Thermodynamic and transport properties of mesoscopic conductors are strongly influenced by the proximity of a superconductor: An interplay between the large scale quantum coherent wave functions in the normal mesoscopic and the superconducting region, respectively, leads to unusual mechanisms of quantum interference. These manifest themselves in both the mean and the mesoscopic fluctuation behaviour of superconductor/normal-metal (SN) hybrid systems being strikingly different from those of conventional mesoscopic systems. After reviewing some established theories of SN-quantum interference phenomena, we introduce a new approach to the analysis of SNmesoscopic physics. Essentially, our formalism represents a unification of the quasi-classical formalism for describing mean properties of SN-systems on the one hand, with more recent field theories of mesoscopic fluctuations on the other hand. Thus, by its very construction, the new approach is capable of exploring both averaged and fluctuation properties of SN-systems on the same microscopic footing. As an example, the method is applied to the study of various characteristics of the single particle spectrum of SNS-structures.

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We construct a supersymmetric field theory for the problem of a two-dimensional electron gas in a random, static magnetic field. We find a new term in the free energy in addition to those present in the conventional unitary sigma model, whose presence relies on the long-range nature of the disorder correlations. Under a perturbative renormalization group analysis of the free energy, the new term contributes to the scaling function at one-loop order and leads to antilocalization.

We consider a two-dimensional electron gas with long range disorder. Assuming that time reversal symmetry is broken either by an external magnetic field or, as in the case of a delta-correlated random magnetic field, by the disorder itself, we derive a supermatrix σ-model. As an intermediate step, we provide a microscopic derivation of the ballistic σ-model, and find that certain corrections to its usual form may become important. We then integrate out degrees of freedom corresponding to short length scales to derive a low-energy supermatrix σ-model. We find an extra term in the free energy that couples to the correlator of local currents. Use of a proper ultraviolet regularisation procedure that preserves gauge invariance indicates that the contribution of the extra term seems finally to become irrelevant. Within the scope of our analyis, we therefore do not find any deviation of the scaling behaviour of the delta-correlated random magnetic field model from that of the conventional unitary ensemble. We then generalize the discussion to include models of even longer-ranged disorder, plus short-range disorder. When the disorder is sufficiently long-ranged that the local currents become delta-correlated, a new term appears in the free energy that does give rise to logarithmic corrections to the conductivity. A renormalisation group analysis of the free energy yields a scaling form for the diffusion coefficient which contains both a positive correction, that represents classical superdiffusion, and a negative correction, which is the usual weak localization correction. The fact that both corrections are of the same order and opposite sign leads to the interesting possibility of a quantum phase transition at weak disorder in two dimensions, tuned by the relative strengths of the short and long range disorder.PACS numbers: 72.15. Rn,73.20Fz, Typeset using REVT E X 1