V. Born

Scanning tunneling spectroscopy of optimally doped and underdoped YBa2Cu3Ox thin films

Abstract Recently, experimental and theoretical evidence was given that grain boundaries in cuprate high-temperature superconductors are charge carrier depleted zones. In order to examine, if a charge carrier reduction in YBa 2 Cu 3 O x (YBCO) thin films is detectable by scanning tunneling spectroscopy (STS), we extensively studied optimally doped and underdoped (charge carrier reduced) YBCO films on SrTiO 3 substrates. The current–voltage ( I – U ) characteristics of these pulsed laser deposited YBCO films were recorded by means of STS at growth island plateaus in an ultra-high vacuum (UHV). The outcome of our experiments shows that d I /d U spectra of optimally doped and underdoped films differ significantly. Since even in UHV the YBCO surface degrades, the influence of a degraded surface layer on the tunnel process is studied. It is discussed, to what extent the d I /d U spectra and the changes on doping, respectively, are related to the local density of states of the films.

Improved grain boundary currents in textured YBa2Cu3Oδ thin films on bicrystalline Ni substrates

Using magneto optical imaging a comparative study of YBa2Cu3Oδ thin films grown on Gd-10at% doped CeO2 buffered Ni and SrTiO3 bicrystals with tilt angles ranging from 3° to 16° has been made. The Ni substrates have been chosen as a model system for the RABiTS coated conductor technology. It is found that the YBa2Cu3Oδ grain boundaries on Ni exhibit improved superconducting transport properties, while for the intra-granular critical currents a moderate decrease is observed. The improved Jgb/Jc ratio could be detected for all studied grain boundary angles as well as over the whole accessible temperature range from 8 to 60 K. Extensive x-ray measurements on the in- and out-of-plane texture of all layers have been performed to explain the change in the intra- and inter-granular critical currents.

Self-enhanced flux penetration into small angle grain boundaries in YBCO thin films

Grain boundaries in high-temperature superconductors are well known to strongly decrease the critical current. However, most information about their superconducting properties is based on transport measurements which always give non-local, averaged data over the whole sample size. In this paper, we report on the spatially resolved magneto-optical measurements of the magnetic flux and current density distribution in bicrystalline Y Ba2Cu3O7−x thin films with small angle grain boundaries. The current distributions are obtained by a model-independent inversion of Biot–Savarts law. In contrast to Bean-like models of the current pattern at the grain boundary, we find an inhomogeneous current density across the grain boundary, which is strongly influenced by the magnetic stray fields of the currents in the grains. The self-field effects are also responsible for a self-enhanced flux penetration where the penetration depth is significantly increased compared to a Bean-like model.

The visualization of current-limiting defects in YBa2Cu3O7 films on ion-beam assisted deposition buffer layers of yttrium-stabilized ZrO2 and Gd2Zr2O7

For the production of high-current-carrying, long-length superconducting wires or tapes, it is necessary to use biaxially textured metallic substrates or buffer layers. Though being highly textured, the deposited superconducting film exhibits a complex defect structure which (locally) suppresses the critical current and alternates characteristically the magnetic flux distribution seen in magneto-optical imaging. In this paper, we report on pulsed laser deposited YBaCuO films on biaxially textured yttrium-stabilized ZrO2?(YSZ) and Gd2Zr2O7?(GZO) buffers which were grown by ion beam assisted deposition?(IBAD) on polycrystalline substrates. The current-limiting defect structure turns out to resemble closely a combination of a dense distribution of pinhole-like induced growth distortions and a fine grain boundary network. The current suppression is caused on the one hand by the dense packing of pinhole-like defects. On the other hand, we observe a substantial current anisotropy being related to the surface morphology of the buffer layers and the direction of the IBAD-beam.

Magneto-optical study of grain boundaries, interfaces and grain boundary networks in YBaCuO

Using quantitative magneto-optics and an inversion scheme of Biot-Savarts law the local current carrying capability of various types of grain boundaries (GBs) and interfaces in YBaCuO thin films and bulk material was investigated. In all GBs and interfaces a spatial variation of the local critical current density jc was observed which, however, has different reasons, such as microstructural inhomogeneities, magnetic field dependence of jc and size effects in extended GBs and networks of low angle grain boundaries (LAGBs).

MAGNETO OPTICAL IMAGING OF SMALL ANGLE GRAIN BOUNDARIES ON DIFFERENT BI-CRYSTALLINE SUBSTRATES

With quantitative high resolution magneto-optical imaging the percolative nature of magnetisation currents in granular coated conductors has been studied. As a model system for these types of grain boundaries we have investigated the transport properties of artificially grown grain boundaries in YBa2Cu3O7-δ on different types of bicrystalline substrates. The flux screening properties of a grain boundary network, as can be found in YBa2Cu3O7-δ thin films grown on rolling assisted biaxially textured nickel tapes (RABiTS), are compared with flux distributions of isolated grain boundaries. YBa2Cu3O7-δ thin films with artificial grain boundaries were grown on Gd- 10at% doped CeO2 (CGO) buffered Ni and SrTiO3 bi-crystals with tilt angles ranging from 3 ° to 16 °. The Ni- CGO substrates have been chosen for a better representation of the properties of RABiTS substrates. It is found that in comparison to the films grown on SrTiO3, the YBa2Cu3O7-δ films on bi-crystalline Ni-CGO exhibit improved superconducting transport properties across the grain boundary, while for the intra-granular critical currents a moderate decrease is observed. The improved J gb /J c ratio holds for all studied grain boundary angles as well as over the whole accessible temperature range from 8–60 K. X-ray measurements on the in- and out-of plane texture of all layers have been performed to get abetter understanding of film’s structure at the grain boundary.

Modifying the Current Distribution of Grain Boundaries in YBCO Films

The current distribution in thin superconducting films can be influenced by magnetic surroundings. A soft magnet put parallel to a thin film edge can reduce or prevent flux entry and therefore stabilise Meissner screening currents in the film. This is particularly interesting for the investigation of currents across grain boundaries in high temperature superconductors, where the critical current density strongly depends on the flux which penetrates into the grain boundary. Furthermore, using special magnetic arrangements, asymmetric flux and current distributions can be obtained. An increase of the grain boundary critical current density is obtained in the Meissner state, compared to flux penetrated states. We show, that flux penetration into grain boundaries can be suppressed up to a certain external field H*, which depends on the temperature, allowing larger intergranular current densities. All investigations are done by magneto-optical imaging and the inversion of Biot and Savart.

Grain Boundary Critical Currents in High‐Tc Superconductors: A Magneto‐Optical Study

Using magneto‐optical imaging the magnetic field and current distribution in decreasing external fields in bi‐crystalline YBa2Cu3O7−δ thin films containing small angle grain boundaries have been studied with high spatial resolution. In this work, we focus on the influence of an external magnetic field on the remanent state current distribution in these films. It is found, that the external field has strong influence on the inter‐ to intra‐grain current ratio. For a symmetric 4° tilt grain boundary the inter‐ to intra‐grain current ratio was increased from 0.45 to 0.63 by the application of an external field of 80 mT at high temperatures. This shift is based on two effects: Firstly, we find a suppression of the intra‐granular currents and, secondly, an increase of the currents crossing the grain boundary.