Quantum Criticality Stabilizes High T_c Superconductivity Against   Competing Symmetry-Breaking Instabilities
  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. 
Josef Ashkenazi and Neil F. Johnson
cond-mat.supr-con cond-mat.str-el quant-ph