A Tale of Two Metals: contrasting criticalities in the pnictides and hole-doped cuprates

TL;DR

This paper compares the critical behaviors of iron-based superconductors and cuprates, challenging the idea that both are governed by a quantum critical point, and proposes a new perspective on the pseudogap phase in cuprates.

Contribution

It critically examines the role of quantum critical points in pnictides and cuprates, highlighting differences and proposing a new scenario for the pseudogap phase in cuprates.

Findings

Evidence supports a QCP in iron pnictides.

The pseudogap in cuprates is linked to quasiparticle breakdown.

A new model emphasizes momentum-dependent quasiparticle loss in cuprates.

Abstract

The iron-based high temperature superconductors share a number of similarities with their copper-based counterparts, such as reduced dimensionality, proximity to states of competing order, and a critical role for 3d electron orbitals. Their respective temperature-doping phase diagrams also contain certain commonalities that have led to claims that the metallic and superconducting properties of both families are governed by their proximity to a quantum critical point (QCP) located inside the superconducting dome. In this review, we critically examine these claims and highlight significant differences in the bulk physical properties of both systems. While there is now a large body of evidence supporting the presence of a (magnetic) QCP in the iron pnictides, the situation in the cuprates is much less apparent, at least for the end point of the pseudogap phase. We argue that the opening of the normal state pseudogap in cuprates, so often tied to a putative QCP, arises from a momentum-dependent breakdown of quasiparticle coherence that sets in at much higher doping levels but which is driven by the proximity to the Mott insulating state at half filling. Finally, we present a new scenario for the cuprates in which this loss of quasiparticle integrity and its evolution with momentum, temperature and doping plays a key role in shaping the resultant phase diagram.