Percolative superconductor-to-insulator transition in Eu_1.5Ce_0.5NbSr₂Cu₂O₁₀

We employed scanning tunneling microscopy and spectroscopy combined with macroscopic resistivity and magnetization measurements in the study of the superconductor-to-insulator transition in ceramic Eu_1.5Ce_0.5NbSr₂Cu₂O₁₀. This superconductor (T_C = 29 K) is made insulating by hydrogen doping in a nondestructive way, allowing for a reversible transition. We found that although the samples were nominally homogeneous the resistivity and magnetization data are indicative of a granular system. Tunneling measurements directly show that the effect of hydrogen loading is to induce a random insulator-superconductor phase separation, resulting in a percolative granularlike transition.