DISAPPEARANCE OF SUPERCONDUCTIVITY IN ULTRA-THIN AMORPHOUS NB FILMS

The disappearance of superconductivity in ultra thin amorphous Nb film s has been studied by the measurements of the electrical sheet resista nce R(square) and I-V characteristics. The mean field transition tempe rature T(c0) and the Kosterlitz- Thouless transition temperature T(KT) have been determined for films with various thicknesses d. We have fo und that T(c0)'s decrease linearly with d-1 and become zero near R(squ are) approximately h/4e2 = 6.45kOMEGA/square, the quantum resistance o f pairing electrons. However T(KT)'s go to zero for the films with R(s quare)(N)lower than h/(2e) although their T(c0)'s are finite. It indic ates that near the superconductor-insulator transition in uniformly di sordered films the quantum fluctuations of the phase are of significan ce as well as the reduction of the amplitude of the superconducting or der parameter.