Magnetic Order in YBa$_2$Cu$_3$O$_{6+x}$ Superconductors

TL;DR

This study uses neutron diffraction to investigate magnetic order in YBa2Cu3O6+x superconductors, revealing complex magnetic phases possibly related to orbital ordering and their behavior across different doping levels.

Contribution

The paper provides detailed neutron diffraction analysis of magnetic order in YBa2Cu3O6+x, identifying multiple magnetic phases and their relation to pseudogap and superconducting states, with implications for understanding high-temperature superconductivity.

Findings

High-temperature magnetic phase likely from impurities with in-plane moments.

A second magnetic order near the pseudogap temperature with possible orbital origin.

No magnetic signal detected in YBa2Cu3O7, small signal in YBa2Cu3O6.45.

Abstract

Polarized and unpolarized neutron diffraction has been used to search for magnetic order in YBa$_2$Cu$_3$O$_{6+x}$ superconductors. Most of the measurements were made on a high quality crystal of YBa$_2$Cu$_3$O$_{6.6}$. It is shown that this crystal has highly ordered ortho-II chain order, and a sharp superconducting transition. Inelastic scattering measurements display a very clean spin-gap and pseudogap with any intensity at 10 meV being 50 times smaller than the resonance intensity. The crystal shows a complicated magnetic order that appears to have three components. A magnetic phase is found at high temperatures that seems to stem from an impurity with a moment that is in the $a$-$b$ plane, but disordered on the crystal lattice. A second ordering occurs near the pseudogap temperature that has a shorter correlation length than the high temperature phase and a moment direction that is at least partly along the c-axis of the crystal. Its moment direction, temperature dependence, and Bragg intensities suggest that it may stem from orbital ordering of the $d$-density wave (DDW) type. An additional intensity increase occurs below the superconducting transition. The magnetic intensity in these phases does not change noticeably in a 7 Tesla magnetic field aligned approximately along the c-axis. Searches for magnetic order in YBa$_2$Cu$_3$O$_{7}$ show no signal while a small magnetic intensity is found in YBa$_2$Cu$_3$O$_{6.45}$ that is consistent with c-axis directed magnetic order. The results are contrasted with other recent neutron measurements.