Polarized-neutron diffraction (PND) experiments 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 at a high-symmetry momentum point in a structurally simple compound has been lacking. Such a measurement would be crucial, as it would help to narrow down the scenarios for the microscopic origin of the IUC magnetism. Here we report PND results with unprecedented accuracy for the IUC magnetic order in the simple-tetragonal single-CuO2-layer compound HgBa2CuO4+δ. At the pseudogap temperature, we find evidence for magnetic critical scattering. Deep in the ordered state, we determine the moment direction to be 70° ± 10° away from the normal to the CuO2 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 CuO6 octahedra.
Bibliographical noteFunding Information:
The work at the University of Minnesota was funded by the Department of Energy through the University of Minnesota Center for Quantum Materials under DE-SC-0016371. We also acknowledge financial support at LLB from the projects UNESCOS (Contract No. ANR-14-CE05-0007) and NirvAna (Contract No. ANR-14-OHRI-0010) of the ANR. We acknowledge decisive discussions with Dalila Bounoua, Sergio di Matteo, Thierry Giamarchi, Stephen Lovesey, Mike Norman, and Chandra Varma on the topic of this article.
© 2018 American Physical Society.