PROPERTIES OF THE SUPERCONDUCTING STATE OF THE GRAINS IN INSULATING GRANULAR FILMS

Abstract

We have estimated the mean zero-temperature superconducting energy gap, DELTA0(0), of metallic indium grains imbedded in insulating In/InOx films (26 kOMEGA/class<R(N)<130 kOMEGA/class) from the saturated value of the effective activation energy of the conduction at low temperatures. The value of DELTA0(0)/k(B) varies from approximately 5.5 K to approximately 11.5 K, depending on the sheet resistance of the samples. Upon decreasing the temperature near the superconducting transition of the grains, the magnetoresistance changes from being positive with a small fractional change in magnitude to being negative with a large fractional change. The transition temperature of the grains, T(c), is determined from the crossover of the temperature dependence of the magnetoresistance. The values of DELTA0(0) are larger than those for pure bulk indium and increase with the sheet resistance of the films, indicating that the indium grains are disordered in the films. The value of 2DELTA0(0)/k(B)T(c) indicates that the grains are in the strong-coupling superconductivity limit for the samples with high sheet resistances.