  The occurrence of high-T_c superconductivity in systems including the
cuprates and the iron-based superconductors, is known to coincide with the
existence of anomalous normal-state properties which have been associated with
quantum criticality. We argue here that this observation results from the fact
that quantum criticality can allow the occurrence of very-strong-coupling
superconductivity by preventing its suppression due to competing
symmetry-breaking instabilities. Treating the electrons through a large-U
ansatz yields their separation into boson quasiparticles which are directly
involved in the formation of these instabilities, represented as their Bose
condensates, and charge-carrying fermion quasiparticles which are affected by
them indirectly. Within the critical regime, condensates corresponding to the
different broken-symmetry states are combined; consequently their negative
effect on the pairing of the fermions is strongly diminished, enabling high-T_c
superconductivity to occur. The observed phase diagram of the hole-doped
cuprates then derives from a hidden T=0 quantum phase transition between a
Fermi-liquid and a non-Fermi-liquid broken-symmetry striped state. The
pseudogap range within this diagram is found to include two distinct regimes,
with partial pairing occurring in one of them.

Superconductivity;Strongly Correlated Electrons;Quantum Physics