Ferro-octupolar order and low-energy excitations in d$^2$ double perovskites of Osmium

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-Tellerdistorted order of quadrupoles or the hitherto elusive octupolar order remains debated. We resolve this uncertainty through direct calculations of all-rank inter-site exchange interactions and inelastic neutron scattering (INS) cross-section for the d 2 double perovskite series Ba2M OsO6 (M = Ca, Mg, Zn). Using advanced many-body first principles methods we show that the ground state is formed by ferro-ordered octupoles coupled within the ground-stated Eg doublet. Computed ordering temperature of the single second-order phase-transition and gapped excitation spectra are fully consistent with observations. Minuscule distortions of the parent cubic structure are shown to qualitatively modify the structure of magnetic excitations.

Octupolar order and Ising quantum criticality tuned by strain and dimensionality: Application to $d$-orbital Mott insulators

“…Despite the seeming success of our phenomenological proposal, our previous work did not identify the microscopic origin of the octupolar order. A very recent study combining ab initio and dynamical mean field theory calculations [37] has found unequivocal evidence of FO exchange -however, the essential ingredients favoring octupolar order were not fully clarified in this work. At the same time, several competing theories have emerged for the phase transition observed in these osmates.…”