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See detailRaman spectroscopy of rare-earth orthoferrites RFeO3 (R=La, Sm, Eu, Gd Tb, Dy)
Weber, Mads Christof; Guennou, Mael UL; Zhao, Hong Jian et al

in PHYSICAL REVIEW B (2016), 94(21),

We report a Raman scattering study of six rare-earth orthoferrites La, Sm, Eu, Gd, Tb, Dy. The use of extensive polarized Raman scattering of SmFeO3 and first-principles calculations enable the assignment ... [more ▼]

We report a Raman scattering study of six rare-earth orthoferrites La, Sm, Eu, Gd, Tb, Dy. The use of extensive polarized Raman scattering of SmFeO3 and first-principles calculations enable the assignment of the observed phonon modes to vibrational symmetries and atomic displacements. The assignment of the spectra and their comparison throughout the whole series allow correlating the phonon modes with the orthorhombic structural distortions of RFeO3 perovskites. In particular the positions of two specific A(g) modes scale linearly with the two FeO6 octahedra tilt angles, allowing the distortion to be tracked throughout the series. At variance with literature, we find that the two octahedra tilt angles scale differently with the vibration frequencies of their respective A(g) modes. This behavior, as well as the general relations between the tilt angles, the frequencies of the associated modes, and the ionic radii are rationalized in a simple Landau model. The reported Raman spectra and associated phonon-mode assignment provide reference data for structural investigations of the whole series of orthoferrites. [less ▲]

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See detailMultiple strain-induced phase transitions in LaNiO3 thin films
Weber, M. C.; Guennou, Mael UL; Dix, N. et al

in PHYSICAL REVIEW B (2016), 94(1),

Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or ... [more ▼]

Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or fully relaxed films, appear under both compressive and tensile strains indicating symmetry reductions. Interestingly, the Raman spectra and the underlying crystal symmetry for tensile and compressively strained films are different. Extensive mapping of LaNiO3 phase stability is addressed by simulations, showing that a variety of crystalline phases are indeed stabilized under strain. The calculated Raman frequencies reproduce the principal features of the experimental spectra, supporting the validity of the multiple strain-driven structural transitions predicted by the simulations. [less ▲]

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See detailUltrafast acousto-optic mode conversion in optically birefringent ferroelectrics
Lejman, Mariusz; Vaudel, Gwenaelle; Infante, Ingrid C. et al

in NATURE COMMUNICATIONS (2016), 7

The ability to generate efficient giga-terahertz coherent acoustic phonons with femtosecond laser makes acousto-optics a promising candidate for ultrafast light processing, which faces electronic device ... [more ▼]

The ability to generate efficient giga-terahertz coherent acoustic phonons with femtosecond laser makes acousto-optics a promising candidate for ultrafast light processing, which faces electronic device limits intrinsic to complementary metal oxide semiconductor technology. Modern acousto-optic devices, including optical mode conversion process between ordinary and extraordinary light waves (and vice versa), remain limited to the megahertz range. Here, using coherent acoustic waves generated at tens of gigahertz frequency by a femtosecond laser pulse we reveal the mode conversion process and show its efficiency in ferroelectric materials such as BiFeO3 and LiNbO3. Further to the experimental evidence, we provide a complete theoretical support to this all-optical ultrafast mechanism mediated by acousto-optic interaction. By allowing the manipulation of light polarization with gigahertz coherent acoustic phonons, our results provide a novel route for the development of next-generation photonic-based devices and highlight new capabilities in using ferroelectrics in modern photonics. [less ▲]

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See detailEvolution of defect signatures at ferroelectric domain walls in Mg-doped LiNbO3
Nataf, Guillaume F.; Guennou, Mael UL; Haussmann, Alexander et al

in PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS (2016), 10(3), 222-226

The domain structure of uniaxial ferroelectric lithium niobate single crystals is investigated using Raman spectroscopy mapping. The influence of doping with magnesium and poling at room temperature is ... [more ▼]

The domain structure of uniaxial ferroelectric lithium niobate single crystals is investigated using Raman spectroscopy mapping. The influence of doping with magnesium and poling at room temperature is studied by analysing frequency shifts at domain walls and their variations with dopant concentration and annealing conditions. It is shown that defects are stabilized at domain walls and that changes in the defect structures with Mg concentration can be probed by the shift of Raman modes. We show that the signatures of polar defects in the bulk and at the domain walls differ. (C) 2016 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim [less ▲]

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