Berlin 2024 – scientific programme
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MA: Fachverband Magnetismus
MA 4: Magnetic Heusler Compounds and Complex Magnetic Oxides
MA 4.9: Talk
Monday, March 18, 2024, 11:45–12:00, EB 107
Multipolar interactions as the origin of excitation gap in d3 spin-orbit double perovskites — •Leonid Pourovskii — CPHT, CNRS, École polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France — Collège de France, Université PSL, 11 pl. Marcelin Berthelot, 75005 Paris, France
In Mott insulators with a half-filled t2g shell the Hund’s rule coupling JH induces a spin S=3/2 orbital-singlet ground state. The spin-orbit interaction is not effective within this ground state and conventional spin orders are expected. This is the case in d3 cubic double perovskites (DP) of heavy transition metals. However, their inelastic neutron scattering (INS) spectra feature unexpectedly large gaps. Even in the cubic DP Ba2YB′O6 (B′=Os, Ru), where single-ion anisotropy is expected to be negligible, the measured gaps are remarkably large. We employ an ab initio many-body force-theorem method to obtain effective magnetic Hamiltonians for these two systems. The calculated Hamiltonains feature unexpectedly significant multipolar - dipole-octupolar (DO) - intersite exchange interactions. The DO terms break continuous symmetry of the S=3/2 Heisenberg terms opening an excitation gap. The theoretical gap magnitudes and calculated INS spectra agree with experiment. The DO intersite coupling arises due to excited states of the t2g3 manifold admixed by spin-orbit into the S=3/2 ground state. Their large relative magnitude stems from a characteristic anisotropy of t2g hopping terms in DPs and scales as a square of the ratio of spin-orbit coupling and JH.
Keywords: magnetic excitations; multipolar moments; correlated insulators; ab initio calculations; exchange interactions