Domain wall-localized phonons in BiFeO3: spectrum and selection rules

Abstract Ferroelectric domain walls (DWs) are nanoscale topological defects that can be easily tailored to create nanoscale devices. Their excitations, recently discovered to be responsible for GHz DW conductivity, hold promise for faster signal transmission and processing compared to the existing technology. Here we find that DW phonons have unprecedented dispersion going from GHz all the way to THz frequencies, and resulting in a surprisingly broad GHz signature in DW conductivity. Puzzling activation of nominally forbidden DW sliding modes in BiFeO3 is traced back to DW tilting and resulting asymmetry in wall-localized phonons. The obtained phonon spectra and selection rules are used to simulate scanning impedance microscopy, emerging as a powerful probe in nanophononics. The results will guide the experimental discovery of the predicted phonon branches and design of DW-based nanodevices operating in the technologically important frequency range

Les Femmes parisiennes, ou Le furet de société. Tome 1 / . Par Ponet, auteur du Jeune major...

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The energy landscape of magnetic materials

Abstract Magnetic materials can display many solutions to the electronic-structure problem, corresponding to different local or global minima of the energy functional. In Hartree-Fock or density-functional theory different single-determinant solutions lead to different magnetizations, ionic oxidation states, hybridizations, and inter-site magnetic couplings. The vast majority of these states can be fingerprinted through their projection on the atomic orbitals of the magnetic ions. We have devised an approach that provides an effective control over these occupation matrices, allowing us to systematically explore the landscape of the potential energy surface. We showcase the emergence of a complex zoology of self-consistent states; even more so when semi-local density-functional theory is augmented - and typically made more accurate - by Hubbard corrections. Such extensive explorations allow to robustly identify the ground state of magnetic systems, and to assess the accuracy (or not) of current functionals and approximations

Mechanical softness of ferroelectric 180° domain walls

Using scanning probe microscopy, we measure the out-of-plane mechanical response of ferroelectric 180° domain walls and observe that, despite separating domains that are mechanically identical, the walls appear mechanically distinct—softer—compared to the domains. This effect is observed in different ferroelectric materials (LiNbO3, BaTiO3, and PbTiO3) and with different morphologies (from single crystals to thin films), suggesting that the effect is universal. We propose a theoretical framework that explains the domain wall softening and justifies that the effect should be common to all ferroelectrics. The lesson is, therefore, that domain walls are not only functionally different from the domains they separate, but also mechanically distinct.C. S. thanks BIST for the PREBIST Grant. This projecthas received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 754558. E. L. acknowledges the funding received from the European Union’s Horizon 2020 research and innovation program through the Marie Skłodowska-Curie Actions: Individual Fellowship-Global Fellowship (Ref. No. MSCA-IF-GF708129). M. S. and K. S. acknowledge the support of the European Research Council under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529), Ministerio de Economia, Industria y Competitividad through Grants No. MAT2016- 77100-C2-2-P and No. SEV-2015-0496, and the Generalitat de Catalunya (Grant No. 2017SGR 1506). G. C. acknowledge the support of the Ministerio de Economía, Industria y Competitividad, Agencia Estatal de Investigación/Fondo Europeo de Desarrollo Regional and European Union through Grant No. MAT2016-77100-C2-1-P (MINECO/AEI/FEDER, UE).Peer reviewe