We studied the electronic transport properties of Pb x (SiO 2 ) 1Àx 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 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 x s c for superconductorinsulator transition is almost identical to the percolation threshold for metal-insulator transition. For the film with x just above x s c , 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. Rapid Research Letter1700070 (4 of 5) Y.-J. Xi et al.: Quasireentrant superconductivity and enhanced critical field in granular Pb-SiO 2
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