Measurements on superconducting films

In this thesis, three distinct types of experiments are discussed having the common theme of superconducting films. The far infrared absorption of thin superconducting films under pair-breaking conditions is discussed. It has been known for a long time that the pairing of electrons (Cooper pairs) is the basis of the phenomenon of superconductivity. It is also well-known that certain external perturbations such as paramagnetic impurities, magnetic fields, electric currents and the so-called ''proximity effect'' (a non-superconducting metal in good electrical contact with a superconductor) have a pair-breaking influence. The far infared absorption of superconductors has been measured under the influence of two different pair-breaking effects, i.e.: a superconducting film in a perpendicular magnetic field and a superconducting film in proximity effect contact with a normal metal. The experimental results are compared with the general pair-breaking theory of Abrikosov and Gorkov (AG). It turns out that the agreement is rather poor due to a spatial dependence of the superconducting properties in these pair-breaking situations not contained in the AG theory. An extension of the AG theory, taking into account this spatial dependence, leads to far better agreement. The nucleation field of a wedge-shaped superconductor is discussed. The critical or nucleation field of a superconductor is defined as the field where nucleation starts when the magnetic field is decreased. This field depends on the geometry of the superconductor. In particular, the nucleation field increases near an interface. The nucleation field has been calculated and measured for a wedge-shaped superconductor as a function of the orientation of the magnetic field. The agreement here between theory and experiment is also good. Finally, there is a report on the ''depinning of vortices'' and the influence on it by the proximity effect. A magnetic field penetrates a thin superconducting film in the form of quantized vortices (small non-superconducting areas); an electric current through the film exerts a Lorentz force on the vortices. When the Lorentz force exceeds the so-called ''Pinning force,'' the vortices will move. The phenomenon of depinning has been investigated as a function of the orientation of the magnetic field under the influence of the proximity effect of a normal metal