  A grand challenge in many-body quantum physics is to explain the apparent
connection between quantum criticality and high-temperature superconductivity
in the cuprates and similar systems, such as the iron pnictides and
chalcogenides. Here we argue that the quantum-critical regime plays an
essential role in activating a strong-pairing mechanism: although pairing
bosons create a symmetry-breaking instability which suppresses pairing, the
combination of these broken-symmetry states within the critical regime can
restore this symmetry for the paired quasiparticles. This condition is shown to
be met within a large-U ansatz. A hidden quantum phase transition then arises
between a Fermi-liquid and a non-Fermi-liquid broken-symmetry striped state,
and a critical regime in which the broken-symmetry states are combined.
