinsic and defect barriers with low conduction electron density. The influence of pinning is negligible. H m is not an intrinsic property. Average sizes of the superconducting blocks between and of the Josephson loops around defect barriers can be extracted from the anomalous temperature and field dependence of the spontaneous flux expulsion.

Concentric superconducting shells with weak links in an external magnetic field

A model calculation for a system of concentric superconducting shells with one weak link in each shell is considered to help understand the behavior of granular high-temperature superconductors. In this type of superconducting material the contacts between grains often resemble the weak links in classical superconductors. The behavior of a system of concentric superconducting shells, containing weak links, in an external magnetic field is studied. The dependence of inner magnetic flux on external magnetic flux is calculated, as well as the distribution of magnetic field in the space between the shells. An additional modulation appears in the phi /sub i/ vs. phi /sub ex/ curve. The modulation period depends on the spacing between the shells but not on the number of shells. The cooperative behavior of the system of shells is considered as well.

The non-ohmic behaviour of bulk high temperature superconductors

The current voltage characteristics of granular high temperature superconductors become non-ohmic in a way which depends on both the temperature and the magnetic field. We have studied this behaviour in the lead stabilized 2:2:2:3 phase of bismuth strontium calcium copper oxide and have found that there are some samples where a single family of V( I) characteristics may be swept through either by changing the temperature or by changing the magnetic field. The magnetic field and temperature dependence of these materials can thus be described by a single magnetic field and temperature dependent parameter. This result spans both the low magnetic field regime (fields less than one flux quantum per grain boundary) and high magnetic fields and both the region of the characteristics where log( V) varies linearly with log( I) and the region of non-linear behaviour. The dissipation is almost independent of the angle between the magnetic field and the measurement current. These results impose a number of severe constraints upon possible theories for the dissipation.

Frequency dependence of flux profile in ceramic superconductors

We have observed the frequency dependence of the so called flux profile in two granular high temperature superconductor ceramics at liquid nitrogen temperature. We show that the penetration depth decreases while the granular fraction and intergrain (or matrix) critical current increases with frequency. The magnitude depends on the granularity of the sample. We explain such features along the lines of the thermally assisted flux creep model.

Dynamical Properties of Weakly Coupled Josephson Systems

ABSTRACTWe review recent work on the dynamical behavior of coupled resistively-shunted Josephson junctions, with emphasis on our own calculations. We present a model which allows for the inclusion of finite temperature, disorder, d.c. and a. c. applied currents, and applied magnetic fields. We discuss applications to (a) calculations of critical currents and IV characteristics; (b) harmonic generation and microwave absorption by finite clusters of Josephson junctions; (c) critical energies for vortex depinning; and (d) quantized voltage plateaus in arrays subjected to combined d.c. and a. c. currents. Possible connections to the behavior of granular high-temperature superconductors are briefly discussed.

Detection of weak link superconductivity by magnetically modulated electrical resistance

Abstract A new technique, based on observing the effect of magnetic field modulation on the DC resistance of superconductors, sensitively detects the presence of weak links in granular high temperature superconductors. The phase detected response to the magnetic modulation shows a peak at T c for both metallic and ceramic superconductors, analogous to that observed in magnetically modulated microwave absorption. A second peak, attributed to weak links, is observed below T c in the ceramic oxide superconductors when tailing is present in the resistance vs. temperature curve.

Magnetic-history-dependent critical currents in granular high-temperature superconductors: Dependence on defect structures and morphology

Critical current densities, Jc’s, in conventional low temperature superconductors are controlled by defect structures that provide pinning for the magnetic flux lattice. High critical current densities and strong pinning have been observed in single crystals of the new oxide, high temperature superconductors (HTS) and have been ascribed to the presence of twin planes and to stoichiometry variations in the YBCO compound. Neutron irradiation studies have shown that intragranular Jc’s in YBCO can be strongly enhanced by the point defects introduced by irradiation. However, transport Jc’s determined by intergranular current transfer have remained dissappointingly low in bulk ceramic HTS materials. This has suggested a model of strongly superconducting grains separated by “weak links” that behave as Josephson tunnel junctions.The extraordinarily small superconducting coherence length ζo ∼ 10 A° in the HTS make them considerably more sensitive to small scale defects. Grain boundaries that act as pinning centers in conventional superconductors thus become barriers to current flow in the HTS.

Reduced fluxon viscosity in a Josephson junction

Fluxon viscosities within weak Josephson junctions are orders of magnitude smaller than in bulk. Highly mobile fluxons within weak junctions, driven by surface microwave currents, are responsible for the large low-field microwave absorption observed in the granular high-temperature superconductors. While typical fluxon displacements in bulk are in the Angstrom range, displacements in weak Josephson junctions are in the micron range.