Browsing by Author "Born, V."

Using magneto optical imaging a comparative study of YBa2Cu3Odelta thin films grown on Gd-10at% doped CeO2 buffered Ni and SrTiO3 bicrystals with tilt angles ranging from 3degrees to 16degrees has been made. The Ni substrates have been chosen as a model system for the RABiTS coated conductor technology. It is found that the YBa2Cu3Odelta grain boundaries on Ni exhibit improved superconducting transport properties, while for the intra-granular critical currents a moderate decrease is observed. The improved J(gb)/J(c) 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.

Using space resolved magneto-optical microscopy we have studied the development of the critical state in high temperature superconducting small-angle grain boundaries in magnetisation experiments. It has been found that with respect to the position in the grain boundary plane thiS critical State develops in a highly inhomogeneous manner. Towards the centre of the grain boundary there exists a distinct suppression of the inter-granular currents flowing across the grain boundary. A strong correlation between the static (critical current) and the dynamical properties (e.g. electric field or flux-line velocity) of the vortex system inside the grain boundary appears to be the principal mechanism for the observed inhomogeneous current distribution in wide bi-crystalline current bridges.

We have performed a deliberate study on the transport properties of Y-123 thin films on bi-crystalline substrates containing low angle grain boundaries (LAGBs). By magneto-optical (MO) imaging we are able to map the magnetic field and current distribution of wide current bridges. A quantitative analysis of the distribution of currents over the entire grain boundary (GB) is presented. We discuss the effects of magnetic history on the transport properties of Y-123 thin films with LAGBs. The results are compared with artificially created two-dimensional (2D) current distributions. (C) 2004 Elsevier B.V. All rights reserved.

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 YBa2Cu3Ox (YBCO) thin films is detectable by scanning tunneling spectroscopy (STS), we extensively studied optimally doped and underdoped (charge carrier reduced) YBCO films on SrTiO3 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 dI/dU 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 dI/dU spectra and the changes on doping, respectively, are related to the local density of states of the films. (C) 2002 Elsevier Science B.V. All rights reserved.