Orientation of the intra-unit-cell magnetic moment in the high-Tc superconductor HgBa2CuO$_{4+{\delta}}$

TL;DR

This study uses polarized-neutron diffraction to precisely measure the intra-unit-cell magnetic moment orientation in HgBa2CuO4+δ, revealing a non-trivial angle that challenges existing theoretical models of IUC magnetism in cuprates.

Contribution

First precise measurement of the IUC magnetic moment direction in HgBa2CuO4+δ, providing new constraints on microscopic models of pseudogap magnetism in high-Tc superconductors.

Findings

Magnetic critical scattering observed at the transition temperature.

Moment direction determined to be approximately 70° from the normal to CuO2 layers.

Results exclude purely planar loop currents and high-symmetry Dirac multipoles as the sole origin.

Abstract

Polarized-neutron diffraction experiments (PND) have revealed that the pseudogap state of the cuprates exhibits unusual intra-unit-cell (IUC) magnetism. At a qualitative level, the data indicate a moment direction that is neither perpendicular nor parallel to the CuO2 layers, yet an accurate measurement of a structurally simple compound has been lacking. Here we report PND results with unprecedented accuracy for the IUC magnetic order in the simple-tetragonal single-CuO$_2$-layer compound HgBa2CuO$_{4+{\delta}}$. At the transition temperature, we find evidence for magnetic critical scattering. Deep in the ordered state, we determine the moment direction to be 70{\deg} $\pm$ 10{\deg} away from the normal to the CuO$_2$ layers, which rules out both purely planar loop currents and high-symmetry Dirac multipoles, the two most prominent theoretical proposals for the microscopic origin of the IUC magnetism. However, the data are consistent with Dirac multipoles of lower symmetry or, alternatively, with a particular configuration of loop currents that flow on the faces of the CuO$_6$ octahedra.