Granular insulator-superconductor transition in ultrathin Bi films

We have studied the insulator-superconductor transition (IST) by tuning the thickness in quench-condensed Bi films. The resistive transitions of the s uperconducting films are smooth and can be considered to represent "homogen eous" films. The observation of an IST very close to the quantum resistance for pairs, R-square(N) similar to h/4e(2) on several substrates supports t his idea. The relevant length scales here are the localization length, and the coherence length. However, at the transition, the localization length i s much higher than the superconducting coherence length, contrary to expect ation for a "homogeneous" transition. This suggests the invalidity of a pur ely fermionic model for the transition. Furthermore, the current-voltage ch aracteristics of the superconducting films are hysteretic, and show the fil ms to be granular. The relevant energy scales here are the Josephson coupli ng energy and the charging energy. However, Josephson coupling energies (E- c) and the charging energies (E-c) at the IST, are found to obey the relati on E-J < E-c. This is again contrary to expectation, for the IST in a granu lar or inhomogeneous, system. Hence, a purely bosonic picture of the transi tion is also inconsistent with our observations. We conclude that the IST o bserved in our experiments may be either an intermediate case between the f ermionic and bosonic mechanisms, or in a regime of charge and vortex dynami cs for which a quantitative analysis has not yet been done. (C) 2001 Elsevi er Science Ltd. All rights reserved.