Fate of multipolar physics in $5d^2$ double perovskites

Abstract

In a cubic environment, the ground state of spin-orbit coupled $5d^2$ ions is a non-Kramers $E_g$ doublet, which hosts quadrupole and octupole moments. A series of $5d^2$ osmium double perovskites Ba${}_{2}M$OsO${}_6$ (M = Mg, Ca, Zn, Cd) have recently been proposed to exhibit multipolar orders. We investigate the structural properties of these materials using $\text{ab}$-$\text{initio}$ calculations and find that the cubic structure is unstable for the Cd compound while the Mg, Ca, and Zn materials retain $Fm\bar{3}m$ symmetry. We show that Ba${}_2$CdOsO${}_6$ favours a rhombohedral $R\bar{3}$ structure characterized by $a^{-}{a}^{-}{a}^{-}$ octahedral tiltings as indicated by unstable ${\mathcal{T}}_{1g}$ phonon modes. Trigonal distortions split the excited $T_{2g}$ triplet into an $E_g^{\prime }$ doublet and an $A_g$ singlet, which may cross energy levels with the $E_g$ doublet and suppress the multipolar physics. We find a window where $E_g$ remains the lowest energy state under trigonal distortion, enabling the emergence of multipole phases in non-cubic crystal environments.

arXiv comment9 pages, 6 figures, journal version with minor revisions

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• Emergence of quadrupolar order under magnetic field in $5d^2$ double perovskites
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