On self-protecting singlets in cuprate superconductors

TL;DR

This paper proposes a model where self-protecting singlets in cuprate superconductors explain the observed incompressibility and doping-dependent behavior of the Cu-Cu lattice, linking electronic structure to lattice interactions.

Contribution

It introduces a classical liquid model of self-protecting singlets to explain lattice behavior and doping effects in cuprate superconductors, highlighting a new perspective on their electronic states.

Findings

Incompressible 'hump' in basal area around optimal doping

Self-protecting singlets form closest packing at critical doping

Singlet states are bosonic but not magnetic polarons

Abstract

The basal area (Cu-Cu grid) of the cuprate superconductors not only tends to shrink on hole doping, as expected from single electron quantum chemistry, but exhibits also an electronically incompressible "hump'' around optimum doping n_opt = 0.16. The hump collapses near critical doping n_crit = 0.19. We analyze the origin of the hump in terms of a classical liquid of interacting incompressible particles in a container with antiferromagnetic walls. Oxygen holes interacting with the wall form singlets, protect themselves against other holes by an incompressible "spin fence'', and thus interact also with the lattice. Occupation of the CuO_2 lattice with holes must therefore follow a non-double-occupant constraint also for the oxygen cage enclosing the copper hole. Closest packing of self-protecting singlets is found to occur around critical doping; closest packing of paired self-protecting singlets around optimum doping. These singlet-states are bosonic, but are not magnetic polarons.