Octupolar order in d-orbital systems

Conventional magnetism in solids deals with ordering patterns of the electron magnetic dipole moment, which can be probed, for instance, via neutron diffraction. However, materials such as f-electron heavy fermion systems are well-known candidates where more complex forms of “hidden” symmetry breaking, involving higher-order magnetic or electric multipoles, are likely to be important. In this talk, I will discuss our recent theoretical proposal for Ising octupolar order in spin-orbit coupled d-orbital systems [1,2,3], which appears to explain a wide range of experiments in 5d transition metal oxides such as Ba2ZnOsO6, Ba2CaOsO6, and Ba2MgOsO6. These include elastic neutron diffraction and X-ray diffraction, inelastic neutron scattering, and muon spin relaxation experiments. The proposed Ising ferro-octupolar order in these Mott insulators is shown to be linked to a specific type of uniform plaquette loop-current order which can explain the muon spin rotation experiments. We propose field-dependent Raman scattering and NMR experiments under strain as further tests of our proposal. This appears to be the first candidate for octupolar order in a d-orbital system. Our work highlights the intimate connection between heavy d-orbital oxides and f-electron based heavy fermion materials.