CRITICAL SHEET RESISTANCE FOR GLOBAL SUPERCONDUCTIVITY IN ANTIGRANULOCYTES ALUMINUM FILMS

The temperature and magnetic field dependence of sheet resistance was measured in aluminum granular films which consisted of 10 A grains and thin oxides on grain surfaces. For the samples with normal resistance less than 6 kohm, a superconducting transition was observed, while the others behaved as insulators. The transition between superconductor and insulator was induced also by a magnetic field, and the normal sheet resistance at the boundary was 6.7 kohm. The results are discussed in connection with the theories for the universal critical resistance R(Q) ( = h/4e2), and with the results of previous experiments for films of larger grains.