Phase separation in the vicinity of "quantum critical" doping concentration: implications for high temperature superconductors

TL;DR

This paper introduces a quantitative measure for phase separation tendency in systems with phase transitions, applies it to high-temperature cuprate superconductors, and suggests cuprates are near the threshold of phase separation.

Contribution

It develops a new measure for phase separation propensity and applies it to cuprates, linking antiferromagnetic correlations and Coulomb interactions to phase separation.

Findings

Cuprates are close to the phase separation threshold.

Coulomb repulsion accelerates decay of antiferromagnetic correlations.

Phase separation could be nanoscale or macroscopic in cuprates.

Abstract

A general quantitative measure of the tendency towards phase separation is introduced for systems exhibiting phase transitions or crossovers controlled by charge carrier concentration. This measure is devised for the situations when the quantitative knowledge of various contributions to free energy is incomplete, and is applied to evaluate the chances of electronic phase separation associated with the onset of antiferromagnetic correlations in high-temperature cuprate superconductors. The experimental phenomenology of lanthanum- and yittrium-based cuprates was used as input to this analysis. It is also pointed out that Coulomb repulsion between charge carriers separated by the distances of 1-3 lattice periods strengthens the tendency towards phase separation by accelerating the decay of antiferromagnetic correlations with doping. Overall, the present analysis indicates that cuprates are realistically close to the threshold of phase separation -- nanoscale limited or even macroscopic with charge density varying between adjacent crystal planes.