Phase transitions and magnetic properties of superconducting-normal sandwiches

When a superconductor S and a normal metal N are put into contact, the Cooper pairs which make up superconductivity in S can leak into N and induce superconductivity into N by proximity effects. It is demonstrated that the order parameter at the outer surface of N, where the supercurrent is required to vanish, can take one of two possible values. This arbitrariness leads to two different modes for the superconducting in the sandwich, whose respective energies are differently affected by variation of the external conditions such as the temperature and the magnetic field. In this thesis, within the domain of validity of the local Ginzburg-Landau GL theory, the occurrence of first order phase transitions in superconducting-normal S/N sandwiches is investigated. Experimentally, such a phase transition has been observed as a response to the external magnetic field, whereby induced superconductivity in which the pair potential is finite throughout the N, gives way at and above a given field (the breakdown field, Hsub(b)) to a second mode for which the field penetrates into N and destroys the induced superconductivity. In the absence of a magnetic field, a first order phase transition (which has not yet been observed experimentally) is predicted due to competition between the two possible modes. In addition to the calculation of upper critical field of type II superconductors the extreme type II S in a magnetic field is studied. The effective penetration depth and the barrier field are also evaluated. (author)