Quasi-static magnetoelectric quadrupoles as the order parameter for the pseudo-gap phase in cuprate superconductors

TL;DR

This paper proposes that quasi-static magnetoelectric quadrupoles, arising from coupling between spin moments and phonons, serve as the order parameter for the pseudo-gap phase in cuprate superconductors, supported by first-principles calculations.

Contribution

It introduces a novel mechanism linking spin fluctuations and phonons to explain the pseudo-gap phase, proposing magnetoelectric multipoles as the order parameter, supported by first-principles calculations.

Findings

Mechanism consistent with experimental pseudo-gap data

Magnetoelectric multipoles as the order parameter

Embraces aspects of orbital current models

Abstract

We introduce a mechanism in which coupling between fluctuating spin magnetic dipole moments and polar optical phonons leads to a non-zero ferroic ordering of quasi-static magnetoelectric multipoles. Using first-principles calculations within the LSDA$+U$ method of density functional theory, we calculate the magnitude of the effect for the prototypical cuprate superconductor, HgBa$_2$CuO$_4$. We show that our proposed mechanism is consistent, to our knowledge, with all experimental data for the onset of the pseudo-gap phase and therefore propose the quasi-static magnetoelectric multipole as a possible pseudo-gap order parameter. Finally, we show that our mechanism embraces some key aspects of previous theoretical models, in particular the description of the pseudo-gap phase in terms of orbital currents.