Localized superconducting to normal transition at the grain boundary for transport critical current in YBa2Cu3O7−δ

Local measurements of the critical current (Ic) at 77 K have been made on YBa2Cu3O7−δ consisting of mm sized textured domains denoted “grains”. The samples were cut and polished into thin slices less than 500μm thick in order to isolate various grain boundaries. In one case a current path across the grain boundary was observed to strongly reduce Ic, compared to the values obtained for current paths in the neighboring grains. The superconducting to normal transition at Ic was also shown to be localized at the grain boundary. In two other cases, the grain boundary was observed to modify only slightly the superconducting characteristics of the material. In a polycrystalline form of the ceramic superconductor YBa2Cu3O7−δ the presence of grain boundaries has been deduced to be one of the major factors limiting the critical current (Ic). This conclusion is essentially based on the three following observations. First, the critical current at 77 K, Ic, decreases strongly on application of an external magnetic field which is the characteristic of a Josephson junction. The grain boundaries are thus considered to be at the origin of such weak link behavior [1]. Second, experiments carried out on thin film bicrystal show strong attenuation in transport Ic when the superconducting current path crosses the grain boundary [2]. Third, forms of YBa2Cu3O7−δ containing a reduced number of grain boundaries, such as textured material or thin films, yield measured values of critical current density (Jc) which are higher than the values obtained for small grain ceramics by one order of magnitude or more [3, 4]. This letter discusses the local electrical response around the grain boundaries in ‘bicrystals’ cut out of large grain ceramics fabricated by a method close to melt textured growth. A particular attention is paid to the local nature of the transition to the normal state at the boundary when the transport critical current is just exceeded. With this type of behavior the sample can be considered to be consti-