R. Warthmann

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.

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.

Anisotropic defect structure and transport properties of YBa2Cu3O7−δ films on vicinal SrTiO3(001)

The microstructure of YBa2Cu3O7−δ thin films grown on vicinal SrTiO3(001) has been studied as a function of the vicinal angle by x-ray diffraction using the two-dimensional q-scan technique. Our results reveal a strong correlation between the miscut of a SrTiO3(001) substrate and the anisotropic defect structure of the film. Furthermore, we observed an anisotropy of the corresponding critical current density up to 4.6 depending on the angle of miscut.

Anisotropic flux pinning in thin YBCO-films by substrate modifications

Abstract The magnetooptical Faraday effect is an excellent tool to investigate the spatially resolved flux density distribution and, by a numerical inversion of the Biot–Savart law, the local current density in a superconducting film with a spatial resolution better than 10 μm [Ch. Jooss, R. Warthmann, A. Forkl, H. Kronmuller, Physica C 299 (1998) 215]. Irradiating the surface of SrTiO 3 (001) substrates with a focused ion beam in a line pattern leads to an anisotropy of the flux and current density distribution in a well-defined area. While we measure an enhancement of j c parallel to the lines, a reduction of the value of the current perpendicular to the lines is found. This is in qualitative agreement with experiments on YBCO films grown on vicinal substrate surfaces [T. Haage, J. Zegenhagen, J.Q. Li, H.-U. Habermeier, M. Cardona, Ch. Jooss, R. Warthmann, A. Forkl, H. Kronmuller, Phys. Rev. B 56 (1997) 8404] and based on the data gained on these systems, a pinning model for the films on the irradiated substrates is suggested.

Influence of substrate irradiation on critical current density and microstructure in YBCO thin films

Abstract Strontiumtitanate substrates have been irradiated in defined areas with a focused ion beam (FIB) of gallium ions. This leads to a charge of the surface structure of the substrates. Afterwards YBCO was grown epitactically on these substrates by pulsed laser deposition. The effect of the modification on the flux density distribution in the YBCO thin films has been observed magnetooptically and the local current distribution has been determined. The correlation between the change in jc and the microstructure of the YBCO samples, investigated by transmission electron microscopy (TEM), is discussed.

Pinning mechanism of vortices at antiphase boundaries in YBaCuO films

Antiphase boundaries represent dislocation free interfaces in YBaCuO with small structural width of less than 1 nm. We show, that strong pinning at such boundaries takes places by quasiparticle scattering induced variation of the condensation energy, characteristic of superconductors with anisotropic gap. The length scale of the order parameter suppression at antiphase boundaries will be discussed.

Current densities in low-angle grain boundaries of YBCO

Abstract Magnetooptical imaging has been used to investigate low-angle grain boundaries (LAGBs) in YBCO thin films at T = 5K. This technique allows us to visualize the flux density distribution with a spatial resolution of a few micrometers. By a numerical analysis of the experimental data the local current distribution in a LAGB has deen determined. It is shown that the current density acros the grain boundary depends strongly on the local flux density which leads to a crossover in the pinning scenario at the LAGB.

Nanoscale engineering: Tailored transport properties by self-organization in YBa2Cu3O7-δ thin films

Abstract We have grown YBa2Cu3O7-δ (YBCO) films on vicinal SrTiO3(001) surfaces. Scanning tunneling microscopy reveals that their morphology is reminiscent of the regular array of steps on the substrate surface. The resultant film microstructure leads to a pronounced in-plane anisotropy of the resistivity and the critical current density jc. We observed a substantial enhancement of jc up to 8 × 1011A/m2 in the nanoscale engineered films.

Anisotropic flux penetration and current distribution in (001) tilted YBa2Cu3O7−δ thin films

Abstract The growth of YBCO thin films on miscut SrTiO 3 substrates with various tilt angles towards [010] allows the controlled generation of planar defects, which are aligned to the surface steps of the substrate in [100] (L)-direction. Due to the anisotropic microstructure we observe a strong anisotropy of flux penetration and critical current density by means of magneto-optics. The critical current density in the (ab)-plane of these c-axis oriented thin films is determined by a new method of high resolution inversion of Biot-Savarts law. New features like periodic streaky flux patterns in L-direction and a strongly different magnetic field dependence of the critical current density longitudinal to the defect planes, j c, L , and transversal, j c, T , are observed. This gives rise to a magnetic field dependence of the anisotropy ratio of the critical current densities.