Normal state properties of high angle grain boundaries in (Y,Ca)Ba2Cu3O7-delta.pdf

Submitted to Physical Review B Normal state properties of high angle grain boundaries in (Y,Ca)Ba2Cu3O7-δ S.H. Mennema, J.H.T. Ransley, G. Burnell, J.L. MacManus-Driscoll, E.J. Tarte, M.G. Blamire Department for Materials Science and Metallurgy, University of Cambridge, Pembroke Street, CB2 3QZ, Cambridge, United Kingdom. By lithographically fabricating an optimised Wheatstone bridge geometry, we have been able to make accurate measurements of the resistance of grain boundaries in Y1-xCaxBa2Cu3O7-δ between the superconducting transition temperature, Tc, and room temperature. Below Tc the normal state properties were assessed by applying sufficiently high currents. The behaviour of the grain boundary resistance versus temperature and of the conductance versus voltage are discussed in the framework charge transport through a tunnel barrier. The influence of misorientation angle, oxygen content, and calcium doping on the normal state properties is related to changes of the height and shape of the grain boundary potential barrier. I. INTRODUCTION Grain boundaries in high-Tc cuprates have become a subject of intensive study after it became clear that these defects are the cause of severe reduction of the attainable critical currents in polycrystalline materials.1-4 Elucidating the electronic properties of grain boundaries is of great importance for the development of recipes to enhance their transport properties for the purpose of multigrain applications, like coated conductors. A widely discussed framework to describe the exponential decrease of critical current density and normal state conductivity with misorientation angle is the so-called model of band bending, as proposed by Mannhart et al.5,6 Band bending at the grain boundary is expected to occur as a result of work-function differences, charging, and Fermi-level pinning at interface states, and leads to hole-depleted layers in the superconducting electrodes next to the boundary. Acco