Ferro-octupolar Order and Low-Energy Excitations in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">d</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>Double Perovskites of Osmium
Leonid V. Pourovskii, Dario Fiore Mosca, Cesare Franchini
Abstract
Conflicting interpretations of experimental data preclude the understanding of the quantum magnetic state of spin-orbit coupled ${d}^{2}$ double perovskites. Whether the ground state is a Janh-Teller--distorted order of quadrupoles or the hitherto elusive octupolar order remains debated. We resolve this uncertainty through direct calculations of all-rank intersite exchange interactions and inelastic neutron scattering cross section for the ${d}^{2}$ double perovskite series ${\text{Ba}}_{2}M{\mathrm{OsO}}_{6}$ ($M=\mathrm{Ca}$, Mg, Zn). Using advanced many-body first-principles methods, we show that the ground state is formed by ferro-ordered octupoles coupled by superexchange interactions within the ground-state ${E}_{g}$ doublet. Computed ordering temperature of the single second-order phase transition is consistent with experimentally observed material-dependent trends. Minuscule distortions of the parent cubic structure are shown to qualitatively modify the structure of gaped magnetic excitations.