Percolative superconductor-to-insulator transition in Eu1.5Ce0.5NbSr2Cu2O10

We employed scanning tunneling microscopy and spectroscopy combined with ma croscopic resistivity and magnetization measurements in the study of the su perconductor-to-insulator transition in ceramic Eu1.5Ce0.5NbSr2Cu2O10. This superconductor (T-C = 29 K) is made insulating by hydrogen doping in a non destructive way, allowing for a reversible transition. We found that althou gh 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-su perconductor phase separation, resulting in a percolative granularlike tran sition. [S0163-1829(99)51646-9].