Untangling the structural, magnetic dipole, and charge multipolar orders in Ba2MgReO6 and Cs2TaCl6

We present density-functional calculations of hidden multipolar orders in 5d transition-metal double perovskites, using Ba2MgReO6 and Cs2TaCl6 as examples. Recent studies have suggested the existence of unconventional types of ordering, not detectable using typical probes, in these 5d double perovskites, stemming from the interplay of spin-orbit coupling, crystal-field splitting, and electron-electron repulsion. These "hidden orders" are formed from ordered anisotropies in the electronic charge and magnetization density distributions that can be described within multipole expansions beyond the usual dipolar distributions and include ordered charge quadrupoles and magnetic octupoles, as well as higher-order multipoles.

We also present an experimental study using resonant elastic x-ray scattering to directly probe the existence of charge quadrupoles in Cs2TaCl6. In addition to shedding light on the detailed behavior of Ba2MgReO6 and Cs2TaCl6, our analysis provides a novel insight into the crystal structure-multipolar order relationship of 5d double perovskites.

Materials Theory, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland

Institute of Physics, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland}