  The change-over from phase- to amplitude-fluctuation driven superconductivity
is examined for a composite system of free electrons (Fermions with
concentration n_F) and localized electron-pairs (hard-core Bosons with
concentration n_B) as a function of doping-changing n_B. The coupling together
of these two subsystems via a charge exchange term induces electron pairing
below a certain T^* (showing up in form of a pseudogap) and ultimately
superconductivity in the Fermionic subsystem. T^* steadily decreases with
decreasing n_B. Below T^* this electron pairing leads to electron-pair resonant
states (Cooperons) with quasi-particle features which strongly depend on $n_B$.
For high concentrations, (n_B \simeq 0.5), correlation effects between the
hard-core Bosons lead to itinerant Cooperons having a heavy mass m_p, but are
long-lived. Upon reducing n_B, the mass as well as the lifetime of those
Cooperons is considerably reduced. For high values of n_B, a superconducting
state sets in at a T_c, being controlled by the phase stiffness D_\phi=\hbar^2
n_p/m_p of those Cooperons, where n_p denotes their density. Upon reducing n_B,
the phase stiffness steadily increases, and eventually exceeds the pairing
energy k_B T^*. The Cooperons loose their well defined itinerant quasi-particle
features and superconductivity gets controlled by amplitude fluctuations. The
resulting phase diagram with doping is reminiscent of that of the phase
fluctuation scenario for high T_c superconductivity, except that in our
scenario the determinant factors are the mass and the lifetime of the Cooperons
rather than their density.
