Interface transparency and proximity effect in Nb/Cu triple layers realized by sputtering and molecular beam epitaxy

We have investigated, in the framework of the proximity effect theory, the interface transparency T between Nb and Cu in the case of high quality Nb/Cu trilayers fabricated by molecular beam epitaxy (MBE) and sputtering deposition techniques. The obtained T values do not seem to be strongly influenced by the fabrication methods but more by the intrinsic properties of the two metals; a slightly higher value for T has even been deduced for the MBE prepared samples. The proximity effect in these samples has also been studied in the presence of an external magnetic field. In the parallel configuration a significant shift towards lower values of the 2D-3D crossover temperature has been observed for MBE samples, in good agreement with very recent theoretical predictions. In the perpendicular case a positive curvature of the temperature dependence of the upper critical field has been detected, which was less pronounced for sputtered samples. Both the effects have been observed only for trilayers with low Nb thickness (<600 Å) which confirms the crucial influence of the interface transparency on the values of the upper critical field in such samples.