Fundamental Questions About Superconductivity in the Pnictides (Former title: Electromagnetic and Nanostructural Studies of Rare Earth Copper Oxide Grain Boundaries Grain Boundaries in High Temperature Superconductors)

In early 2008 the Hosono group at Tokyo Institute of Technology discovered high temperature superconductivity in a new extensive family of pnictides based on doped As-Fe. This exciting discovery encouraged us to transition our work on grain boundary studies of YBCO to this new class of superconductors. We showed that they indeed exhibit high critical temperatures Tc, extremely high upper critical fields Hc2, an irreversibility field H close to Hc2, high critical current density Jc, and small electronic anisotropy Hc2abHc2c. Our recent pioneering investigation of 122 bicrystals has shown that grain boundaries in pnictides exhibit weak link behavior similar to that of cuprates. Moreover, our recent experiments introducing controlled magnetic and nonmagnetic disorder by particle irradiation have shown a remarkable resilience of multiband superconductivity to nonmagnetic and Kondo scattering, suggesting new possibilities of their optimization. In this respect the pnictides are both similar and rather dissimilar to the cuprates, issues that deserve understanding since doping from an initial non-superconducting state is so far the only way to achieve a transition temperature of more than 50 K.