R.C. Surdeanu

Fast determination of 2D current patterns in flat conductors from measurement of their magnetic field

Abstract An extremely fast method is presented to calculate the local current–density vector in a flat conductor from the z -component of the magnetic field measured above its surface, e.g. by means of magneto-optical indicators, Hall-probe arrays or scanning SQUIDs. The method may be used for samples of arbitrary thickness provided that the current vector has only x - and y -components. The method combines the conjugate gradient (CG) method and fast Fourier transform to invert the relevant Toeplitz matrix equation. For a current map of n × n pixels, the number of operations needed is of order n 2.8 only, compared to n 4.5 or higher for earlier methods. The increase in speed for 512×512 pixels is found to be a factor 135 with respect to the fastest existing CG method.

Comparative study of global and local magnetization measurements on single crystalline high-Tc superconductors

The mixed state of several RE Ba Cu O single crystals REs Y,Yb was investigated by various measuring 23 7 yd

Pattern formation due to non-linear vortex diffusion

Abstract Penetration of magnetic flux in YBa 2 Cu 3 O 7 superconducting thin films in an external magnetic field is visualized using a magneto-optic technique. A variety of flux patterns due to non-linear vortex diffusion is observed: (1) Roughening of the flux front with scaling exponents identical to those observed in burning paper including two distinct regimes where respectively spatial disorder and temporal disorder dominate. In the latter regime Kardar-Parisi-Zhang behavior is found. (2) Fractal penetration of flux with Hausdorff dimension depending on the critical current anisotropy. (3) Penetration as ‘flux-rivers’. (4) The occurrence of commensurate and incommensurate channels in films with anti-dots as predicted in numerical simulations by Reichhardt, Olson and Nori. It is shown that most of the observed behavior is related to the non-linear diffusion of vortices by comparison with simulations of the non-linear diffusion equation appropriate for vortices.

Magneto-optical observation of the influence of an artificial periodic magnetic pattern on the pinning of a YBa2Cu3O7−δ thin film

Abstract In a recent paper [1], it has been shown from transport measurements that artificial reversible pinning can be created by the application of a magnetic tape on the surface of a thin superconducting film. We study the influence of a periodic array of magnetic stripes, generated from a signal pre-recorded on a magnetic tape, on the pinning of vortices. With our high resolution magneto-optical technique [2], we visualize in two dimensions the penetration of the flux in a thin YBa 2 Cu 3 O 7−δ film partially covered by such a magnetic tape. In particular, we investigate the evolution of flux entry in the superconducting strip under and next to the magnetic tape. We present results obtained for various temperatures and various applied fields.

Relation between micro-structure and transport properties of epitaxial YBa2Cu3O7−δ thin films

Abstract In order to understand the origin of the high critical current densities observed in thin superconducting films compared to single crystals we measured in several epitaxial YBa 2 Cu 3 O 7−δ thin films the magnetic field and temperature dependence of the superconducting current density j s and the dynamical relaxation rate Q ≡ d ln j s / d ln( d B/ dt ). For all samples we found that (i) the superconducting current density j s is independent of magnetic field up to a certain field B Φ . The value of this field, which is sample dependent, decreases with increasing temperature; (ii) for fields B>B Φ , the superconducting current density falls off as B −1/2 over several decades; (iii) Q exhibits a sharp decrease for B B Φ and (iv) Q is essentially constant for B > B Φ . These results can be understood in terms of strong pinning. Based on structural information obtained by AFM and STM we propose a model to explain both the high value and the magnetic field dependence of j s and Q .

Measurement of local currents in superconductors using an in-situ high field magneto-optical microscope

A contactless method is used to measure the local current density in superconductors at fields up to 7 T with a spatial resolution better than 1 {mu}m. The local currents are derived from magneto-optical images using a powerful inversion scheme. The interpretation of global experimental data on flux flow, flux creep and relaxation phenomena is hindered if the magnetic moment of the sample as a whole is measured, while the current flow in the sample is not uniform. This problem can be overcome by the local measurement of the currents flowing in the sample that is discussed below.

Bulk pinning by planar defects in Ti-doped Bi-2212 single crystals

Abstract We demonstrate that Ti-doping in Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) single crystals results in significantly enhanced pinning properties due to the formation of a peculiar planar defect structure induced by the dopant.

Crossover from fractal to non-fractal flux penetration in vicinal Tl2Ba2CuO6+δ thin films

Abstract The role of anisotropy on magnetic flux penetration and critical current density j c in c-axis epitaxial Tl 2 Ba 2 CuO 6+δ (Tl2201) films is investigated by magneto-optics. Thin films of Tl2201 are deposited by a sputtering technique on vicinal (001) SrTiO 3 substrates with miscut angles of 0.5 and 2.5 degrees and patterned into various shapes. Circular samples are used to determine the current anisotropy axis, whereas square samples allow to evaluate the amplitude of this anisotropy. A cross-over from fractal to uniform anisotropic flux penetration is observed with increasing miscut angle.