Increased resistance below the superconducting transition in granular metals

Granular metal systems can show an increased electrical resistance when the metal in the grains becomes superconducting. It is shown that extreme increase of resistance is explained quantitatively on the assumption that charge is transferred between grains by tunnelling of quasiparticles. Detailed microscopic analysis and the semi-phenomenological two-fluid model give essentially the same result when relevant charging energies are not much greater than the superconducting energy gap. Weaker increase of resistance, decreased resistance and eventual superconductivity of the granular metal are attributed to progressive increase of Josephson coupling as the separation between grains is decreased. (author)