Characterization of localized states in thin PrBa2Cu3O7?x tunnel barriers

Characteristic power-law dependence of junction conductance on temperature was confirmed at low temperatures fora,b-axis oriented Au/PrBa₂Cu₃O_7−x /YBa₂Cu₃O_7−y junctions with a PrBa₂Cu₃O_7−x layer thinner than 30 nm, indicating that current transport via a small number of localized states was predominant. The radius and the density of the localized states were estimated to be 1.1 nm and 5.0x10¹⁹ eV⁻¹ cm⁻³, respectively. Transport measurement at 1.7 K in a magnetic field of up to 6 T revealed no indication of on-site Coulomb repulsion of electrons in the localized states. These results support the possibility of resonant tunneling of Cooper pairs in superconductor-insulator-superconductor junctions with a thin PrBa₂Cu₃O_7−x tunnel barrier.