Administration Locale:- Membre d'un pool d'experts
- Responsable de diplôme (M2)
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Domaines de Recherche: - Physique/Matière Condensée/Systèmes désordonnés et réseaux de neurones
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Dernieres productions scientifiques :
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Sotf mode in cubic PbTiO3 by hyper-Raman scattering 
Auteur(s): Hlinka J., HEHLEN B., Kania A., Gregora I.
(Article) Publié:
Physical Review B, vol. p.064101 (2013)
Ref HAL: hal-00797857_v1
DOI: 10.1103/PhysRevB.87.064101
Résumé: Hyper-Raman scattering experiments allowed collecting the spectra of the lowest F1u-symmetry mode of PbTiO3 crystal in the paraelectric phase up to ≈930 K as well as down to about 1 K above the phase transition. It is realized that this mode is fully responsible for the Curie-Weiss behavior of its dielectric permittivity above Tc. Near the phase transition, this phonon frequency softens down to 17 cm-1 and its spectrum can be well modeled as a response of a single damped harmonic oscillator. It is concluded that PbTiO3 constitutes a clean example of a soft-mode-driven ferroelectric system.
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Structural investigation of strontium titanate nanoparticles and the core-shell model
Auteur(s): Kiat J-m., Bogicevic Christine, Gemeiner P., AL-ZEIN A., Karolak F., Guiblin N., Porcher Florence, HEHLEN B., Yedra Li., Estradé S., Peiro F., Haumont R.
(Article) Publié:
Physical Review B, vol. 87 p.024106 (2013)
Ref HAL: hal-00777658_v1
DOI: 10.1103/PhysRevB.87.024106
Résumé: Nanoparticles of strontium titanate (SrTiO3) have been synthesized in the 15 nm to 1 μm size range and studied using a combination of dielectric, Raman, x-ray, and neutron measurements. When diminishing the grain size, a strong reduction of dielectric permittivity, an enhancement of (normally forbidden) Raman polar modes and a progressive decoupling (or nonlinear coupling) between the antiferrodistortive (AFD) order parameter and the spontaneous strain, is observed. A qualitative explanation of all these effects could be achieved using the Petzelt core-shell model of SrTiO3 nanoparticles, with a core constituted of nonferroelectric AFD phase and a shell with frozen polarization. Depending on the route of synthesis, a strong increase of the AFD ferroelastic critical temperature Tc is observed. This behavior cannot be explained by considering only pure size or external strain effects, and a more complicated mechanism involving defects induced by the synthesis process should probably be considered. Interestingly, those are able to strongly affect the core grain structure, modifying thereby the macroscopic physical properties of SrTiO3 nanoparticle-based-materials.
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Spectroscopie vibrationnelle appliquée à la détermination de la structure locale des verres silicatés
Auteur(s): HEHLEN B.
Conférence invité: Les journées de l'USTV, Union pour la Science et la Technologie Verrière (Montpellier, FR, 2012-11-08)
Ref HAL: hal-00809286_v1
Résumé: L'étude des vibrations atomiques par diffusion inélastique de la lumière (Brillouin, Raman, et hyper-Raman) constitue un moyen indirect mais très efficace pour sonder la structure locale des matériaux. Toutefois, dans les verres, le désordre structural empêche souvent une analyse fine et détaillée des données spectroscopiques. A travers plusieurs exemples pris dans la famille des silicates et des borosilicates je montrerai comment, à partir des réponses spectrales, il est possible d'atteindre des informations quantitatives ou semi-quantitatives, d'une part sur la nature des vibrations mises en jeu dans le processus de diffusion, et d'autre part sur les modifications de la structure locale.
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Soft-mode dynamics in micrograin and nanograin ceramics of strontium titanate observed by hyper-Raman scattering
Auteur(s): HEHLEN B. , Al-Zein A., Bogicevic Christine, Gemeiner P., Kiat J-m.
(Article) Publié:
Physical Review B, vol. 87 p.014303 (2013)
Ref HAL: hal-00787294_v1
DOI: 10.1103/PhysRevB.87.014303
Résumé: The low-frequency vibrations of micro- and nanograin ceramics of strontium titanate are investigated by hyper-Raman spectroscopy. The combination of a confocal optical microscope with an ultranarrow Notch filter enabled an accurate spectroscopy on both the Stokes and anti-Stokes side down to about 8 cm−1. A splitting of the soft mode into E-u- and A-2u-symmetry vibrations is clearly observed in the micrograin sample. The peculiar temperature dependence of ω^2_A2u−ω^2_Eu emphasizes a nonlinear coupling between the square of the TiO6 tilting angle and the spontaneous strain c/a − 1. In addition, the very different temperature behavior of the linewidth of the two soft components is assigned to the growth of a disorder anisotropy on cooling, probably on the titanium site.
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The Vibrations of Vitreous Silica Observed in Hyper-Raman Scattering
Auteur(s): HEHLEN B., Simon G.
(Article) Publié:
Journal of Raman Spectroscopy, vol. 43 p.1941 (2012)
Ref HAL: hal-00767212_v1
DOI: 10.1002/jrs.4110
Résumé: Hyper-Raman scattering (HRS) is performed in v-SiO2, and the results are compared to RS and infrared absorption data. Mode analyses are tested based on two models, either the local C2v symmetry of the Si-O-Si bonds or the Td symmetry of the SiO4 tetrahedra. The HRS boson-peak (BP) signal is dominated by the quasi-local librations of tetrahedra. However, we find that it also contains a weak dipolar contribution with a different spectral signature, suggesting that the BP modes are not fully hybridized. The transverse optic (TO) modes can be assigned to molecular-like excitations according to either model, with the exception of the lowest frequency TO4 which involves highly cooperative Si-O-Si motions. On the opposite, the strong scattering from all longitudinal optic (LO) modes clearly follows the average isotropic medium ∞∞m selection rules. This long range cooperativity presumably results from the associated internal electric field. Finally, the strong LO HRS signals enabled observing a new longitudinal component at 1078 cm−1. Through the example of v-SiO2, the article also highlights new perspectives provided by HRS for the description of the vibrations in disordered materials in general.
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Bernard.Hehlen
univ-montp2.fr 
0467143464
Bureau: 240, Bât: 13 - Site : Campus Triolet