Quantum Criticality Stabilizes High T_c Superconductivity Against Competing Symmetry-Breaking Instabilities

TL;DR

Quantum criticality can enhance high-temperature superconductivity by suppressing competing symmetry-breaking instabilities, explaining the phase diagram features of cuprates and iron-based superconductors.

Contribution

This work introduces a theoretical framework showing how quantum criticality stabilizes high-T_c superconductivity against competing orders.

Findings

Quantum criticality allows strong-coupling superconductivity by mitigating symmetry-breaking effects.

The phase diagram of cuprates reflects a quantum phase transition between Fermi-liquid and non-Fermi-liquid states.

Partial pairing occurs within the pseudogap regime, indicating complex interplay of phases.

Abstract

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.