Papers by Philippe Ghosez
First-principles study of PbTiO3/SrTiO3 multilayers
Peer reviewe
Making simple rocksalt binary oxide ferroelectric
Peer reviewe
Ferroelectricity in PbTiO3 Thin Films: A First Principles Approach
Journal of Electroceramics, 2000
The ground-state ferroelectric distortion of PbTiO3 thin films is studied using a first-principle... more The ground-state ferroelectric distortion of PbTiO3 thin films is studied using a first-principles effective Hamiltonian to describe the structural energetics at a microscopic level. Under short-circuit electrical and zero-stress mechanical boundary conditions, (0 0 1) films as thin as one unit cell are found to support a stable perpendicular polarization. Size effects in the layer-by-layer ferroelectric distortion are discussed. The continuum limit of the first-principles effective Hamiltonian has the same form as the widely-used Landau-Ginzburg expression for the free energy near Tc, so that the present results can be directly related to those of previous phenomenological studies. In particular, a microscopic interpretation of the extrapolation length δ is proposed.
Applied Physics Letters, 2019
HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific r... more HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Advanced Materials, 2017
Transition-metal oxides with an ABO 3 perovskite structure exhibit strongly entangled structural ... more Transition-metal oxides with an ABO 3 perovskite structure exhibit strongly entangled structural and electronic degrees of freedom and thus one expects to unveil exotic phases and properties by acting on the lattice through various external stimuli. Using the Jahn-Teller active praseodymium vanadate Pr 3+ V 3+ O 3 compound as a model system, we show that PrVO 3 Néel temperature T N can be raised by 40 K with respect to the bulk when grown as thin films. Using advanced experimental techniques, this enhancement is unambiguously ascribed to a tetragonality resulting from the epitaxial compressive strain experienced by the films. First-principles simulations not only confirm experimental results, but they also reveal that the strain promotes an unprecedented orbital ordering of the V 3+ d electrons, strongly favoring antiferromagnetic interactions. These results show that an accurate control of structural aspects of oxides is the key for unveiling unexpected phases in oxides.
The electron localization tensor, Born effective charges, dielectric constants and phonon dispers... more The electron localization tensor, Born effective charges, dielectric constants and phonon dispersion relations of the skutterudite CoSb3 and the filled skutterudite TlFeCo3Sb12 have been studied using density functional perturbation theory. The origin of the low energy peak in the phonon density of states of TlFeCo3Sb12 observed recently by Neutron inelastic scattering [R. P. Hermann et al., Phys. Rev. Lett. 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.
Physical Review B, 2015
Using a combination of first-principles calculations and experimental transport measurements, we ... more Using a combination of first-principles calculations and experimental transport measurements, we study the electronic and magnetic structure of the unfilled skutterudite FeSb 3 . We employ the hybrid functional approach for exchange correlation. The ground state is determined to be antiferromagnetic with an atomic magnetic moment of 1.6 μ B /Fe. The Néel temperature T N is estimated at 6 K, in agreement with experiments which found a paramagnetic state down to 10 K. The ground state is semiconducting, with a small electronic gap of 33 meV, also consistent with previous experiments on films. Charge carrier concentrations are estimated from Hall resistance measurements. The Seebeck coefficient is measured and mapped using a scanning probe at room temperature that yields an average value of 38.6 μV K -1 , slightly lower than the theoretical result. The theoretical conductivity is analyzed as a function of temperature and concentration of charge carriers.
Physical Review Letters, 2015
Thermoelectrics are promising to address energy issues but their exploitation is still hampered b... more Thermoelectrics are promising to address energy issues but their exploitation is still hampered by low efficiencies. So far, much improvement has been achieved by reducing the thermal conductivity but less by maximizing the power factor. The latter imposes apparently conflicting requirements on the band structure: a narrow energy distribution and a low effective mass. Quantum confinement in nanostructures or the introduction of resonant states were suggested as possible solutions to this paradox but with limited success. Here, we propose an original approach to fulfill both requirements in bulk semiconductors. It exploits the highly-directional character of some orbitals to engineer the band-structure and produce a type of low-dimensional transport similar to that targeted in nanostructures, while retaining isotropic properties. Using first-principles calculations, the theoretical concept is demonstrated in Fe2YZ Heusler compounds, yielding power factors 4-5 times larger than in classical thermoelectrics at room temperature. Our findings are totally generic and rationalize the search of alternative compounds with a similar behavior. Beyond thermoelectricity, these might be relevant also in the context of electronic, superconducting or photovoltaic applications.
First-Principles Studies of Ferroelectric Oxides
Topics in Applied Physics
The application of first-principles methods to the study of ferroelectric oxides is reviewed. Whi... 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
Physical Review B, 2014
The behavior of PbTiO 3 under uniaxial strains and stresses is investigated from first-principles... more The behavior of PbTiO 3 under uniaxial strains and stresses is investigated from first-principles calculations within density functional theory. We show that irrespective of the uniaxial mechanical constraint applied, the system keeps a purely ferroelectric ground state, with the polarization aligned either along the constraint direction (FE z phase) or along one of the pseudocubic axes perpendicular to it (FE x phase). This contrasts with the cases of isotropic and biaxial mechanical constraints for which novel phases combining ferroelectric and antiferrodistortive motions have been previously reported. Under uniaxial strain, PbTiO 3 switched from an FE x ground state under compressive strain to an FE z ground state under tensile strain beyond a critical strain η c zz ≈ +1%. Under uniaxial stress, PbTiO 3 exhibits either an FE x ground state under compression (σ zz < 0) or an FE z ground state under tension (σ zz > 0). Here, however, an abrupt jump of the structural parameters is also predicted under both compressive and tensile stresses at critical values σ zz ≈ +2 and -8 GPa. This behavior appears to be similar to that predicted under negative isotropic pressure and might turn out to be practically useful for enhancing the piezoelectric response in nanodevices.
Polarization Patterns In GeTe From Bulk To Ferroelectric Nanoclusters
In this study, we investigated the ferroelectric and structural properties of GeTe crystal and na... more In this study, we investigated the ferroelectric and structural properties of GeTe crystal and nanoclusters, an alternative type of ferroelectric material, up to 1500 atoms from first-principles calculations based on density functional theory (DFT). Firstly, the dynamical, dielectric and elastic properties of GeTe in ferroelectric phase have been investigated [1]. Next, we demonstrate, for the first time at the DFT
Since its introduction in the 90's, the first-principles effective-Hamiltonian method has been su... more Since its introduction in the 90's, the first-principles effective-Hamiltonian method has been successfully used to simulate temperature-driven phenomena in increasingly complex ferroelectrics, from classic compound BaTiO 3 to multiferroic BiFeO 3 . Currently, the emergence of nano-structured materials -e.g., in the form of ultra-thin films or short-period superlattices -poses new challenges to the simulations, and the development of predictive models seems to require a reconsideration of the traditional approach. Of particular interest are cases in which novel interfacial effects determine the behavior, as in the PbTiO 3 -SrTiO 3 superlattices of Bousquet et al. [Nature 452, 7188 (2008)]. In such situations a large number of structural distortions may become active, and it may be difficult to decide which ones need to be included in the model. In order to tackle these difficulties, we are extending the first-principles effective-Hamiltonian method so as to retain a full atomistic description of the material, thus removing the so-called local mode approximation. I will describe our new approach and show preliminary results for PbTiO 3 . 1 Work funded by the Spanish DGI and the FP7 program of the EU.
Dalton Transactions, 2015
Recent theoretical and experimental studies showing how polar structures or ferroelectricity aris... more Recent theoretical and experimental studies showing how polar structures or ferroelectricity arise in layered perovskites are highlighted.
The Journal of Physical Chemistry C, 2014
The structural, dielectric, dynamical, elastic, piezoelectric and nonlinear optical (second-order... more The structural, dielectric, dynamical, elastic, piezoelectric and nonlinear optical (second-order susceptibility and Pockels tensors) properties of Bi 2 WO 6 in its P2 1 ab 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 confirms the elastic stability of the crystal and allow us to estimate the Young and shear moduli of polycrystalline samples. The piezoelectric constants have significant intrinsic values comparable to those of prototypical ABO 3 ferroelectrics. The electro-optic response is strongly dominated by the ionic contribution of transverse optic modes, yielding sizable Pockels coefficients around 9 pm/V along the polar direction.
Physical Review Letters, 2011
We describe the intrinsic mechanism of 2-dimensional electron confinement at the n-type SrTiO 3 =... more We describe the intrinsic mechanism of 2-dimensional electron confinement at the n-type SrTiO 3 =LaAlO 3 interface as a function of the sheet carrier density n s via advanced first-principles calculations. Electrons localize spontaneously in Ti 3d xy levels within a thin (& 2 nm) interface-adjacent SrTiO 3 region for n s lower than a threshold value n c $ 10 14 cm À2 . For n s > n c a portion of charge flows into Ti 3d xz -d yz levels extending farther from the interface. This intrinsic confinement can be attributed to the interface-induced symmetry breaking and localized nature of Ti 3d t 2g states. The sheet carrier density directly controls the binding energy and the spatial extension of the conductive region. A direct, quantitative relation of these quantities with n s is provided.
Physical Review Letters, 2009
From rst-principles calculations, we investigate the structural instabilities of CaMnO3. We point... more From rst-principles calculations, we investigate the structural instabilities of CaMnO3. We point out that, on top of a strong antiferrodistortive instability responsible for its orthorhombic ground-state, the cubic perovskite structure of CaMnO3 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 CaMnO3 multiferroic. We nally highlight that the FE instability of CaMnO3 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.
First-principles study of the dynamical and nonlinear optical properties of urea single crystals
Phys. Chem. Chem. Phys., 2010
Nature, 2008
In paraelectric/ferroelectric heterostructures with thick constituent layers electrostatics is th... more In paraelectric/ferroelectric heterostructures with thick constituent layers electrostatics is the dominant interaction between layers and we have previously demonstrated that the key ferroelectric parameters, polarization and critical temperature can be tuned over a very large range in PbTiO 3 /SrTiO 3 superlattices by varying the ratio of the layer thicknesses [1]. However, as the layers become thinner, a departure from the electrostatic model is observed, which manifests itself as an unusually high ferroelectric polarization and transition temperature and a high, but temperature independent, dielectric constant. Detailed examination of the phase transitions with temperature reveal that along with these enhanced characteristics there is a fundamental change in the nature of the ferroelectricity. The microscopic origin of this change, a form of improper ferroelectricity, is revealed by first principles calculations to occur through a coupling of oxygen rotations and the polarization mode at the interfaces in the material.
Journal of Solid State Chemistry, 2009
Journal of Physics: Condensed Matter, 2008
The structural, dielectric and dynamical properties of the low temperature antiferromagnetic orth... more 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.
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Papers by Philippe Ghosez