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Electronic and thermoelectric properties of Fe2VAl: The role of defects and disorder Bilc, Daniel ; Ghosez, Philippe in Physical Review. B, Condensed Matter and Materials Physics (2011), 83 Using first-principles calculations, we show that Fe2VAl is an indirect band-gap semiconductor. Our calculations reveal that its semimetallic character (which is sometimes assigned) is not an ntrinsic ... [more ▼] Using first-principles calculations, we show that Fe2VAl is an indirect band-gap semiconductor. Our calculations reveal that its semimetallic character (which is sometimes assigned) is not an ntrinsic property but originates from magnetic antisite defects and site disorder, which introduce localized in-gap and resonant states changing the electronic properties close to the band gap. These states negatively affect the thermopower S and the power factor equal to S2σ, decreasing the good thermoelectric performance of intrinsic Fe2VAl. [less ▲] Detailed reference viewed: 59 (5 ULg)Interface Physics in Complex Oxide Heterostructures ; ; et al in Annual Review of Condensed Matter Physics (2011), 2 Complex transition metal oxides span a wide range of crystalline structures and play host to an incredible variety of physical phenomena. High dielectric permittivities, piezo-, pyro-, and ... [more ▼] Complex transition metal oxides span a wide range of crystalline structures and play host to an incredible variety of physical phenomena. High dielectric permittivities, piezo-, pyro-, and ferroelectricity are just a few of the functionalities offered by this class of materials, while the potential for applications of the more exotic properties like high temperature superconductivity and colossal magnetoresistance is still waiting to be fully exploited. With recent advances in deposition techniques, the structural quality of oxide heterostructures now rivals that of the best conventional semiconductors, taking oxide electronics to a new level. Such heterostructures have enabled the fabrication of artificial multifunctional materials. At the same time they have exposed a wealth of phenomena at the boundaries where compounds with different structural instabilities and electronic properties meet, giving unprecedented access to new physics emerging at oxide interfaces. Here we highlight some of these exciting new interface phenomena. [less ▲] Detailed reference viewed: 85 (2 ULg)First-principles calculations of the structural and dynamical properties of ferroelectric YMnO3 Prikockyte, Alina ; Bilc, Daniel ; Hermet, Patrick 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 ▲] Detailed reference viewed: 31 (5 ULg)Coupling of three lattice instabilities Ghosez, Philippe ; in Nature Materials (2011), 10 Detailed reference viewed: 23 (10 ULg)Strain-induced ferroelectricity in simple rocksalt binary oxides Bousquet, Eric ; ; Ghosez, Philippe in Physical Review Letters (2010), 104 Using first-principles density functional calculations, we show that ferroelectricity can be induced in simple alkaline-earth-metal binary oxides such as barium oxide (BaO) using appropriate epitaxial ... [more ▼] Using first-principles density functional calculations, we show that ferroelectricity can be induced in simple alkaline-earth-metal binary oxides such as barium oxide (BaO) using appropriate epitaxial strains. Going beyond the fundamental discovery, we highlight that the functional properties (polarization, dielectric constant, and piezoelectric response) of such strained binary oxides are comparable in magnitude to those of typical ferroelectric perovskite oxides, making them of direct interest for applications. Finally, we show that magnetic binary oxides such as EuO, with the same rocksalt structure, behave similarly to the alkaline-earth-metal oxides, suggesting a route to new multiferroics combining ferroelectric and magnetic properties. [less ▲] Detailed reference viewed: 80 (8 ULg)First-principles study of competing ferroelectric and antiferroelectric instabilities in BaTiO3/BaO superlattices Bousquet, Eric ; ; Ghosez, Philippe in Physical Review. B, Condensed Matter and Materials Physics (2010), 82 We report a first-principles study of (BaTiO3) m/ (BaO) n superlattices for a wide range of periodicities m/n. We show that such a system develops a polar zone-center instability for sufficiently large m ... [more ▼] We report a first-principles study of (BaTiO3) m/ (BaO) n superlattices for a wide range of periodicities m/n. We show that such a system develops a polar zone-center instability for sufficiently large m/n ratio, which can be understood, at least qualitatively, from a simple electrostatic model and should lead to a ferroelectric ground state. However, the analysis of the phonon-dispersion curves also points out the appearance of stronger antiferroelectric instabilities at the zone boundaries around m=4, before the critical ratio for ferroelectricity is reached and which still dominate beyond it. The dominant character of the antiferroelectric instability is explained from the depolarizing field which hardens the ferroelectric mode. This analysis allows us to predict that, (BaTiO3) m/ (BaO) n superlattices should present an antiferroelectric ground state for m larger than 4, which should smoothly evolve to a multidomain structure for increasing m values and only become ferroelectric for large m. [less ▲] Detailed reference viewed: 28 (6 ULg)First-principles study of the dynamical and nonlinear optical properties of urea single crystals Hermet, Patrick ; Ghosez, Philippe 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 ▲] Detailed reference viewed: 17 (3 ULg)Polarization vortices in germanium telluride nanoplatelets: A theoretical study Durgun, Engin ; Ghosez, Philippe ; et al in Physical Review Letters (2009), 103 Using first-principles calculations based on density functional theory, we study the properties of germanium telluride crystalline nanoplatelets and nanoparticles. Above a diameter of 2.7 nm, we predict ... [more ▼] Using first-principles calculations based on density functional theory, we study the properties of germanium telluride crystalline nanoplatelets and nanoparticles. Above a diameter of 2.7 nm, we predict the appearance of polarization vortices giving rise to an unusual ferrotoroidic ground state with a spontaneous and reversible toroidal moment of polarization. We highlight the crucial role of inhomogeneous strain in stabilizing polarization vortices. Combined with the phase-change properties of germanium telluride, the ferrotoroidic properties could be of practical interest for ternary logic applications. [less ▲] Detailed reference viewed: 49 (6 ULg)Hybrid functional study of prototypical multiferroic bismuth ferrite Goffinet, Marco ; Hermet, Patrick ; Bilc, Daniel 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 ▲] Detailed reference viewed: 84 (2 ULg)Raman scattering intensities in BaTiO3 and PbTiO3 prototypical ferroelectrics from density functional theory Hermet, Patrick ; ; Ghosez, Philippe 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 ▲] Detailed reference viewed: 65 (0 ULg)Comment on "Electronic structure and zone-center phonon modes in multiferroic bulk BiFeO3" [J. Appl. Phys. 103, 083712 (2008)] Hermet, Patrick ; Goffinet, Marco ; Ghosez, Philippe 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 ▲] Detailed reference viewed: 51 (2 ULg)Engineering Multiferroism in CaMnO3 ; Bousquet, Eric ; Ghosez, Philippe in Physical Review Letters (2009), 102 Structural instabilities of CaMnO3 are investigated from first principles. We point out that, on top of a strong antiferrodistortive instability responsible for its orthorhombic ground state, the cubic ... [more ▼] Structural instabilities of CaMnO3 are investigated from first principles. We point out that, on top of a strong antiferrodistortive instability responsible for its orthorhombic ground state, the cubic perovskite structure of CaMnO3 also exhibits a weak ferroelectric instability. Although ferroelectricity is suppressed by antiferrodistortive motions, we show that it can be favored using strain or chemical engineering in order to make CaMnO3 multiferroic. We finally highlight that the ferroelectric instability of CaMnO3 is Mn-dominated. This illustrates that, contrary to common belief, ferroelectricity and magnetism are not necessarily exclusive but can be driven by the same cation. [less ▲] Detailed reference viewed: 52 (1 ULg)ABINIT: First-principles approach to material and nanosystem properties ; ; et al in Computer Physics Communications (2009), 180 ABINIT allows one to study, from first-principles, systems made of lectrons and nuclei (e.g. periodic solids, molecules, nanostructures, etc.), on the basis of Density-Functional Theory (DFT) and many ... [more ▼] ABINIT allows one to study, from first-principles, systems made of lectrons and nuclei (e.g. periodic solids, molecules, nanostructures, etc.), on the basis of Density-Functional Theory (DFT) and many-Body Perturbation Theory. beyond the computation of the total energy, charge density and electronic structure of such systems, ABINIT also implements many dynamical, dielectric, thermodynamical, mechanical, or electronic properties, at different levels of approximation. The present paper provides an exhustive account of the capabilities of ABINIT. It should be helpful to scienttists that are not familirized with ABINIT, as well as to already regular users. First, we give a broad overview of ABINIT, including the list of the capabilities and how to access them. Then, we present in more details the recent, advance, developments of ABINIT, with adequate references to the underlying theory, as well as the relevant input variables, tests and, if available, ABINIT tutorials. [less ▲] Detailed reference viewed: 231 (12 ULg)Correlation between optical constants and crystal chemical parameters of ZrW2O8 ; ; et al in Journal of Solid State Chemistry (2009), 182(10), 2762-2768 The refractive indices of ZrW2O8, measured at wavelengths of 435.8-643.8 nm, were used to calculate n(D) at lambda = 589.3 nm and n(infinity) at lambda = infinity from a one-term Sellmeier equation ... [more ▼] The refractive indices of ZrW2O8, measured at wavelengths of 435.8-643.8 nm, were used to calculate n(D) at lambda = 589.3 nm and n(infinity) at lambda = infinity from a one-term Sellmeier equation. Refractive indices, n(D) and dispersion values, A, are, respectively, 1.8794 and 114 x 10(-16) m(2). The high dispersion, relative to other molybdates, tungstates and Zr-containing compounds, is attributed to the low value of E-o = 7.7 eV and mean cation coordination number. Total electronic polarizabilities, alpha(total), were calculated from n(infinity) and the Lorenz-Lorentz equation. The unusually large difference between the observed polarizability of 20.087 angstrom(3) and the calculated total polarizability alpha(T) of 17.59 angstrom(3) (Delta = +12.4%) is attributed to (1) a large M-O-W angle, (2) a high degree of W 5d-O(terminal) 2p and Zr nd-O 2p hybridization, and (3) unusually high oxygen displacement factors, B(O), normalized to B(W). (C) 2009 Elsevier Inc. All rights reserved. [less ▲] Detailed reference viewed: 27 (1 ULg)Dynamical, dielectric, and elastic properties of GeTe investigated with first-principles density functional theory ; Durgun, Engin ; Raty, Jean-Yves et al in Physical Review. B, Condensed Matter and Materials Physics (2008), 78 The dynamical, dielectric, and elastic properties of GeTe, a ferroelectric material in its low-temperature rhombohedral phase, have been investigated using first-principles density functional theory. We ... [more ▼] The dynamical, dielectric, and elastic properties of GeTe, a ferroelectric material in its low-temperature rhombohedral phase, have been investigated using first-principles density functional theory. We report the electronic energy bands, phonon-dispersion curves, electronic and low-frequency dielectric tensors, infrared reflectivity, Born effective charges, and elastic and piezoelectric tensors and compare them with the existing theoretical and experimental results, as well as with similar quantities available for other ferroelectric materials, when appropriate. [less ▲] Detailed reference viewed: 39 (4 ULg)A new hybrid exchange-correlation functional for accurate prediction of the electronic and structural properties of ferroelectric oxides Bilc, Daniel ; ; et al in Physical Review. B, Condensed Matter and Materials Physics (2008), 77 Using a linear combination of atomic orbitals approach, we report a systematic comparison of various density functional theory (DFT) and hybrid exchange-correlation functionals for the prediction of the ... [more ▼] Using a linear combination of atomic orbitals approach, we report a systematic comparison of various density functional theory (DFT) and hybrid exchange-correlation functionals for the prediction of the electronic and structural properties of prototypical ferroelectric oxides. It is found that none of the available functionals is able to provide, at the same time, accurate electronic and structural properties of the cubic and tetragonal phases of BaTiO3 and PbTiO3. Some, although not all, usual DFT functionals predict the structure with acceptable accuracy, but always underestimate the electronic band gaps. Conversely, common hybrid functionals yield an improved description of the band gaps, but overestimate the volume and atomic distortions associated with ferroelectricity, giving rise to an unacceptably large c/a ratio for the tetragonal phases of both compounds. This supertetragonality is found to be induced mainly by the exchange energy corresponding to the generalized gradient approximation (GGA) and, to a lesser extent, by the exact exchange term of the hybrid functional. We thus propose an alternative functional that mixes exact exchange with the recently proposed GGA of Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] which, for solids, improves over the treatment of exchange of the most usual GGA's. The new functional renders an accurate description of both the structural and electronic properties of typical ferroelectric oxides. [less ▲] Detailed reference viewed: 25 (5 ULg)First-principles study of the dielectric and dynamical properties of orthorhombic CaMnO3 ; Bousquet, Eric ; Ghosez, Philippe in Journal of Physics : Condensed Matter (2008), 20 The structural, dielectric and dynamical properties of the low temperature antiferromagnetic orthorhombic phase of CaMnO3 have been computed from first principles, using a density functional theory ... [more ▼] The structural, dielectric and dynamical properties of the low temperature antiferromagnetic orthorhombic phase of CaMnO3 have been computed from first principles, using a density functional theory approach within the local spin density approximation. The theoretical structural parameters are in good agreement with experiment. The full set of zone-center phonons is reported, allowing new assignment of experimental Raman data and providing reference values for the interpretation of future infrared phonon measurements. It is shown that the static dielectric constant is very large and comparable in amplitude to that of isostructural CaTiO3. In contrast to the pseudocubic structure, it is also highly anisotropic. These features are discussed in relationship to the anomalous Born effective charges and the presence of low frequency polar modes. [less ▲] Detailed reference viewed: 28 (1 ULg)New insight into the concept of ferroelectric correlation volume ; Bousquet, Eric ; Ghosez, Philippe in Journal Of Computational And Theoretical Nanoscience (2008), 5(4), 517-520 Using an effective Hamiltonian derived from first-principles calculations, we discuss the concept of "ferroelectric correlation volume" and show that it cannot be uniquely defined: it is not the size but ... [more ▼] Using an effective Hamiltonian derived from first-principles calculations, we discuss the concept of "ferroelectric correlation volume" and show that it cannot be uniquely defined: it is not the size but the anisotropic shape of the polar region that is crucial for stabilizing a ferroelectric distortion into a ferroelectric materials. To provide basic understanding of this result, the stability of a polar region is analyzed in term of intra-chain and inter-chain interactions. [less ▲] Detailed reference viewed: 42 (1 ULg)First-Principles Study of Ferroelectric Oxide Epitaxial Thin Films and Superlattices: Role of the Mechanical and Electrical Boundary Conditions ; Ghosez, Philippe in Journal Of Computational And Theoretical Nanoscience (2008), 5(11), 2071-2088 In this review, we propose a summary of the most recent advances in the first-principles study of ferroelectric oxide epitaxial thin films and multilayers. We discuss in detail the key roles of mechanical ... [more ▼] In this review, we propose a summary of the most recent advances in the first-principles study of ferroelectric oxide epitaxial thin films and multilayers. We discuss in detail the key roles of mechanical and electrical boundary conditions, providing to the reader the basic background for a simple and intuitive understanding of the evolution of the ferroelectric properties in many nanostructures. Going further we also highlight promising new avenues and future challenges within this exciting field or researches. [less ▲] Detailed reference viewed: 57 (0 ULg)Improper ferroelectricity in perovskite oxide artificial superlattices Bousquet, Eric ; ; et al in Nature (2008), 452(7188), 732-736 Detailed reference viewed: 63 (25 ULg) |
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