M. Leghissa

Flux-lines of inversed sign in YBa2Cu3O7−δ thin films

Abstract Using the high-resolution Faraday technique (HRF), we observed directly the nucleation of flux-lines of opposite sign in the remanent state of YBaCuO thin films. These negative flux-lines are nucleated by the stray fields caused by the pinned vortices. Due to the large demagnetizing factor of the thin film, this flux is able to penetrate the sample. Such a coupled behaviour can be observed directly. Experimental evidence for the existence of vortices of opposite sign is given. The influence of various polarizer/ analyzer settings on these observations is discussed. Furthermore, we show effects of flux-creep in thin films and the role of the negative flux-lines in this behavior is analyzed.

Magneto-optical study of flux penetration in heavy-ion irradiated high-Tc single crystals

Abstract Single crystals of Bi2Sr2CaCu2O8+δ (Bi-2212) and of DyBa2Cu3O7−δ have been exposed to high-energy heavy-ion irradiation (230 MeV 58Ni-ions, 0.5 GeV 127I-ions and 1.0 GeV 208Pb-ions) with various fluences perpendicular to the sample surface and hence parallel to the c-axis of the crystals. The irradiation with Ni-ions produces spherical regions of amorphized material, whereas the iodine and lead ions create columnar defects along the paths of the projectiles in the superconductors. Using the high-resolution Faraday effect (HRF) technique, domain patterns of the Shubnikov phase are investigated. From the measured flux-density profiles, the acting local pinning forces and critical current densities are determined. To allow direct comparison of the irradiation effects under the same experimental conditions, one half of each crystal was covered by an absorber during the irradiation. Fluence-dependent measurements of the pinning forces are performed and the influence of irradiation on flux-creep effects is also investigated. All heavy-ion irradiated samples show an enhancement of the measured critical current densities up to a factor of 33 as compared to the unirradiated region of the same crystal. The effective activation energies for flux creep remain unchanged after each kind of irradiation in both types of single crystals.

Influence of Irradiation-Induced Latent Tracks on Local Flux Pinning in Bi-2212 Crystals

For the first time the influence of irradiation-induced columnar defects (0.5 GeV I-ions) on flux line pinning in Bi-2212 crystals has been investigated by means of the high-resolution magneto-optical Faraday effect. By irradiating only a part of each sample, the local change in the flux pinning has been determined. Irradiated and nonirradiated parts have been investigated simultaneously under the same experimental conditions. At T = 5 K and μ0Hext = (0 ÷ 300) mT a large enhancement of jc, m is found in the irradiated region. The observation of the time dependence of the flux density distribution yields small values for the pinning potential U, supporting the model of two-dimensional pancake vortex pinning.

Dissipative flux motion in YBa2Cu3O7−δ films — Investigation by means of transport I-V curves

The nature of dissipative flux motion in epitaxial YBa2Cu3O7−δ thin films has been investigated by means of resistivity ϱ(T, B) and transport current-voltage characteristicsE(j, T, B) covering up to 5 orders of magnitude in voltage for a wide range of temperatures (4.2–85 K) and magnetic fields (0.5–7.0 T). Activation energiesU(T, B, j) have been determined in the framework of the collective flux creep theory. It is shown thatU(T, B, j) decreases with increasing temperature, magnetic field and current density. In order to study the influence of a well-defined defect structure, columnar defects have been generated by irradiation with 340 MeV Xe-ions. From a scaling law relating theI–V curves after irradiation with the curves before irradiation it is found that vortices pinned by the columnar defects do not contribute to dissipation. Due to irradiation the irreversibility line is shifted to higher magnetic fields, indicating its connection to the pinning mechanisms present in the samples.

VORTEX-LINE LIQUID PHASES : LONGITUDINAL SUPERCONDUCTIVITY IN THE LATTICE LONDON MODEL

We study the vortex-line lattice and liquid phases of a clean type-II superconductor by means of Monte Carlo simulations of the lattice London model. Motivated by a recent controversy regarding the presence, within this model, of a vortex-liquid regime with longitudinal superconducting coherence over long length scales, we directly compare two different ways to calculate the longitudinal coherence. For an isotropic superconductor, we interpret our results in terms of a temperature regime within the liquid phase in which longitudinal superconducting coherence extends over length scales larger than the system thickness studied. We note that this regime disappears in the moderately anisotropic case due to a proliferation, close to the flux-line lattice melting temperature, of vortex loops between the layers. {copyright} {ital 1997} {ital The American Physical Society}

Large fishtail effect in DyBa2Cu3O7−δ single crystals containing columnar defects

Abstract Using capacitance torque magnetometry, hysteresis loops were measured on a Pb-ion irradiated DyBa 2 Cu 3 O 7−δ single crystal as a function of magnetic field, temperature and angle Θ between the field direction and the c axis of the sample. The superconducting current densities, j c ( T , B , Θ), determined using the extended Bean model, show a clear fishtail effect at temperatures above 30 K despite the large enhancement of j c due to the columnar tracks. The enhancement of j c due to irradiation was magneto-optically determined to j c ( φt )/ j c (0) = 10 at T = 30 K. The pinning energy U 0 is, however, only slightly increased by the irradiation. Our measurements indicate clearly that the fishtail effect is a general feature of the high- T c superconductors and not related to a special arrangement of pinning centres.