Two-component scenarios for non-conventional (Exotic) superconductors

The possibility of superconductivity is analyzed within models where two components are present. Here both components may be non-superconducting on their own, but become superconducting when coupled to each other. Also the possibility exists that only one system is superconducting and induces via coupling superconductivity in the second component. Another scenario is that the physical properties of the considered systems are totally different: while one is assumed to consist of itinerant fermions, the second may consist of preformed pairs, "hard core bosons". All scenarios are shown to lead to superconductivity where different pairing symmetries are possible. Typically two superconducting gaps are observed, and enhancements of the superconducting transition temperature T-c as compared to BCS theory are a consequence. The consequences for other superconducting characteristics but also for the normal state are investigated and possible applications to new non-conventional high temperature superconductors discussed.