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 ${\mathit{T}}_{\mathit{c}}$ 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-${\mathit{T}}_{\mathit{c}}$ 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 ${\mathit{T}}_{\mathit{c}}$ changes relatively slowly. The model is directly related to such phenomena as residual microwave losses, zero-bias anomalies, the ‘‘plateau’’ effect, etc.