DMFT-QE Symposium: November 25

Talk 1:

Multipolar Order and Excitations in 5d Double Perovskites

Bruce D. Gaulin, McMaster University and CIFAR Quantum Materials

Double perovskites are a common class of materials with the chemical composition A 2 BB’O 6. Cubic double perovskites possess a crystalline architecture wherein both the B and B’ sublattices form a network of edge sharing tetrahedra – that is, they form FCC sublattices. We have carried out neutron spectroscopic and diffraction studies on powder samples of several 5d families of cubic double perovskites, that involve Os and Re in different oxidation states.  This includes 5d 3 systems, such as Ba 2 YOsO 6 with Os 5+ , 5d 2 systems such as Ba 2 MgOsO 6 with Os 6+ , and the 5d 1 system Ba 2 MgReO 6 with Re 6+ .  The 5d 3 double perovskites all display well defined Neel order with sizeable dipole magnetic moments, while the 5d1 system displays both a structural and a magnetic transition on lowering temperature, consistent with expectations for coupled quadrupolar and dipolar degrees of freedom.  The 5d 2 cubic double perovskites are particularly intriguing as these display clear phase transitions at T* ~ 45 K where ~ R ln(2) in entropy is released on passing through T*. However, while magnetic inelastic scattering is clearly observed, there is no evidence for the ordering of dipole moments in neutron diffraction, with an upper limit on any possible ordered dipole moment of ~ 0.1 muB.  We understand these results in terms of ferro-octupolar order below T*, and argue that this is consistent with the full set of experimental observations on these 5d 2 cubic double perovskites. We believe this likely constitutes the first compelling case for octupolar order in a d-orbital Mott insulator.

Talk 2:

Multipolar interactions, hidden orders and magnetic excitations in spin-orbit double perovskites

Leonid V. Pourovskii, CPHT-École polytechnique, Institut Polytechnique de Paris, Collège de France, Université PSL

Spin-orbit (SO) cubic double perovskites (DP) of heavy transition metals exhibit exotic “hidden”-order low-temperature ordered phases as well as unusual “conventional” magnetic orders and excitations. Those orders arise due to high-rank multipolar moments of charge and magnetic density hosted by SO-entangled ground states of transition metal ions. Previous studies of ordered phases in SO DP employed simplified tight-binding models to derive superexchange coupling between multipolar moments. Electron-lattice coupling between Jahn-Teller (JT) active ions was also considered on a model level.

We apply an ab initio force-theorem method based on DFT+DMFT in a quasi-atomic approximation to derive realistic superexchange Hamiltonians for SO DP with various d-shell occupancies. The Hamiltonians are subsequently solved within a single-site mean-field theory; magnetic excitations are evaluated within a random phase approximation. We review the results obtained within this framework for d 2 and d 3 DP. We find that purely electronic exchange mechanisms do not account for observed ordered phases in JT-active d 1 DP systems; electron-lattice coupling thus should be included. We determine all the parameters of this extended low-energy model – superexchange, JT and elastic coupling couplings – from first principles and solve it in within a consistent mean-field approximation. The results obtained within this approach for d 1 DP of Rhenium are discussed.

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