arXiv subjects

Philippe Ghosez

Publications and source records attributed to Philippe Ghosez.

89 records · Page 5Linked to original sources

arXiv2012

First-principles study of the ferroelectric Aurivillius phase Bi2WO6

In order to better understand the reconstructive ferroelectric-paraelectric transition of Bi2WO6, which is unusual within the Aurivillius family of compounds, we performed first principles calculations of the dielectric and dynamical properties on two possible high-temperature paraelectic structures: the monoclinic phase of A2/m symmetry observed experimentally and the tetragonal phase of I4/mmm symmetry, common to most Aurivillius phase components. Both paraelectric structures exhibits various unstable modes, which after their condensation bring the system toward more stable structures of lower symmetry. The calculations confirms that, starting from the paraelectric A2/m phase at high temperature, the system must undergo a reconstructive transition to reach the P2_1ab ferroelectric ground state.

cond-mat.mtrl-sci↗

arXiv2012

Electrical properties of improper ferroelectrics from first principles

We study the interplay of structural and polar distortions in hexagonal YMnO3 and short-period PbTiO3/SrTiO3 superlattices by means of first-principles calculations at constrained electric displacement field D. We find that in YMnO3 the tilts of the oxygen polyhedra produce a robustly polar ground state, which persists at any choice of the electrical boundary conditions. Conversely, in PTO/STO the antiferrodistortive instabilities alone do not break inversion symmetry, and open-circuit bundary conditions restore a non-polar state. We suggest that this qualitative difference naturally provides a route to rationalizing the concept of "improper ferroelectricity" from the point of view of first-principles theory. We discuss the implications of our arguments for the design of novel multiferroic materials with enhanced functionalities, and for the symmetry analysis of the phase transitions.

cond-mat.mtrl-sci↗

arXiv2012

First-principles modeling of the thermoelectric properties of SrTiO$_3$/SrRuO$_3$ superlattices

Using a combination of first-principles simulations, based on the density functional theory and Boltzmann's semiclassical theory, we have calculated the transport and thermoelectric properties of the half-metallic two dimensional electron gas confined in single SrRuO$_3$ layers of SrTiO$_3$/SrRuO$_3$ 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 $μ$V/K, and the minority spin channel shows good in-plane conductivity $σ$ = 2.5 (m$Ω$cm)$^{-1}$. However, contrary to the expectation of Hicks and Dresselhaus model about enhanced global thermoelectric properties due to the confinement of the metallic electrons, we find that the total power factor and thermoelectric figure of merit for reduced doping is too small for practical applications. The reason for this failure can be traced back on the electronic structure of the interfacial gas, which departs from the free electron behaviour on which the model was based. 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 can be even smaller than that of the spacial confinement of the carriers.

cond-mat.mtrl-sci↗

arXiv2012

Ab initio investigation of competing instabilities in ferroelectric perovskite PbTiO3

We have developed first-principles models, based on a general parametrization of the full potential-energy surface, to investigate the lattice-dynamical properties of perovskite oxides. We discuss the application of our method to prototypic ferroelectric PbTiO3, showing that its properties are drastically affected by a competition between structural instabilities. Indeed, we confirm previous indications that the destructive interaction between the ferroelectric and antiferrodistortive (involving rotations of the O6 octahedra) soft modes shifts PbTiO3's Curie temperature by hundreds of degrees. Our theory provides unique insight into this gigantic effect and its dynamical character.

cond-mat.mtrl-sci↗

arXiv2012

Highly-confined spin-polarized two-dimensional electron gas in SrTiO$_{3}$/SrRuO$_{3}$ superlattices

We report first principles characterization of the structural and electronic properties of (SrTiO$_{3}$)$_{5}$/(SrRuO$_{3}$)$_{1}$ superlattices. We show that the system exhibits a spin-polarized two-dimensional electron gas, extremely confined to the 4$d$ orbitals of Ru in the SrRuO$_{3}$ layer. Every interface in the superlattice behaves as a minority-spin half-metal ferromagnet, with a magnetic moment of $μ$ = 2.0 $μ_{\rm B}$/SrRuO$_{3}$ unit. The shape of the electronic density of states, half metallicity and magnetism are explained in terms of a simplified tight-binding model, considering only the $t_{2g}$ orbitals plus (i) the bi-dimensionality of the system, and (ii) strong electron correlations.

cond-mat.mtrl-sci↗

arXiv2011

Electronic and thermoelectric properties of Fe2VAl: The role of defects and disorder

Using first-principles calculations, we show that Fe2VAl is an indirect band gap semiconductor. Our calculations reveal that its, sometimes assigned, semimetallic character is not an intrinsic property but originates from the antisite defects and site disorder, which introduce localized ingap and resonant states changing the electronic properties close to band gap. These states negatively affect the thermopower S and power factor PF=S^2σ, decreasing the good thermoelectric performance of intrinsic Fe2VAl.

cond-mat.mtrl-sci↗

arXiv2010

First-principles study of competing ferroelectric and antiferroelectric instabilities in BaTiO3/BaO superlattices

We report a first-principles study of (BaTiO${_3}$)$_m$/(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 \textit{m}/\textit{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 anti-ferroelectric instability is explained from the depolarizing field which hardens the ferroelectric mode. This analysis allows us to predict that, (BaTiO${_3}$)$_m$/(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 $m$.

cond-mat.mtrl-sci↗

arXiv2009

Strain-induced ferroelectricity in simple rocksalt binary oxides

The alkaline earth binary oxides adopt a simple rocksalt structure and form an important family of compounds because of their large presence in the earth's mantle and their potential use in microelectronic devices. In comparison to the class of multifunctional ferroelectric perovskite oxides, however, their practical applications remain limited and the emergence of ferroelectricity and related functional properties in simple binary oxides seems so unlikely that it was never previously considered. Here, we show using first-principles density functional calculations that ferroelectricity can be easily induced in simple alkaline earth binary oxides such as barium oxide (BaO) using appropriate epitaxial strains. Going beyond the fundamental discovery, we show 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, so 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 oxides, suggesting a route to new multiferroics combining ferroelectric and magnetic properties.

cond-mat.mtrl-sci↗

arXiv2008

Engineering multiferroism in CaMnO$_3$

From first-principles calculations, we investigate the structural instabilities of CaMnO$_3$. We point out that, on top of a strong antiferrodistortive instability responsible for its orthorhombic ground-state, the cubic perovskite structure of CaMnO$_3$ also exhibit a weak ferroelectric instability. Although ferroelectricity is suppressed by antiferrodistortive oxygen motions, we show that it can be favored using strain or chemical engineering in order to make CaMnO$_3$ multiferroic. We finally highlight that the FE instability of CaMnO$_3$ is Mn-dominated. This illustrates that, contrary to the common believe, ferroelectricity and magnetism are not necessarily exclusive but can be driven by the same cation.

cond-mat.mtrl-sci↗

arXiv2008

First-principle calculation of the dielectric and dynamical properties of orthorhombic CaMnO$_{3}$

The structural, dielectric and dynamical properties of the low temperature antiferromagnetic orthorhombic phase of CaMnO$_3$ 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 infrared and Raman zone-center phonons is reported and compared to experimental data. It is shown that coherently with the anomalous Born effective charges and the presence of low frequency polar modes, the static dielectric constant is very large and highly anisotropic.

cond-mat.mtrl-sci↗

arXiv2007

First-principles study of ferroelectric oxide epitaxial thin films and superlattices: role of the mechanical and electrical boundary conditions

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.

cond-mat.mtrl-sci↗

arXiv2006

First-Principles Modeling of Ferroelectric Oxide Nanostructures

The aim of this Chapter is to provide an account of recent advances in the first-principles modeling of ferroelectric oxide nanostructures. Starting from a microscopic description of ferroelectricity in bulk materials and considering then, successively, different kinds of nanostructures (films, multilayers, wires, and particles), we try to identify the main trends and to provide a coherent picture of the role of finite size effects in ferroelectric oxides.

cond-mat.mtrl-sci↗

arXiv2006

Finite-size effects in BaTiO3 nanowires

The size dependence of the ferroelectric properties of BaTiO$_3$ nanowires is studied from first-principles. We show that the ferroelectric distortion along the wire axis disappears below a critical diameter of about 1.2 nm. This disappearance is related to a global contraction of the unit cell resulting from low atomic coordinations at the wire surface. It is shown that a ferroelectric distortion can be recovered under appropriate tensile strain conditions.

cond-mat.mtrl-sci↗

arXiv2004

Ferroelectricity and tetragonality in ultrathin PbTiO3 films

The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 films with thicknesses ranging from 500 down to 24 Angstroms. High resolution x-ray pointed out a systematic decrease of the c-axis lattice parameter with decreasing film thickness below 200 Angstroms. Using a first-principles model Hamiltonian approach, the decrease in tetragonality is related to a reduction of the polarization attributed to the presence of a residual unscreened depolarizing field. It is shown that films below 50 Angstroms display a significantly reduced polarization but still remain ferroelectric.

cond-mat.mtrl-sci↗

arXiv2004

First-principles study of the electrooptic effect in ferroelectric oxides

We present a method to compute the electrooptic tensor from first principles, explicitly taking into account the electronic, ionic and piezoelectric contributions. It allows us to study the non-linear optic behavior of three ferroelectric ABO_3 compounds : LiNbO_3, BaTiO_3 and PbTiO_3. Our calculations reveal the dominant contribution of the soft mode to the electrooptic coefficients in LiNbO_3 and BaTiO_3 and identify the coupling between the electric field and the polar atomic displacements along the B-O chains as the origin of the large electrooptic response in these compounds.

cond-mat.mtrl-sci↗

arXiv2004

First-principles study of filled and unfilled skutterudite antimonies

The electron localization tensor, Born effective charges, dielectric constants and phonon dispersion relations of the skutterudite CoSb$_3$ and the filled skutterudite TlFeCo$_3$Sb$_{12}$ have been studied using density functional perturbation theory. The origin of the low energy peak in the phonon density of states of TlFeCo$_3$Sb$_{12}$ observed recently by Neutron inelastic scattering [R. P. Hermann {\it et al.}, Phys. Rev. Lett. {\bf 90}, 135505 (2003)] is attributed to the vibrations of Tl that show only weak coupling with the normal modes of the host crystal. Moreover, the dielectric properties of these materials show unsual features such as giant Born effective charges and a strong increase of the optical and static dielectric tensor in the filled compound.

cond-mat.mtrl-sci↗

arXiv2002

First-principles calculation of the band offset at BaO/BaTiO$_3$ and SrO/SrTiO$_3$ interfaces

We report first-principles density-functional pseudopotential calculations on the atomic structures, electronic properties, and band offsets of BaO/BaTiO$_3$ and SrO/SrTiO$_3$ 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 and BaTiO$_{3}$/SrRuO$_{3}$ interfaces we extrapolate a value for the band alignments along the whole gate stacks of technological interest: Si/SrO/SrTiO$_3$ and Si/BaO/BaTiO$_3$/SrRuO$_3$ heterostructures.

cond-mat.mtrl-sci↗