Induced superconducting state and two-gap structure: Application to cuprate superconductors and conventional multilayers

An induced superconducting state caused by charge transfer between intrinsically superconducting ({α}) and intrinsically normal ({β}) subsystems is studied. A most interesting case is a layered system with some layers being normal. An analysis of the general Hamiltonian describing the phenomenon allows us to evaluate Tc and the spectrum, which displays a two-gap structure. A superconducting state can be induced through different charge transfer channels (intrinsic proximity effect; inelastic two-band channel). A very important contribution comes from the ``mixed'' channel. Systems with various strengths of the coupling are described. The presence of magnetic impurities leads to an induced gapless superconductivity. This model is applied to high-Tc cuprates (in particular, to Y-Ba-Cu-O), as well as to conventional systems. The spectroscopy of Y-Ba-Cu-O appears to be very sensitive to the oxygen content whereas Tc changes relatively slowly. The model is directly related to such phenomena as residual microwave losses, zero-bias anomalies, the ``plateau'' effect, etc.