Quasireentrant superconductivity and enhanced critical field in thick Pb-SiO2 granular films.

We studied the electronic transport properties of Pb x(SiO2)1− x nanogranular films with Pb volume fraction x ranging from ∼0.51 to ∼0.87 and thickness t > 500 nm. It has been found that those x ≳ 0.60 films reveal metallic behavior at normal state and change to superconductors below ∼7 K, while those x ≤ 0.53 films show insulator properties over the whole measured temperature range. The quasireentrant superconductivity behaviors were observed in the films with intermediate x values, x [ABSTRACT FROM AUTHOR]