Superconducting films and interfaces: Novel features from spin imbalance and Rashba spin-orbit coupling

In recent years, the manufacturing of controlled ultra-thin superconducting films has made impressive progress.Important examples are monoatomic or monomolecular layers on a substrate, superconducting layers in a superlattice, or superconducting interfaces and surfaces. These systems have in common the absence of inversion symmetry and hence the presence of Rashba-type spin-orbit energy $\lambda$. The latter can be modified to some extent by varying the thickness, the number of layers, or by applying an electric voltage. The subtle interplay of spin imbalance created by a magnetic field and the Rashba spin-orbit interaction gives rise to novel phenomena in quasi-2D superconductors which, in turn, could provide new functionalities. I discuss the results of a microscopic theory of superconducting films with population imbalance which are subject to Rashba spin- orbit interaction. The full range from small to large spin-orbit interaction is covered. A quantum phase transition at a critical value of the spin-orbit energy ${\lambda }_c$ is predicted where abrupt changes in the superconducting state are expected. It is tempting to speculate, that this feature might give rise to novel phenomena. For example, the Josephson interference effects should change dramatically near this point. These and other open issues together with recent work and promising future directions will be discussed.