Ferro-type order of magneto-electric quadrupoles in the pseudo-gap phase of YBa2Cu3O6 + x unearthed by neutron diffraction

TL;DR

This paper presents a comprehensive theory explaining neutron diffraction observations in the pseudo-gap phase of YBa2Cu3O6+x, proposing magneto-electric quadrupoles as the primary order parameter, which challenges conventional magnetic models.

Contribution

It introduces a novel theoretical framework that attributes neutron diffraction signals to magneto-electric quadrupoles, not spin or orbital dipoles, in the pseudo-gap phase of cuprate superconductors.

Findings

Magneto-electric quadrupoles explain neutron diffraction data.

Absence of spin and orbital dipoles aligns with superconductivity.

Magneto-electric quadrupoles are proposed as the primary order parameter.

Abstract

A theory that fully describes previously observed neutron Bragg spots indicative of magnetic order in the pseudo-gap phase of YBa2Cu3O6 + x is presented. Magnetic diffraction intensities, unveiled by polarization analysis, are explained by parity-violating magnetic (magneto-electric) quadrupoles at in-plane Cu sites arranged in a simple ferro-type motif. An absence of conventional spin and orbital angular momentum dipoles in the theory is compatible with the occurrence of superconductivity and strongly suggests, moreover, that magneto-electric quadrupoles are the primary order-parameter in the pseudo-gap phase.