The change-over from phase- to amplitude-fluctuation-driven superconductivity is examined for a composite system of free electrons (fermions with concentration
nF) and localized electron pairs (hard-core bosons with concentration
nB) as a function of doping---changing the total concentration of charge carriers
(ntot=nF+2nB). The coupling together of these two subsystems via a charge exchange term induces electron pairing and ultimately superconductivity in the fermionic subsystem. The difference in statistics of the two species of charge carriers has important consequences on the doping mechanism, showing an onset temperature
T∗ of incoherent electron pairing in the fermionic subsystem (manifest in form of a pseudogap), which steadily decreases with decreasing
ntot. Below
T∗ this electron pairing leads, in the normal phase, to electron-pair resonant states (Cooperons) with quasiparticle features which strongly depend on
ntot. For high concentrations, where ${n}_{B}\ensuremath{\simeq}0.5,$ correlation effects between the hard-core bosons lead to itinerant Cooperons having a heavy mass
mp, but are long lived. Upon reducing the concentration of charge carriers and consequently
nB, the mass as well as the lifetime of those Cooperons is considerably reduced. As a result, for high values of
nB, a superconducting state below
T∗ sets in at a
Tc, being controlled by the phase stiffness ${D}_{\ensuremath{\varphi}}={\ensuremath{\Elzxh}}^{2}{n}_{p}{/m}_{p}$ of those Cooperons, where
np denotes their density. Upon reducing
ntot, the phase stiffness steadily increases, and eventually exceeds the pairing energy
kBT∗. There, 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-
Tc superconductivity, except that in our scenario the determinant factors are the mass and the lifetime of the Cooperons rather than their density.
Effects of random impurities on one-dimensional interacting electron systems are studied by use of the quantum Monte Carlo method and the phase variable representation of charge density and superconducting fluctuations. The impurities pin the phase variable of charge density fluctuation, whereas the quantum fluctuation around the pinned state reduces the pinning effect yielding increase of the superconducting fluctuation since the charge density and the superconducting fluctuations are complementary to each other. Such interplay between randomness and interactions is shown explicitly by calculating spatial dependence of quantum fluctuations, metastable states and correlation functions for superconductivity. Quantum character of metastable states is discussed. It is found that superconducting state is more sensitive to impurities than charge density wave state when the interaction between electrons is weak.
Engineering quantum phases using light is a novel route to designing functional materials, where lightinduced superconductivity is a successful example.Although this phenomenon has been realized experimentally, especially for the high-T c cuprates, the underlying mechanism remains mysterious.Using the recently developed variational non-Gaussian exact diagonalization method, we investigate a particular type of photoenhanced superconductivity by suppressing a competing charge order in a strongly correlated electron-electron and electron-phonon system.We find that the d-wave superconductivity pairing correlation can be enhanced by a pulsed laser, consistent with recent experiments based on gap characterizations.However, we also find that the pairing correlation length is heavily suppressed by the pump pulse, indicating that light-enhanced superconductivity may be of fluctuating nature.Our findings also imply a general behavior of nonequilibrium states with competing orders, beyond the description of a mean-field framework.