References of "Hermet, Patrick"
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See detailFirst-principles model potentials for lattice-dynamical studies: general methodology and example of application to ferroic perovskite oxides
Wojdeł, Jacek; Hermet, Patrick ULg; Ljungberg, Mathias et al

in Journal of Physics : Condensed Matter (2013), 25

We present a scheme to construct model potentials, with parameters computed from first principles, for large-scale lattice-dynamical simulations of materials. We mimic the traditional solid-state approach ... [more ▼]

We present a scheme to construct model potentials, with parameters computed from first principles, for large-scale lattice-dynamical simulations of materials. We mimic the traditional solid-state approach to the investigation of vibrational spectra, i.e., we start from a suitably chosen reference configuration of the compound and describe its energy as a function of arbitrary atomic distortions by means of a Taylor series. Such a form of the potential-energy surface is general, trivial to formulate for any material, and physically transparent. Further, such models involve clear-cut approximations, their precision can be improved in a systematic fashion, and their simplicity allows for convenient and practical strategies to compute/fit the potential parameters. We illustrate our scheme with two challenging cases in which the model potential is strongly anharmonic, namely, the ferroic perovskite oxides PbTiO3 and SrTiO3. Studying these compounds allows us to better describe the connection between the so-called effective-Hamiltonian method and ours (which may be seen as an extension of the former), and to show the physical insight and predictive power provided by our approach—e.g., we present new results regarding the factors controlling phase-transition temperatures, novel phase transitions under elastic constraints, an improved treatment of thermal expansion, etc. [less ▲]

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See detailFirst-principles calculations of the structural and dynamical properties of ferroelectric YMnO3
Prikockyte, Alina ULg; Bilc, Daniel ULg; Hermet, Patrick ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2011), 84

We report the structural and dynamical properties of the ground state ferroelectricP63cm structure of hexagonal YMnO3. The lattice parameters, atomic positions, local magnetic moment of Mn atoms, band gap ... [more ▼]

We report the structural and dynamical properties of the ground state ferroelectricP63cm structure of hexagonal YMnO3. The lattice parameters, atomic positions, local magnetic moment of Mn atoms, band gap, and -point phonons are calculated within the local spin-density approximation plus Hubbard U term and the B1-WC hybrid functional. Our results are discussed in comparison to theoretical and experimental values available in the literature. Based on our first-principles calculations, we propose a reassignment of certain modes. We also discuss the relation between the phonon modes of the ferroelectric P63cm phase and those of the paraelectric P63/mmc phase that condense at the phase transition. [less ▲]

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See detailFirst-principles study of the dynamical and nonlinear optical properties of urea single crystals
Hermet, Patrick ULg; Ghosez, Philippe ULg

in Physical Chemistry Chemical Physics [=PCCP] (2010), 12

Raman and infrared spectra of urea single crystals have been calculated using the density functional theory. This allowed us to assign the remaining experimental low-frequency phonon modes. Then, we have ... [more ▼]

Raman and infrared spectra of urea single crystals have been calculated using the density functional theory. This allowed us to assign the remaining experimental low-frequency phonon modes. Then, we have determined the sign of the second-harmonic nonlinear optical susceptibility coefficient in urea to be negative, clarifying a long debate in the literature. Finally, we computed for the first time the electro-optic coefficients of urea. We found that the electronic and ionic contributions have a similar order of magnitude and an opposite sign, yielding a smaller value than that expected, and necessitating further experimental clarifications. [less ▲]

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See detailHybrid functional study of prototypical multiferroic bismuth ferrite
Goffinet, Marco ULg; Hermet, Patrick ULg; Bilc, Daniel ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2009), 79(1),

We report a systematic comparison of various exchange-correlation functionals for the prediction of the structural, magnetic, electronic, and dynamical (phonons and Born effective charge tensors ... [more ▼]

We report a systematic comparison of various exchange-correlation functionals for the prediction of the structural, magnetic, electronic, and dynamical (phonons and Born effective charge tensors) properties of bismuth ferrite, a prototypical multiferroic compound. We have not only considered the usual approximations to density-functional theory such as the local-density approximation (LDA), generalized (GGA), and LDA+U, but also hybrid approaches such as B3LYP and B1. The recent B1-WC hybrid functional of Bilc [Phys. Rev. B 77, 165107 (2008)], with the GGA functional of Wu and Cohen and an exact exchange mixing parameter of 0.16, provides very good overall agreement with experiments and can be considered as a valuable alternative to LDA, GGA, and DFT+U for the study of bismuth ferrite. This does not only allow a reliable interpretation of the physical properties of this specific compound but also opens perspectives for further and more predictive first-principles investigations of multiferroic materials. [less ▲]

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See detailRaman scattering intensities in BaTiO3 and PbTiO3 prototypical ferroelectrics from density functional theory
Hermet, Patrick ULg; Veithen, M.; Ghosez, Philippe ULg

in Journal of Physics : Condensed Matter (2009), 21

Nonlinear optical susceptibilities and Raman scattering spectra of the ferroelectric phases of BaTiO3 and PbTiO3 are computed using a first-principles approach based on density functional theory and ... [more ▼]

Nonlinear optical susceptibilities and Raman scattering spectra of the ferroelectric phases of BaTiO3 and PbTiO3 are computed using a first-principles approach based on density functional theory and taking advantage of a recent implementation based on the nonlinear response formalism and the 2n + 1 theorem. These two prototypical ferroelectric compounds were chosen to demonstrate the accuracy of the Raman calculation based both on their complexity and their technological importance. The computation of the Raman scattering intensities has been performed not only for the transverse optical modes, but also for the longitudinal optical ones. The agreement between the measured and computed Raman spectra of these prototypical ferroelectrics is remarkable for both the frequency position and the intensity of Raman lines. This agreement presently demonstrates the state-of-the-art in the computation of Raman responses on one of the most complex systems, ferroelectrics, and constitutes a step forward in the reliable prediction of their electro-optical responses. [less ▲]

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See detailComment on "Electronic structure and zone-center phonon modes in multiferroic bulk BiFeO3" [J. Appl. Phys. 103, 083712 (2008)]
Hermet, Patrick ULg; Goffinet, Marco ULg; Ghosez, Philippe ULg

in Journal of Applied Physics (2009), 105

In a recent paper, Tutuncu and Srivastava [J. Appl. Phys. 103, 083712 (2008)] report the electronic structure and the assignment of the zone-center phonon modes of the R3c phase of BiFeO3 using the ... [more ▼]

In a recent paper, Tutuncu and Srivastava [J. Appl. Phys. 103, 083712 (2008)] report the electronic structure and the assignment of the zone-center phonon modes of the R3c phase of BiFeO3 using the density functional theory. In the present comment, we point out some physical inaccuracies in their mode assignment. We review their interpretation and analyze which conclusions are actually justified. [less ▲]

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See detailImproper ferroelectricity in perovskite oxide artificial superlattices
Bousquet, Eric ULg; Dawber, Matthew; Stucki, Nicolas et al

in Nature (2008), 452(7188), 732-736

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See detailRaman and infrared spectra of multiferroic bismuth ferrite from first principles
Hermet, Patrick ULg; Goffinet, Marco ULg; Kreisel, J. et al

in Physical Review. B, Condensed Matter and Materials Physics (2007), 75

The entire zone-center phonon spectrum of the R3c ferroelectric antiferromagnetic phase of bismuth ferrite is computed using a first-principles approach based on density functional theory. Two phonon ... [more ▼]

The entire zone-center phonon spectrum of the R3c ferroelectric antiferromagnetic phase of bismuth ferrite is computed using a first-principles approach based on density functional theory. Two phonon modes exhibiting eigendisplacement vectors that strongly overlap with the atomic distortions taking place at the ferroelectric structural phase transition are identified and give support to a transition with displacive character. Both Raman and infrared reflectivity spectra are also computed, providing benchmark theoretical results for the assignment of experimental spectra. [less ▲]

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