Ch. Jooss

Magneto-optical studies of current distributions in high-Tc superconductors

In the past few years magneto-optical flux imaging (MOI) has come to take an increasing role in the investigation and understanding of critical current densities in high-Tc superconductors (HTS). This has been related to the significant progress in quantitative high-resolution magneto-optical imaging of flux distributions together with the model-independent determination of the corresponding current distributions. We review in this article the magneto-optical imaging technique and experiments on thin films, single crystals, polycrystalline bulk ceramics, tapes and melt-textured HTS materials and analyse systematically the properties determining the spatial distribution and the magnitude of the supercurrents. First of all, the current distribution is determined by the sample geometry. Due to the boundary conditions at the sample borders, the current distribution in samples of arbitrary shape splits up into domains of nearly uniform parallel current flow which are separated by current domain boundaries, where the current streamlines are sharply bent. Qualitatively, the current pattern is described by the Bean model; however, changes due to a spatially dependent electric field distribution which is induced by flux creep or flux flow have to be taken into account. For small magnetic fields, the Meissner phase coexists with pinned vortex phases and the geometry-dependent Meissner screening currents contribute to the observed current patterns. The influence of additional factors on the current domain patterns are systematically analysed: local magnetic field dependence of jc(B), current anisotropy, inhomogeneities and local transport properties of grain boundaries. We then continue to an overview of the current distribution and current-limiting factors of materials, relevant to technical applications like melt-textured samples, coated conductors and tapes. Finally, a selection of magneto-optical experiments which give direct insight into vortex pinning and depinning mechanisms are reviewed.

Polaron melting and ordering as key mechanisms for colossal resistance effects in manganites.

Polarons, the combined motion of electrons in a cloth of their lattice distortions, are a key transport feature in doped manganites. To develop a profound understanding of the colossal resistance effects induced by external fields, the study of polaron correlations and the resulting collective polaron behavior, i.e., polaron ordering and transition from polaronic transport to metallic transport is essential. We show that static long-range ordering of Jahn–Teller polarons forms a polaron solid which represents a new type of charge and orbital ordered state. The related noncentrosymmetric lattice distortions establish a connection between colossal resistance effects and multiferroic properties, i.e., the coexistence of ferroelectric and antiferromagnetic ordering. Colossal resistance effects due to an electrically induced polaron solid–liquid transition are directly observed in a transmission electron microscope with local electric stimulus applied in situ using a piezo-controlled tip. Our results shed light onto the colossal resistance effects in magnetic field and have a strong impact on the development of correlated electron-device applications such as resistive random access memory (RRAM).

High-resolution magneto-optical imaging of critical currents in YBa2Cu3O7−δ thin films

Abstract Magneto-optical investigation of flux penetration into type-II superconductors allows the determination of the local critical current density j c by inversion of the Biot–Savart law. Due to the required computational effort, in the past, this method was limited to low spatial resolution of the current density. In this paper, we present a fast inversion scheme using Fast-Fourier-Transformation, which allows high spatial resolution imaging of current distributions. Due to the nonlocal relation between magnetic field and current density, it is necessary to use a method that images field and current distribution of superconductors as a whole, and enables high resolution at the same time. To demonstrate the power of our method, the local current density in a YBa 2 Cu 3 O 7− δ square and disk are imaged with high resolution and described in detail for increasing and decreasing external magnetic fields. At low fields, the dependencies of j c ( B ) on the local magnetic field B and of the average j c (magnetic moment) on the applied magnetic field show significant differences. Finally, we directly image the local current density near macroscopic defects in a square and a disk. The observed current distributions near defects significantly differ from the predictions of an extended Bean model.

Thickness and roughness dependence of magnetic flux penetration and critical current densities in YBa2Cu3O7−δ thin films

Abstract As a result of the island growth mode all epitactic YBa 2 Cu 3 O 7−δ thin films show surface roughness. To investigate a possible surface pinning mechanism, combined magneto-optical and atomic force microscopy studies were carried out. Measurement of the spatial distribution of magnetic flux density on the surface of thin YBa 2 Cu 3 O 7−δ films by means of the magneto-optical Faraday effect (MOFE) under application of external magnetic fields allows an accurate determination of critical current densities j c . In order to have a quantitative comparison between the Bean model for thin films and experiment, a new nonlinear calibration technique for the flux densities was developed. For determining the thickness and roughness dependence of j c , samples with YBaCuO-strips of different thickness and roughness were patterned from one film. With the roughness determined experimentally by afm measurements, a satisfactory agreement between the measured and calculated thickness dependence of j c is achieved. Surface pinning is found to cause between 10%–30% of the critical current densities of epitactic YBa 2 Cu 3 O 7−δ thin films. Additionally microscopic deviations of the flux profiles from the Bean model are detected. Evidence for matching effects of the vortex line distribution with the density of surface pins is given.

Observation of dendritic flux instabilities in YNi2B2C thin films

Magneto-optical imaging and magnetization measurements performed on thin films of the borocarbide superconductor YNi2B2C reveal the occurrence of magnetic flux instabilities upon reducing the applied magnetic field towards the remanent state. In contrast to other low-Tc materials such as Nb and MgB2, where similar instabilities occur in both increasing and decreasing magnetic fields, dendritic flux patterns are observed in YNi2B2C for decreasing fields only. Also in the magnetization measurements, a distinct asymmetry is evident between increasing and decreasing fields. The effect does not depend on the sweep rate of the field, but is strongly dependent on the maximum field applied before reduction. The observation of spontaneous flux instabilities in this additional family of low-temperature superconductors suggests that the responsible mechanism is universal to this class of materials.Magneto-optical imaging and magnetization measurements performed on thin films of the borocarbide superconductor YNi2B2C reveal the occurrence of magnetic flux instabilities upon reducing the applied magnetic field towards the remanent state. In contrast to other low-Tc materials such as Nb and MgB2, where similar instabilities occur in both increasing and decreasing magnetic fields, dendritic flux patterns are observed in YNi2B2C for decreasing fields only. Also in the magnetization measurements, a distinct asymmetry is evident between increasing and decreasing fields. The effect does not depend on the sweep rate of the field, but is strongly dependent on the maximum field applied before reduction. The observation of spontaneous flux instabilities in this additional family of low-temperature superconductors suggests that the responsible mechanism is universal to this class of materials.

Substrate-mediated anisotropy of transport properties in YBa2Cu3O7−δ thin films

Abstract We have generated an almost periodic step structure of c-axis-oriented YBa 2 Cu 3 O 7−δ films grown on 10° miscut SrTiO 3 (001) substrates. Combined scanning tunneling microscopy and cross-sectional transmission electron microscopy studies reveal that this growth morphology is linked to an anisotropic defect microstructure. We present evidence that translational boundaries contribute to strong flux pinning in our films. The resistivity and critical current density in the miscut-grown YBa 2 Cu 3 O 7−δ films have been found to be anisotropic.

Overcritical Meissner current densities in YBa2Cu3O7 films in soft magnetic environments

Magnetic flux density and current density distributions of YBa2Cu3O7 films in soft magnetic environments are investigated by magneto-optics. Next to the magnets flux penetration is strongly modified. Current density distributions obtained by an inversion of the law of Biot–Savart show overcritical current densities in the current domains next to the magnets.

Bi2Sr2Can-1CunO2(n+2)+δ thin films on c-axis oriented and vicinal substrates

Abstract Bi 2 Sr 2 Ca n −1 Cu n O 2( n +2)+ δ ( n =1,2,3) thin films are prepared on c -axis oriented and vicinal substrates by pulsed-laser deposition. Optimization of substrate temperature, laser fluence and post-annealing conditions produces single-phase, well-oriented and smooth films. The surface morphology, texture, crystallinity and stoichiometry of Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) thin films strongly depend on the fabrication parameters. On ( 0 0 1 ) SrTiO 3 substrates, c -axis oriented Bi-2212 films with J c (60 K )=2×10 6 A/cm 2 and T c0 =82 K are obtained. Vicinal Bi-2212 films grown on miscut substrates reveal anisotropic resistivities independent of film thickness (20–300 nm), high J c anisotropy and strong in-plane and out-of-plane texture.

Transport and ordering of polarons in CER manganites PrCaMnO

The temperature-dependent resistivity and the colossal resistance effect induced by strong electric fields of the small-band Pr1−xCaxMnO3 (PCMO) manganites are analysed with respect to the influence of the Ca doping, post-annealing, the prehistory of the electric stimulation, and the physical dimensions of the sample. Despite the phase separation between charge and orbital ordered and disordered phases, PCMO reveals the properties of a homogeneous medium with a conductivity governed by the hopping of small polarons if the electric field is not too strong. In contrast, high electric fields induce a structural transition which gives rise to a glassy behaviour in the transient regime. In the low resistance state the small activation energy of charge carrier hopping implies a transition to large polaron hopping.

Interplay of point defects, biaxial strain, and thermal conductivity in homoepitaxial SrTiO3 thin films

Separating out effects of point defects and lattice strain on thermal conductivity is essential for improvement of thermoelectric properties of SrTiO3. We study relations between defects generated during deposition, induced lattice strain, and their impact on thermal conductivity κ in homoepitaxial SrTiO3 films prepared by ion-beam sputtering. Lowering the deposition temperature gives rise to lattice expansion by enhancement of point defect density which increases the hardness of the films. Due to a fully coherent substrate-film interface, the lattice misfit induces a large biaxial strain. However, we can show that the temperature dependence of κ is mainly sensitive on the defect concentration.