Thickness tuned two-dimensional superconductor-insulator transitions for amorphous ultra-thin films

Abstract

We have carried out experiments of the thickness-tuned Superconductor- insulator (S-I) transition for amorphous ultra-thin films of various materials. The S-I transition in two-dimensional homogeneous systems has been studied using a wide variety of amorphous films, such as films quench-condensed onto cryogenic substrates and composite material films. The values of critical sheet resistance Rc obtained by many groups for homogeneous films of various materials do not indicate a universal value. Our results indicate that the Rc of Bi, MoRu and AuGe on SiO underlayers have approximately 5.2, 6.7 and 4.9 k Ω respectively. The difference among the values of Rc of the present three films is small in comparison with previous reports of other groups. The decrease in superconducting transition temperature of Bi, MoRu and AuGe films showed similar dependence of normal state sheet resistance below 2kΩ. This behavior can be theoretically explained by the electron-electron effect due to the enhanced Coulomb interactions in a disordered superconductor. On the insulating side, the conductance of the films exhibits temperature dependence as R = Ro exp[(T0/T) x]. The factor x of Bi films have ∼ 1 and that of MoRu and AuGe films are ∼ 0.5 near the S-I transition. This results may reflect the difference of films structure. © 2009 IOP Publishing Ltd.