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

We studied the electronic transport properties of Pbx(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 ≃ 0.54 and 0.57. It is found that the critical metal volume fraction xcs for superconductor–insulator transition is almost identical to the percolation threshold for metal–insulator transition. For the film with x just above xcs, the superconducting states can preserve to relative high temperatures even at a magnetic field with magnitude of 9 T, and the upper critical field at zero temperature is estimated to be close to the Pauli limit (∼15 T). Our results establish a key element for the high field applications of superconducting material.