Magnetic field and currents in HTSC films

Magnetic field measurements above type-II superconductor films in a remanent magnetization state have been used to determine two dimensional surface current distributions. Using the Biot-Savart law for the surface current density, integral equations (of 2-D convolution type) for two components of current have been obtained. These equations have been solved by employing Tikhonov's method of generalized discrepancy, and the surface current pattern in superconductors as well as the magnetic field distribution on the film surface have been obtained. Current peculiarities related to various kinds of inhomogeneities, specific character of the film form, and a laser pulse effect have been observed. In particular, it was discovered that a laser pulse can signficantly redistribute the film currents without changing the total number of current vortices in the film.     

Influence of weak visible light on flux creep in HTSC at low temperatures

We present an experimental study of flux creep in YBCO film at low temperatures (1.3 K < T < 25 K). A ring shaped film was used to obtain directly voltage–current characteristics of the film. It has been found that even very weak illumination greatly accelerates the flux creep at liquid helium temperatures. The influence of illumination is completely different from that caused by the temperature rise. We propose a simple qualitative model to explain the high sensitivity of the flux creep to weak illumination.     

Magnetic flux trapping and broken Abrikosov vortices in yttrium HTSC ceramic samples

Direct evidence of the existence of broken Abrikosov vortices is obtained from measurements of the distribution and values of residual magnetic fields in ceramic yttrium HTSC samples after the switching off of a transport current. In this case, the intergrain magnetic induction averaged over the sample volume has the same direction as the field that was in the sample before the current was switched off.     

Scaling of the current-voltage characteristics of superconducting films in the flux creep model

On the basis of the magnetic flux creep model, taking viscous vortex motion and the spatial shape of the pinning potential into account, we have constructed a model explaining the shape of the current-voltage characteristics (CVC’s) of high-temperature superconducting films and the sign change of the curvature of these characteristics with change in temperature. The model given also allows one to explain the scaling of these curves. Fiz. Tverd. Tela (St. Petersburg) 40, 989–992 (June 1998)     

Flux creep through the surface barrier and columnar defects in HTSC

The rate of flux creep over the surface barrier in clean HTSC samples is found. This rate proves to be significantly different for the cases of vortex entry and escape (relaxation “in” and “out”, respectively). Since a flat surface is topologically similar to the artificial columnar defects, their comparison is useful. The vortex creep over the surface is equivalent to that over the cylinder defect of radius r > λ, whereas the radii of real defects obtained so far are of order . We discuss the effect of the smallness of defect radius on the creep process, which results in very small activation energies U at large critical current J_c. The importance of the increase of the defect radius is predicted.     

Size effects in the nonlinear resistance and flux creep in a virtual Berezinskii-Kosterlitz-Thouless state of superconducting films

We show that the size effects radically affect the electric-field-current (E-I) relation of superconducting films. We calculate E(J) due to thermally activated hopping of single vortices driven by a current I across the film in a magnetic field H, taking into account the interaction of free vortices with their antivortex images and peaks in the Meissner currents at the film edges. The unbinding of the virtual vortex-antivortex pairs not only mimics the transport uniform Berezinskii-Kosterlitz-Thouless behavior, it can also dominate the observed E(J) and result in the field-dependent Ohmic resistance at small I. We show that E(I) can be tuned by changing the film geometry and propose experimental tests of this theory.     

Preparation of oriented thick HTSC films by cryoelectrophoretic technique

Il Nuovo Cimento D - A new technique, specifically designed for the preparation of preferentially oriented HTSC (YBCO and BSCCO) films on metallic substrates has been developed. The method, named...     

Transition from thermally activated to regular flow of magnetic flux vortices in HTSC

The effect of structural inhomogeneities in a superconductor on a vortex medium flow in weak magnetic fields at temperatures varying from 78 to 83 K for various bias current densities is investigated by using transport measurements of Bi₂Sr₂CaCu₂O_8+x thin-film microbridges. The results obtained are analyzed on the basis of the theories of flux creep and the regular flow of vortices. It is shown that the current dependences of the effective potential for vortex pinning can be satisfactorily described in the framework of two statistical models, one of which was proposed earlier by the authors. Both models cover the regimes of thermally activated and regular flow of vortices as limiting cases. The wide transition region in which the creep and regular vortex flow processes simultaneously occur due to a large dispersion in the pinning energy distribution. It is found that when the magnetic field exceeds a certain value, the average value and dispersion of the pinning potential decrease sharply, so that the conditions of regular flow set in even for small values of the bias current. This fact is attributed to the destruction of vortex lines into two-dimensional segments.     

磁链的探讨

由同轴电缆自感系数两种解法的矛盾,引入对磁链的进一步分析.通过3个例子分析磁链与磁通的关系,得出磁链与磁通不仅有整数倍的关系,也可以是分数倍的关系.因此,得出在自感系数L=Ψ/I中的Ψ是指磁场线与电流交链的物理量,不该称呼全磁通或总磁通,而应谓之磁链.     

HTSC grain boundary weak links employing high-quality ICM sputtered YBCO films

Summary Bicrystal grain boundary junctions of YBCO have been fabricated by using the Inverted Cylindrical Magnetron d.c. sputtering technique. The main characteristics of the junction behaviour are reported. Some aspects of the experimental results are discussed in the framework of existing models developed for describing weak-link structures. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.