NUCLEATION OF SUPERCONDUCTIVITY IN A THIN-FILM WITH A LATTICE OF CIRCULAR HOLES

We investigate experimentally the nucleation of superconductivity in a thin Aluminium film with a square lattice of microholes in a uniform perpendicular magnetic field H. It is shown that in a non-zero magneti c field, this system has an elevated critical temperature T(c)(H) in c omparison to the reference film without holes. This effect can be cons idered as a generalisation of the well known surface superconductivity effect for the case of a finite radius of the surface and a multiply connected geometry of the sample. Quantization of the fluxoid around e ach hole leads to oscillations in the T(c)(H) dependence with a period approximately corresponding to one flux quantum through a hole. Also another type of oscillation with a smaller period which is equal to on e flux quantum per unit cell was observed. We discuss the second type of oscillation in terms of an interaction between the holes. We believ e that our results can be useful in the analysis of high-T(c) supercon ductors with columnar defects because, as it is shown here, a comparis on of the T(c)(H) dependence before and after irradiation can give som e special information on the properties of the amorphous tracks produc ed by high-energy ions. While many experimental parameters are differe nt in our system and in HTSC we present arguments why this analogy sho uld be correct.