Quasi-2D behaviour of HTSCs: the consequence of the small effective thickness of superconducting layers

Various aspects of the problem of current transfer in high- superconductors (HTSs) are reviewed. The spatial inhomogeneities of various types are identified as a primary cause of non-uniformity of both normal currents and supercurrents in real samples of HTSs. The role these inhomogeneities play in transport features of the samples is discussed. The case of grain boundaries in polycrystalline samples is elaborated in detail. The local structural and transport properties of isolated grain boundaries are first reviewed and then integrated into the knowledge of global (macroscopic) charge transport. The paper emphasizes the common ingredients characterizing the transport in various forms and families of HTS samples in small magnetic fields. The phenomenon of percolation is identified as the most obvious one and is shown to dominate a large number of observations covered by this report. The experimental results focused on by this report elaborate primarily the problems of critical currents, initial dissipation and current-voltage characteristics, penetration depth, resistive and metal-insulator transition, resistance noise and magneto-optical studies of current paths. Various models for current transfer (disordered bonds, brick wall and railway switch) are also reviewed and discussed.

Section: Initial Dissipation and Current-voltage Characteristicsmentioning

confidence: 99%

Current transfer and initial dissipation in high- superconductors

Prester

1998

“…Figure 4(a) shows the separate melting lines for low-number stacks obtained from AC measurements on a deformed Bi2223/Ag tape. Each melting line starts at H = 0 from the temperature of the Kosterlitz-Thouless transition (T KT ) [16] and extends down to T KT of the next lower stack. In the intact (nondeformed) tape, a finite Josephson interaction (JI) between the stacks modifies the presented lines, reducing the rate of T m decrease to the lower T KT (shown by broken lines in figure 4(b)).…”

Section: Ac Magnetization Data Four Cross-sections Of the J C (H T )mentioning

confidence: 99%

Origin of vortex melting line in Bi2223/Ag tapes

Mikheenko

Dou

1998

Self Cite

Using AC diamagnetic and transport measurements we have derived a three-dimensional diagram for high-quality Bi2223/Ag tapes. The diagram shows an abrupt decrease of critical current caused by the melting of a vortex glass. The melting line was defined as the contour line for the surface at , where showed the steepest decrease with H and T. This line coincided with the locus of the maximum, at a frequency Hz and an AC field amplitude Oe. The melting line reflects the process of the separation of unit-cell stacks. The most important are the low-number stacks that contain one to three unit cells. These stacks have the lowest Kosterlitz-Thouless transition temperatures and the lowest corresponding vortex melting temperatures. A magnetic field causes the separation of the stacks and reduces in each stack to the melting temperature of the lower stack. At the universal , the vortex melting line is the combination of for different, mainly low-number, stacks.

“…The magnetic field destroys superconducting correlations between the stacks and they then act independently. Among these stacks, a separated unit cell, a stack of two unit cells and stacks of more than two unit cells (they can be united to form one group) have very different properties [8,9] including different sensitivities to magnetic fields. If those stacks define the behaviour of the critical current, the weak and strong links are simply stacks of different thickness.…”

Section: Introductionmentioning

confidence: 99%

“…The most prominent difference between low-number stacks is in their Kosterlitz-Thouless transition temperature (T KT ) [8,9], that is in the threshold temperature below which 'real' superconductivity with zero resistance, finite diamagnetic response and finite critical current appears. As was determined by the transport and a.c. measurements [9,11], isolated layers have T KT of 20, 31 and 24 K for YBa 2 Cu 3 O x , Bi 2 Sr 2 Ca 2 Cu 3 O x and Bi 2 Sr 2 Ca 1 Cu 2 O x respectively.…”

Section: Introductionmentioning

confidence: 99%

Grain-boundary links and critical current of tapes

Mikheenko

Horvat

et al. 1997

Self Cite

The nature of grain-boundary links in high-quality Bi 2 Sr 2 Ca 2 Cu 3 O x tapes is clarified. The investigations focus on the role that various types of links play in the restriction of the critical current. The multiple exponential function is found to be a good approximation for the magnetic-field dependence of critical current I c (H ). The behaviour of I c (H ) at different temperatures gives clear evidence of the two-dimensional character of critical current. It is shown that at all temperatures higher than the Kosterlitz-Thouless transition temperature of isolated superconducting layers the critical current is totally defined by the Josephson interaction between the layers.