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See detailA first-order energy-dissipative momentum-conserving scheme for elasto-plasticity using the variational updates formulation
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

in Computer Methods in Applied Mechanics & Engineering (2008), 197(6-8), 706726

In a previous paper [L. Noels, L. Stainier, J.-P. Ponthot, An energy momentum conserving algorithm using the variational formulation of visco-plastic updates, Int. J. Numer. Methods Engrg. 65 (2006) 904 ... [more ▼]

In a previous paper [L. Noels, L. Stainier, J.-P. Ponthot, An energy momentum conserving algorithm using the variational formulation of visco-plastic updates, Int. J. Numer. Methods Engrg. 65 (2006) 904-942] the authors demonstrated the efficiency of the variational formulation of elasto-plastic updates to develop energy-momentum conserving time integration algorithms. Indeed, within such a framework, the stress tensor always derives from an incremental potential, even when plastic behavior is considered. Therefore the verification of the conservation of energy in the non-linear range can easily be demonstrated: the sum of the reversible stored energy and irreversible dissipated energy exactly corresponds to the work of the external forces applied to the structure. Although this formulation was shown to be accurate and robust, the introduction of numerical dissipation for high-frequency numerical modes can be necessary to simulate complex phenomena. In this work, we propose a modification of the variational updates framework to introduce this numerical property, leading to a new energy-dissipative momentum-conserving time-integration algorithm for elasto-plasticity. (c) 2007 Elsevier B.V. All rights reserved. [less ▲]

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See detailFirst-principle study of crystals exhibiting an incommensurate phase transition
Gonze, X.; Caracas, R.; Sonnet, R. et al

in AIP Conference Proceedings (2000), 535

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See detailFirst-principles and experimental studies of hexagonal YMnO3 single crystals and epitaxial films
Prikockyte, Alina ULg

Doctoral thesis (2012)

Multiferroic materials have attracted much interest during the recent years. Our study is devoted to a prototypic system: yttrium manganite. In particular, we focus on the ferroelectric properties in bulk ... [more ▼]

Multiferroic materials have attracted much interest during the recent years. Our study is devoted to a prototypic system: yttrium manganite. In particular, we focus on the ferroelectric properties in bulk and in thin film forms. Yttrium manganite belongs to the class of ABO3 compounds. Most theoretical studies of ferroelectricity to date were concentrated on cubic perovskite ABO3. Yttrium manganite is hexagonal and is an improper ferroelectric. We were interested to study theoretically and experimentally how these two features behave in thin film form. Our study is organized as follows. [less ▲]

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See detailFirst-principles calculation of kinetic barriers and metastability for the corundum-to-Rh/sub 2/O/sub 3/(II) transition in Al/sub 2/O/sub 3/
Xu, Bin ULg; Stokes, Harold; Dong, Jianjun

in Journal of Physics: Condensed Matter (2010), 22(31), 3154036-3154036

We demonstrate that the rhombohedral corundum crystal and the orthorhombic Rh/sub 2/O/sub 3/ (II) crystal are structurally related by a diffusionless monoclinic common subgroup pathway, along which the ... [more ▼]

We demonstrate that the rhombohedral corundum crystal and the orthorhombic Rh/sub 2/O/sub 3/ (II) crystal are structurally related by a diffusionless monoclinic common subgroup pathway, along which the intermediate structures have the space group P2/c symmetry. This transformation pathway is energetically favored because the majority of Al-O bonds are preserved during the transition. Only 1/3 of the four-coordinated anions and 1/2 of the six-coordinated cations break and reform a single bond. No other bonds are broken in the transformation. Our calculated forward (corundum-to-Rh/sub 2/O/sub 3/ (II)) enthalpy barrier (about 140 meV/atom) in Al/sub 2/O/sub 3/ is not pressure-sensitive at least up to 160 GPa, which is consistent with the observed metastability of the corundum phase around room temperature. In contrast, we predict a significantly decreasing backward (Rh/sub 2/O/sub 3/ (II)-to-corundum) enthalpy barrier on decompression, which suggests that the high-pressure Rh/sub 2/O/sub 3/ (II) phase of Al/sub 2/O/sub 3/ is unlikely to be recoverable at ambient conditions. In addition, we propose a general, common subgroup-based approach to compare elastic properties of two phases with different Bravais lattices. By treating both rhombohedral and orthorhombic phases with common monoclinic P2/c unit cells (non-primitive for the corundum phase), we show that the elastic properties of the high-pressure Rh/sub 2/O/sub 3/ (II) phase are very comparable to those of the ambient corundum phase. [less ▲]

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See detailFirst-principles calculation of the band offset at BaO/BaTiO3 and SrO/SrTiO3 interfaces
Junquera, J.; Zimmer, M.; Ordejon, P. et al

in Physical Review. B, Condensed Matter and Materials Physics (2003), 67

We report first-principles density-functional pseudopotential calculations on the atomic structures, electronic properties, and band offsets of BaO/BaTiO3 and SrO/SrTiO3 nanosized heterojunctions grown on ... [more ▼]

We report first-principles density-functional pseudopotential calculations on the atomic structures, electronic properties, and band offsets of BaO/BaTiO3 and SrO/SrTiO3 nanosized heterojunctions grown on top of a silicon substrate. The density of states at the junction does not reveal any electronic induced interface states. A dominant perovskite character is found at the interface layer. The tunability of the band offset with the strain conditions imposed by the substrate is studied. Using previously reported theoretical data available for Si/SrO, Si/BaO, SrTiO3/Pt, and BaTiO3/SrRuO3 interfaces we extrapolate the band alignments along two gate stacks of technological interest: Si/SrO/SrTiO3/Pt and Si/BaO/BaTiO3/SrRuO3 heterostructures. [less ▲]

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See detailFirst-principles calculation of the electronic, dielectric, and dynamical properties of CaF2
Verstraete, Matthieu ULg; Gonze, Xavier

in Physical Review. B : Condensed Matter (2003), 68

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See detailFirst-principles calculations of dielectric and effective charge tensors in Barium Titanate
Ghosez, Philippe ULg; Gonze, X.; Michenaud, J. P.

in Ferroelectrics (1994), 153

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See detailFirst-principles calculations of the nonlinear optical susceptibilities and Raman scattering spectra of lithium niobate
Hermet, P.; Veithen, M.; Ghosez, Philippe ULg

in Journal of Physics : Condensed Matter (2007), 19

Nonlinear optical susceptibilities and nonresonant Raman scattering spectra of the ferroelectric phase of lithium niobate (LiNbO3) are computed using a first-principles approach based on density ... [more ▼]

Nonlinear optical susceptibilities and nonresonant Raman scattering spectra of the ferroelectric phase of lithium niobate (LiNbO3) 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. Infrared reflectivity spectra of the ferroelectric phase of LiNbO3 are also calculated. New assignments are proposed for the E-modes, clarifying a longstanding debate in the literature. In addition, it is shown that knowledge of the nonlinear optical susceptibility tensor of LiNbO3 does not significantly alter the profile of its Raman spectra in a configuration where the longitudinal optic modes are involved. [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 characterization of the four phases of barium titanate
Ghosez, Philippe ULg; Gonze, X.; Michenaud, J. P.

in Ferroelectrics (1999), 220(1-2), 1-15

Using a first-principles approach, the four phases of perovskite barium titanate are studied. We determine the lattice parameter, the bulk modulus and the cohesive energy of the cubic phase; the relaxed ... [more ▼]

Using a first-principles approach, the four phases of perovskite barium titanate are studied. We determine the lattice parameter, the bulk modulus and the cohesive energy of the cubic phase; the relaxed atomic positions in the three ferroelectric structures; and the evolution of the electronic properties from one phase to the other. The role of electronic exchange-correlation energy in the stabilization of the different phases is also investigated. [less ▲]

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See detailFirst-principles characterization of the P21ab ferroelectric phase of Aurivillius Bi2WO6
Djani-Ait, Hania ULg; Hermet, Patrick ULg; Ghosez, Philippe ULg

in Journal of Physical Chemistry C (2014), 118

The structural, dielectric, dynamical, elastic, piezoelectric and nonlinear optical (second-order susceptibility and Pockels tensors) properties of Bi2WO6 in its P21ab ferroelectric ground state are ... [more ▼]

The structural, dielectric, dynamical, elastic, piezoelectric and nonlinear optical (second-order susceptibility and Pockels tensors) properties of Bi2WO6 in its P21ab ferroelectric ground state are determined using density functional theory. The calculation of infrared and Raman spectra on single crystal allowed us to clarify the assignment of experimental phonon modes, considering the good agreement between the calculated and the experimental Raman spectra obtained on polycrystal. The calculation of the elastic constants con rms the elastic stability of the crystal and allow us to estimate the Young and shear moduli of polycrystalline samples. The piezoelectric constants have signi cant intrinsic values comparable to those of prototypical ABO3 ferroelectrics. The electro-optic response is strongly dominated by the ionic contribution of transverse optic modes, yielding sizable Pockels coe cients around 9 pm/V along the polar direction. [less ▲]

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See detailFirst-principles computation of material properties: the ABINIT software project
Gonze, X.; Beuken, J. M.; Caracas, R. et al

in Computational Materials Science (2002), 25(3), 478-492

The density functional theory (DFT) computation of electronic structure, total energy and other properties of materials, is a field in constant progress. In order to stay at the forefront of knowledge, a ... [more ▼]

The density functional theory (DFT) computation of electronic structure, total energy and other properties of materials, is a field in constant progress. In order to stay at the forefront of knowledge, a DFT software project can benefit enormously from widespread collaboration, if handled properly. Also, modern software engineering concepts can considerably ease its development. The ABINIT project relies upon these ideas: freedom of sources, reliability, portability, and self-documentation are emphasised, in the development of a sophisticated plane-wave pseudopotential code. We describe ABINITv3.0, distributed under the GNU General Public License. The list of ABINITv3.0 capabilities is presented, as well as the different software techniques that have been used until now: PERL scripts and CPP directives treat a unique set of FORTRAN90 source files to generate sequential (or parallel) object code for many different platforms; more than 200 automated tests secure existing capabilities; strict coding rules are followed; the documentation is extensive, including online help files, tutorials, and HTML-formatted sources. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailFirst-Principles conductance of nanoscale junctions from the polarizability of finite systems
Verstraete, Matthieu ULg; Bokes, Peter; Godby, Rex W

in Journal of Chemical Physics (2009), 130

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See detailFirst-principles design of efficient solar cells using two-dimensional arrays of core-shell and layered SiGe nanowires
Pekoz, Rengin; Malcioglu, Osman Baris ULg; Raty, Jean-Yves ULg

in Physical Review. B, Condensed Matter and Materials Physics (2011), 83(3), 0353176-03531760353176

Research for third generation solar cell technology has been driven by the need to overcome the efficiency and cost problems encountered by current crystalline Si- and thin-film-based solar cells. Using ... [more ▼]

Research for third generation solar cell technology has been driven by the need to overcome the efficiency and cost problems encountered by current crystalline Si- and thin-film-based solar cells. Using first-principles methods, Ge/Si and Si/Ge core/shell and Si-Ge layered nanowires are shown to possess the required qualities for an efficient use in photovoltaic applications. We investigate the details of their band structure, effective mass, absorption property, and charge-carrier localization. The strong charge separation and improved absorption in the visible spectrum indicate a remarkable quantum efficiency that, combined with new designs, compares positively with bulk Si. [less ▲]

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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 modeling of ferroelectric oxide nanostructures
Ghosez, Philippe ULg; Junquera, J.

in Handbook of theoretical and computational nanotechnology (2006)

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See detailFirst-principles modeling of the thermoelectric properties of SrTiO3/SrRuO3 superlattices
Garcia-Fernandez, Pablo; Verissimo-Alves, Marcos; Bilc, Daniel ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2012), 86

Using a combination of first-principles simulations, based on density functional theory and Boltzmann’s semiclassical theory, we have calculated the transport and thermoelectric properties of the half ... [more ▼]

Using a combination of first-principles simulations, based on density functional theory and Boltzmann’s semiclassical theory, we have calculated the transport and thermoelectric properties of the half-metallic twodimensional electron gas confined in single SrRuO3 layers of SrTiO3/SrRuO3 periodic superlattices. Close to the Fermi energy, we find that the semiconducting majority-spin channel displays a very large in-plane component of the Seebeck tensor at room temperature, S ∼ 1500 μV/K, and the minority-spin channel shows good in-plane conductivity, σ = 2.5 (m cm)−1. However, we find that the total power factor and thermoelectric figure of merit for reduced doping is too small for practical applications. Our results support that the confinement of the electronic motion is not the only thing that matters to describe the main features of the transport and thermoelectric properties with respect the chemical doping, but the shape of the electronic density of states, which in our case departs from the free-electron behavior, is also important. The evolution of the electronic structure, electrical conductivity, Seebeck coefficient, and power factor as a function of the chemical potential is explained by a simplified tight-binding model. We find that the electron gas in our system is composed by a pair of one-dimensional electron gases orthogonal to each other. This reflects the fact the physical dimensionality of the electronic system (1D) can be even smaller than that of the spacial confinement of the carriers (2D). [less ▲]

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See detailFirst-principles studies of ferroelectric oxides
Rabe, K. M.; Ghosez, Philippe ULg

in Physics of Ferroelectrics: A Modern Perspective (2007)

The application of first-principles methods to the study of ferroelectric oxides is reviewed. While the main focus is on the perovskites, particularly the most-studied compounds BaTiO3, PbTiO3, and SrTiO3 ... [more ▼]

The application of first-principles methods to the study of ferroelectric oxides is reviewed. While the main focus is on the perovskites, particularly the most-studied compounds BaTiO3, PbTiO3, and SrTiO3, other oxide families, including LiNbO3, layered perovskites, nitrites and nitrates, and electronic and magnetic ferroelectrics, are included. Results are presented for crystal structure, polarization and dielectric and piezoelectric coefficients. The identification of lattice instabilities through the computation of phonon dispersion relations for a high-symmetry reference phase is presented. Results at nonzero temperature, obtained through effective Hamiltonian and interatomic potential approaches, are given. Calculations for solid solutions, defects, thin films, superlattices and nanostructures are described. Challenges and prospects for future research are identified. [less ▲]

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