Defect Network in Superconducting Ceramic Oxides Studied by Neutron and Proton Irradiation
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DEFECT NETWORK INSUPERCONDUCTING CERAMIC OXIDES STUDIED BY NEUTRON AND PROTON IRRADIATION E. MEZZETTI*, D. ANDREONE**, G. CASTAGNO*, R. CHERUBINI***, S. COLOMBO*, R. GERBALDO**, B. MINETTI* *Dipartimento di Fisica - Politecnico
- 10129 TORINO
"**Istituto Elettrotecnico Nazionale Galileo Ferraris 10135 Torino ***Laboratori Nazionali di Legnaro,
INFN, 35020 Legnaro (Padova).
ABSTRACT This paper investigates both transport properties and nature of superconductivity breakdown or, conversely, enhancement in oxide ceramics, due to radiation-induced defects. Low-fluence neutrons (x107 n cm" 2 at 3 MeV) can sensitively damage the samples, giving experimental evidence that the breakdown of coherent percolating paths produces decoupled domains. A set of preliminary measurements shows that high-fluence proton implantation can either damage or enhance critical current density in a currently non controllable way. In both cases strongly damaged or enhanced superconducting paths short-circuit the unaffected bulk network. INTRODUCTION Superconductivity in polycrystalline oxides is strongly affected by random mixing of superconducting zones, other zones where superconductivity - if present - is weaker, and holes, similar in size to the first ones. Generally the first domains are identified with superconducting grains, the other ones with intergrain domains, weakly connecting the grains. Ion irradiation is a powerful means to study the interplay of superconducting domains and their "defective" interconnections which establish or breakup coherence within the sample. A first set of measurements on ceramic YBCO, having very low critical current density (about 5 A cm- 2 ) irradiated by 3 MeV neutrons at fluences ranging from 104 to 109 n cm- 2 , has shown that neutron damage above 105 n cm- 2 affects the critical current density vs temperature curves, and that it can also modify Tc values and spread out the transition, [1]. The dramatic loss of superconductivity observed in these "poor" samples, due to the small number of scattering events (107 neutrons corresponding to z.104 events mm- 3 x 0.16 grain"1) demonstrates that the coherence of superconducting states within the whole sample is the main characteristic of useful superconducting paths [2],[3]. Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
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Recently, critical current density and resistivity measurements have been performed on a new set of ceramic samples, having critical current densities of about 50 A cmn2 , irradiated by neutrons of =107 n cm" 2 at 3 MeV or by protons of z 101 7 p cm- 2 at 6 MeV. A preliminary data analysis substantially confirms the previous results. Furthermore it shows that critical current densities in these ceramic oxides strongly depend on the sample geometry and are completely determined by self-field limitation. The damage substantially breaks-up the superconductor into coherent subdomains bounded by the flux flow, and leads to a change in the slope of the I-V curves [2]. An enhancement of critical current density [4,5,
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