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Register a new userRaman spectroscopic evidence for multiferroicity in rare earth nickelate single crystals
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The rare earth nickelates RNiO3 are metallic at high temperatures and insulating and magnetically ordered at low temperatures. The low temperature phase has been predicted to be type II multiferroic, i.e. ferroelectric and magnetic order are coupled and occur simultaneously. Confirmation of those ideas has been inhibited by the absence of experimental data on single crystals. Here we report on Raman spectroscopic data of RNiO3 single crystals (R = Y, Er, Ho, Dy, Sm, Nd) for temperatures between 10 K and 1000 K. Entering the magnetically ordered phase we observe the appearance of a large number of additional vibrational modes, implying a breaking of inversion symmetry expected for multiferroic order.
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Heterostructure engineering provides an efficient way to obtain several unconventional phases of LaNiO3, which is otherwise paramagnetic, metallic in bulk form. In this work, a new class of short periodic superlattices, consisting of LaNiO3 and EuNiO3 have been grown by pulsed laser interval deposition to investigate the effect of structural symmetry mismatch on the electronic and magnetic behaviors. Synchrotron based soft and hard X-ray resonant scattering experiments have found that these heterostructures undergo simultaneous electronic and magnetic transitions. Most importantly, LaNiO3 within these artificial structures exhibits a new antiferromagnetic, charge ordered insulating phase. This work demonstrates that emergent properties can be obtained by engineering structural symmetry mismatch across a heterointerface.
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Using a combination of density functional theory and dynamical mean field theory we show that electric polarization and magnetism are strongly intertwined in (TMTTF)$_2$-$X$ (X$=$PF$_6$, As$F_6$, and SbF$_6$) organic crystals and they originate from short-range Coulomb interactions. Electronic correlations induce a charge-ordered state which, combined with the molecular dimerization, gives rise to a finite electronic polarization and to a ferroelectric state. We predict that the value of the electronic polarization is enhanced by the onset of antiferromagnetism showing a sizable magnetoelectric leading to a multiferroic behavior of (TMTTF)$_2$-$X$ compounds.
In rare-earth cage compounds, the guest 4f ion cannot be considered as fixed at the centre of its cage. As result of the electronic degeneracy of the 4f shell, single-ion or collective mechanisms can redistribute the ion inside the cage, which can be described in terms of multipolar components. These mechanisms and their influence are here discussed and illustrated in relation with the rare-earth hexaboride series. Warning: Following our oral presentation, this manuscript should have appeared in the Proceedings of SCES 2014 (SCES 2014, International Conference on Strongly Correlated Electron Systems, held 7 - 11 July 2014 in Grenoble). An infuriated referee decided otherwise stating, in substance, that ... it could corrupt the youth ... (the very few interested in this particular the subject). The casual reader is here free to appreciate how far this corruption goes...
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