Order and quantum phase transitions in the cuprate superconductors

TL;DR

This paper discusses how cuprate superconductors exhibit complex order parameters and quantum phase transitions, emphasizing the role of Mott insulator proximity and various experimental evidence for these phenomena.

Contribution

It introduces a classification of 2D Mott insulators relevant to cuprates and links quantum phase transitions of multiple orders to experimental observations.

Findings

Evidence of collinear spin correlations in cuprates

Detection of bond order and confinement of spin excitations

Proximity to quantum critical points influences physical properties

Abstract

It is now widely accepted that the cuprate superconductors are characterized by the same long-range order as that present in the Bardeen-Cooper-Schrieffer (BCS) theory: that associated with the condensation of Cooper pairs. We argue that many physical properties of the cuprates require interplay with additional order parameters associated with a proximate Mott insulator. We review a classification of Mott insulators in two dimensions, and contend that the experimental evidence so far shows that the class appropriate to the cuprates has collinear spin correlations, bond order, and confinement of neutral, spin S=1/2 excitations. Proximity to second-order quantum phase transitions associated with these orders, and with the pairing order of BCS, has led to systematic predictions for many physical properties. We use this context to review the results of recent neutron scattering, fluxoid detection, nuclear magnetic resonance, and scanning tunnelling microscopy experiments.