Superconducting proximity effect in niobium/graphite complex systems

In the junction of a superconductor (S) and a normal conductor (N), the superconducting proximity effect arises, and the transition temperature (Tc) and critical field (Hc) of S/N bilayer usually lower from those in the superconductor itself (Tcs,Hcs). However, in the niobium/graphite corresponding to S/N bilayers, it was found by the authors that the ratio Tc*/Tcs showed a periodic characteristics with kish graphite (KG) film thickness (dKG) and the peak values of the ratio exceeded 1.00 where niobium (Nb) film thickness (dNb) was 40 nm. In this study, we studied experimentally the relation between Tc* and dKG under some conditions of dNb in order to investigate the superconductivity of niobium/graphite bilayers in detail. As in the case of previous study, thin KG films of various thickness ranging from 20 to 160 nm were prepared. For making a specimen, Nb was deposited on a KG film and a quartz glass substrate simultaneously. Nb film thickness prepared in this study were 27 nm, 40 nm and 47 nm. Each resistance of Nb and Nb/KG films was measured at the same time. As the result, it was found that the ratio Tc*/Tcs vs. dKG showed a periodic characteristics at all dNb in this study, and it is indicated that the larger interference effect can be observed under certain condition of dNb and dKG