References of "Bousquet, Eric"
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See detailUnveiling the Room-Temperature Magnetoelectricity of Troilite FeS
Ricci, Fabio ULg; Bousquet, Eric ULg

in Physical Review Letters (2016), 116

We report on a first-principles study of the troilite phase of iron sulfide (FeS). We show that even if, a few decades ago, this material was thought to be ferroelectric, the structural transition from ... [more ▼]

We report on a first-principles study of the troilite phase of iron sulfide (FeS). We show that even if, a few decades ago, this material was thought to be ferroelectric, the structural transition from the high P63/mmc to the low P6¯2c symmetry phase does not involve polar instabilities, though the space inversion center symmetry is broken. Our calculations and symmetry analysis nevertheless reveal that FeS is magnetoelectric at room temperature with a response larger than the prototypical room-temperature magnetoelectric crystal Cr2O3. We also show that the spin channel decomposition of the polarization exhibits nonzero values in the opposite direction in FeS, which is actually a general hint of the presence of a magnetoelectric monopole in diagonal magnetoelectrics. [less ▲]

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See detailLarge elasto-optic effect and reversible electrochromism in multiferroic BiFeO3
Sando, D.; Yang, Yurong; Bousquet, Eric ULg et al

in Nature Communications (2016), 7

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous ... [more ▼]

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology—a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we apply epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption—reminiscent of piezochromism—which we show can be manipulated by an electric field. This constitutes an electrochromic effect that is reversible, remanent and not driven by defects. These findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses. [less ▲]

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See detailNon-collinear magnetism in multiferroic perovskites
Bousquet, Eric ULg; Cano, Andres

in Journal of Physics : Condensed Matter (2016), 28

We present an overview of the current interest in non-collinear magnetism in multiferroic perovskite crystals. We first describe the different microscopic mechanisms giving rise to the non-collinearity of ... [more ▼]

We present an overview of the current interest in non-collinear magnetism in multiferroic perovskite crystals. We first describe the different microscopic mechanisms giving rise to the non-collinearity of spins in this class of materials. We discuss, in particular, the interplay between non-collinear magnetism and ferroelectric and antiferrodistortive distortions of the perovskite structure, and how this can promote magnetoelectric responses. We then provide a literature survey on non-collinear multiferroic perovskites. We discuss numerous examples of spin cantings driving weak ferromagnetism in transition metal perovskites, and of spin- induced ferroelectricity as observed in the rare-earth based perovskites. These examples are chosen to best illustrate the fundamental role of non-collinear magnetism in the design of multiferroicity. [less ▲]

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See detailSpin texture induced by oxygen vacancies in strontium perovskite (001) surfaces: A theoretical comparison between SrTiO3 and SrHfO3
Garcia Castro, Andrés Camilo ULg; Vergniory, Maia G.; Bousquet, Eric ULg et al

in Physical Review B (2016), 93

The electronic structure of SrTiO3 and SrHfO3 (001) surfaces with oxygen vacancies is studied by means of first-principles calculations. We reveal how oxygen vacancies within the first atomic layer of the ... [more ▼]

The electronic structure of SrTiO3 and SrHfO3 (001) surfaces with oxygen vacancies is studied by means of first-principles calculations. We reveal how oxygen vacancies within the first atomic layer of the SrTiO3 surface (i) induce a large antiferrodistortive motion of the oxygen octahedra at the surface, (ii) drive localized magnetic moments on the Ti 3d orbitals close to the vacancies, and (iii) form a two-dimensional electron gas localized within the first layers. The analysis of the spin texture of this system exhibits a splitting of the energy bands according to the Zeeman interaction, lowering of the Ti 3dxy level in comparison with dxz and dyz, and also an in-plane precession of the spins. No Rashba-like splitting for the ground state or for the ab initio molecular dynamics trajectory at 400 K is recognized as suggested recently by A. F. Santander-Syro et al. [Nat. Mater. 13, 1085 (2014)]. Instead, a sizable Rashba-like splitting is observed when the Ti atom is replaced by a heavier Hf atom with a much larger spin-orbit interaction. However, we observe the disappearance of the magnetism and the surface two-dimensional electron gas when full structural optimization of the SrHfO3 surface is performed. Our results uncover the sensitive interplay of spin-orbit coupling, atomic relaxations, and magnetism when tuning these Sr-based perovskites. [less ▲]

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See detailStrain-Engineered Multiferroicity in Pnma NaMnF3 Fluoroperovskite
Garcia Castro, Andrés Camilo ULg; Bousquet, Eric ULg; Romero, Aldo H.

in Physical Review Letters (2016)

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See detailRecent developments in the ABINIT software package
Gonze, X.; Jollet, F.; Araujo, F. Abreu et al

in Computer Physics Communications (2016), 205

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See detailFerroelectric instability in nanotubes and spherical nanoshells
Qiu, Ruihao ULg; Bousquet, Eric ULg; Cano, Andres

in Europhysics Letters [=EPL] (2015), 112

The emergence of ferroelectricity in nanotubes and spherical nanoshells is studied theoretically. We determine semi-analytically the size and thickness dependence of the ferroelectric instability, as well ... [more ▼]

The emergence of ferroelectricity in nanotubes and spherical nanoshells is studied theoretically. We determine semi-analytically the size and thickness dependence of the ferroelectric instability, as well as its dependence on the properties of the surrounding media and the corresponding interfaces. By properly tuning these factors, we demonstrate possible routes for enhancing the ferroelectric transition temperature and promoting the competition between irrotational and vortex-like states in the ultra-thin limit due to the specific topology of these nanoparticles. [less ▲]

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See detailCoupling and electrical control of structural, orbital and magnetic orders in perovskites
Varignon, Julien ULg; Bristowe, Nicholas ULg; Bousquet, Eric ULg et al

in Scientific Reports (2015), 5

Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional ... [more ▼]

Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional electronic properties exhibited by perovskites is often the so-called Jahn-Teller distortion. For applications, an electrical control of the Jahn-Teller distortions, which is so far out of reach, would therefore be highly desirable. Based on universal symmetry arguments, we determine new lattice mode couplings that can provide exactly this paradigm, and exemplify the effect from firstprinciples calculations. The proposed mechanism is completely general, however for illustrative purposes, we demonstrate the concept on vanadium based perovskites where we reveal an unprecedented orbital ordering and Jahn-Teller induced ferroelectricity. Thanks to the intimate coupling between Jahn-Teller distortions and electronic degrees of freedom, the electric field control of Jahn-Teller distortions is of general relevance and may find broad interest in various functional devices. [less ▲]

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See detailProper Ferroelectricity in the Dion−Jacobson Material CsBi2Ti2NbO10: Experiment and Theory
McCabe, E. E.; Bousquet, Eric ULg; Stockdale, C. P. J. et al

in Chemistry of Materials (2015), 27

A diverse range of materials and properties are exhibited by layered perovskites. We report on the synthesis, characterization, and computational investigation of a new ferroelectric --CsBi2Ti2NbO10-- an ... [more ▼]

A diverse range of materials and properties are exhibited by layered perovskites. We report on the synthesis, characterization, and computational investigation of a new ferroelectric --CsBi2Ti2NbO10-- an n = 3 member of the Dion−Jacobson (DJ) family. Structural studies using variable temperature neutron powder diffraction indicate that a combination of octahedral rotations and polar displacements result in the polar structure. Density functional theory calculations reveal that the wider perovskite blocks in CsBi2Ti2NbO0 stabilize proper ferroelectricity, in contrast to the hybrid-improper ferroelectricity reported for all other DJ phases. Our results raise the possibility of a new class of proper ferroelectric materials analogous to the well-known Aurivillius phases. [less ▲]

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See detailFerromagnetism induced by entangled charge and orbital orderings in ferroelectric titanate perovskites
Bristowe, Nicholas ULg; Varignon, Julien ULg; Fontaine, Denis et al

in Nature Communications (2015), 6

In magnetic materials, the Pauli exclusion principle typically drives anti-alignment between electron spins on neighbouring species resulting in antiferromagnetic behaviour. Ferromagnetism exhibiting ... [more ▼]

In magnetic materials, the Pauli exclusion principle typically drives anti-alignment between electron spins on neighbouring species resulting in antiferromagnetic behaviour. Ferromagnetism exhibiting spontaneous spin alignment is a fairly rare behaviour, but once materialized is often associated with itinerant electrons in metals. Here we predict and rationalize robust ferromagnetism in an insulating oxide perovskite structure based on the popular titanate series. In half-doped layered titanates, the combination of Jahn–Teller and oxygen breathing motions opens a band gap and creates an unusual charge and orbital ordering of the Ti d electrons. It is argued that this intriguingly intricate electronic network favours the elusive inter-site ferromagnetic (FM) ordering, on the basis of intra-site Hund's rules. Finally, we find that the layered oxides are also ferroelectric with a spontaneous polarization approaching that of ​BaTiO3. The concepts are general and design principles of the technologically desirable FM ferroelectric multiferroics are presented. [less ▲]

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See detailNoncollinear magnetism in post-perovskites from first-principles: Comparison between CaRhO3 and NaNiF3
Garcia Castro, Andrés Camilo ULg; Romero, Aldo; Bousquet, Eric ULg

in Physica Status Solidi B. Basic Research (2015), 252(4), 689

Based on first-principles calculations, we study the noncollinear magnetism in the post-perovskites (pPv) phase of NaNiFinline image and CaRhOinline image crystals. We find that the magnetic canting is ... [more ▼]

Based on first-principles calculations, we study the noncollinear magnetism in the post-perovskites (pPv) phase of NaNiFinline image and CaRhOinline image crystals. We find that the magnetic canting is one of the most promising properties of pPv systems, which is allowed by symmetry in all the pPv crystals inline image with a magnetically active B-site. In the pPv phase of NaNiFinline image, which has a inline image antiferromagnetic ground state with a inline image ferromagnetic canting, we obtain a magnetic canting amplitude of about 0.1 inline imageatominline image, which is much larger than in the one obtained in CaRhOinline image (canting amplitude of 0.04 inline image). We also computed the exchange constants (inline image), the single-ion anisotropy (SIA) parameters and the anti-symmetric magnetic coupling described by the Dzyaloshinsky–Moriya (DM) interaction in order to scrutinize the origin of the magnetic canting. We find that the canting in NaNiFinline image is mainly due to the DM interaction, while in CaRhOinline image, both DM and SIA contribute to the magnetic canting. Our calculations thus confirm the noncollinear magnetic ground-state solution experimentally observed in both compounds and the calculated magnetic exchange interactions also confirm the quasi-2D magnetic behavior reported in pPv. [less ▲]

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See detailNovel magneto-electric multiferroics from first-principles calculations
Varignon, Julien ULg; Bristowe, Nicholas ULg; Bousquet, Eric ULg et al

in Comptes Rendus Physique (2015), 16

Interest in first-principles calculations within the multiferroic community has been rapidly on the rise over the last decade. Initially considered as a powerful support to explain experimentally observed ... [more ▼]

Interest in first-principles calculations within the multiferroic community has been rapidly on the rise over the last decade. Initially considered as a powerful support to explain experimentally observed behaviours, the trend has evolved and, nowadays, density functional theory calculations have become also an essential predicting tool for identifying original rules to achieve multiferroism and design new magneto-electric compounds. This chapter aims at highlighting the key advances in the field of multiferroics, to which first-principles methods have contributed significantly. The essential theoretical developments that made this research possible are also briefly presented. [less ▲]

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See detailGeometric ferroelectricity in fluoroperovskites
Garcia Castro, Andrés Camilo ULg; Spaldin, Nicola A.; Romero, A. H. et al

in Physical Review. B, Condensed Matter and Materials Physics (2014), 89

We used first-principles calculations to investigate the existence and origin of the ferroelectric instability in the ABF3 fluoroperovskites. While the ground states of most ABF3 compounds are ... [more ▼]

We used first-principles calculations to investigate the existence and origin of the ferroelectric instability in the ABF3 fluoroperovskites. While the ground states of most ABF3 compounds are paraelectric (Pnma phase), we find that many fluoroperovskites have a ferroelectric instability in their high-symmetry cubic structure that is of similar amplitude to that commonly found in oxide perovskites. In contrast to the oxides, however, the fluorides have nominal Born effective charges, indicating a different mechanism for the instability.We show that the instability originates from ionic size effects, and is therefore in most cases largely insensitive to pressure and strain, again in contrast to the oxide perovskites. An exception is NaMnF3, where coherent epitaxial strain matching to a substrate with equal in-plane lattice constants destabilizes the bulk Pnma structure, leading to a ferroelectric, and indeed multiferroic, ground state with an unusual polarization/strain response. [less ▲]

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See detailFirst-principles study of vibrational and noncollinear magnetic properties of the perovskite to postperovskite pressure transition of $NaMnF_3$
Garcia Castro, Andrés Camilo ULg; Romero, A. H.; Bousquet, Eric ULg

in Physical Review. B, Condensed Matter and Materials Physics (2014), 90

We performed a first-principles study of the structural, vibrational, electronic, and magnetic properties of NaMnF3 under applied isotropic pressure. We found that NaMnF3 undergoes a reconstructive phase ... [more ▼]

We performed a first-principles study of the structural, vibrational, electronic, and magnetic properties of NaMnF3 under applied isotropic pressure. We found that NaMnF3 undergoes a reconstructive phase transition at 8 GPa from the Pnma distorted perovskite structure toward the Cmcm postperovskite structure. This is confirmed by a sudden change of the Mn-F-Mn bondings where the crystal goes from corner-shared octahedra in the Pnma phase to edge-shared octahedra in the Cmcm phase. The magnetic ordering also changes from a G-type antiferromagnetic ordering in the Pnma phase to a C-type antiferromagnetic ordering in the Cmcm phase. Interestingly, we found that the high-spin d-orbital filling is kept at the phase transition which has never been observed in the known magnetic postperovskite structures. We also found a highly noncollinear magnetic ordering in the Cmcm postperovskite phase that drives a large ferromagnetic canting of the spins. We discuss the validity of these results with respect to the U and J parameters of the GGA+U exchange-correlation functional used in our study and conclude that large-spin canting is a promising property of the postperovskite fluoride compounds. [less ▲]

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See detailNew developments in artificially layered ferroelectric oxide superlattices
Dawber, Matthew; Bousquet, Eric ULg

in MRS Bulletin (2013), 38

Artifi cially layered superlattices of oxide materials have been intensely investigated for some time, but continue to reveal new potential as a route to advanced functional materials. As well as ... [more ▼]

Artifi cially layered superlattices of oxide materials have been intensely investigated for some time, but continue to reveal new potential as a route to advanced functional materials. As well as considering electrostatics and strain, a more complete picture of the interfaces in these systems also needs to incorporate the possibility of additional structural distortions, electronic redistributions, and complex polarization domain structures. Here we focus on superlattices composed of two perovskite oxide materials, where one is a ferroelectric, and discuss the important interactions between the component materials that determine the behavior of the new artifi cial material. We discuss interfaces both with and without electronic screening. The fi rst class of interface contains technologically relevant ultrathin ferroelectric capacitors and the more recently studied ferroelectric-metal superlattices. In these systems, the infl uence of the ferroelectric polarization decreases rapidly with distance from the interface. By contrast, in systems where the materials adjacent to the ferroelectric layers are dielectrics, the polarization of the ferroelectric layer infl uences the properties of the adjacent layers over a much longer distance, setting the stage for fascinating competition between the properties of the combined materials. [less ▲]

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See detailStrong coupling of Jahn-Teller distortion to oxygen-octahedron rotation and functional properties in epitaxially strained orthorhombic LaMnO3
Lee, Jun Hee; Delaney, Kris; Bousquet, Eric ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88

First-principles calculations reveal a large cooperative coupling of Jahn-Teller (JT) distortion to oxygenoctahedron rotations in perovskite LaMnO3. The combination of the two distortions is responsible ... [more ▼]

First-principles calculations reveal a large cooperative coupling of Jahn-Teller (JT) distortion to oxygenoctahedron rotations in perovskite LaMnO3. The combination of the two distortions is responsible for stabilizing the strongly orthorhombic A-AFM insulating (I ) eP bnm ground state relative to a metallic ferromagnetic (FM-M) phase. However, epitaxial strain due to coherent matching to a crystalline substrate can change the relative stability of the two states. In particular, coherent matching to a square-lattice substrate favors the less orthorhombic FM-M phase, with the A-AFM phase stabilized at higher values of tensile epitaxial strain due to its larger volume per formula unit, resulting in a coupled magnetic and metal-insulator transition at a critical strain close to 1%. At the phase boundary, a very large magnetoresistance is expected. Tensile epitaxial strain enhances the JT distortion and opens the band gap in the A-AFM-I c-eP bnm phase, offering the opportunity for band-gap engineering. Compressive epitaxial strain induces a transition within the FM-M phase from the c-eP bnm orientation to the ab-eP bnm orientation with a change in the direction of the magnetic easy axis relative to the substrate, yielding strain-controlled magnetization at the phase boundary. Similar behavior is expected in other JT active Pbnm perovskites. [less ▲]

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See detailMonopole-based formalism for the diagonal magnetoelectric response
Spaldin, Nicola A.; Fechner, Michael; Bousquet, Eric ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88

We develop the formalism of the macroscopic monopolization—that is, the magnetoelectric monopole moment per unit volume—in periodic solids, and discuss its relationship to the magnetoelectric effect. For ... [more ▼]

We develop the formalism of the macroscopic monopolization—that is, the magnetoelectric monopole moment per unit volume—in periodic solids, and discuss its relationship to the magnetoelectric effect. For the series of lithium transition metal phosphate compounds, we use first-principles density functional theory to calculate the contributions to the macroscopic monopolization from the global distribution of magnetic moments within the unit cell, as well as from the distribution of magnetization around the atomic sites. We find one example within the series (LiMnPO4) that shows a macroscopic monopolization corresponding to a ferromonopolar ordering consistent with its diagonal magnetoelectric response. The other members of the series (LiMPO4, with M=Co, Fe, and Ni) have zero net monopolization but have antiferromonopolar orderings that should lead to q-dependent diagonal magnetoelectric effects. [less ▲]

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See detailFirst-principles study of the ferroelectric Aurivillius phase Bi2WO6
Djani-Ait, Hania ULg; Bousquet, Eric ULg; Kellou, Abdelhafid et al

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

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 ... [more ▼]

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 of two possible high-temperature paraelectric 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 exhibit various unstable modes, which, after their condensation, bring the system toward more stable structures of lower symmetry. The calculations confirm that, starting from the paraelectric A2/m phase at high temperatures, the system must undergo a reconstructive transition to reach the P21ab ferroelectric ground state. [less ▲]

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See detailFirst-principles study of structural and vibrational properties of SrZrO3
Amisi, Safari ULg; Bousquet, Eric ULg; Katcho, Karume et al

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85

Using first-principles calculations, we investigate the electronic, structural, and vibrational properties of SrZrO3. We start from the high-symmetry cubic perovskite phase, for which the phonon ... [more ▼]

Using first-principles calculations, we investigate the electronic, structural, and vibrational properties of SrZrO3. We start from the high-symmetry cubic perovskite phase, for which the phonon dispersion curves are reported.We point out the coexistence of structural antiferrodistortive instabilities at the R and M zone-boundary points and a ferroelectric instability at the zone center.We showthat the strong antiferrodistortive motions suppress ferroelectricity and are responsible for the orthorhombic ground state as in CaTiO3. The structural properties <br />of possible intermediate phases and of the orthorhombic Pnma ground state are reported. For the latter, an assignment of IR and Raman zone-center phonon modes is proposed. The main features of the ferroelectric instability are also discussed, and we show that a ferroelectric ground state can even be induced in SrZrO3 by strain engineering. [less ▲]

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See detailPhenomenological thermodynamic potential for CaTiO3 single crystals
Gu, Yijia; Rabe, Karin; Bousquet, Eric ULg et al

in Physical Review B (2012), 85

The antiferrodistortive (AFD) structural transitions of calcium titanate (CaTiO3) at ambient pressure have been extensively studied during the last few years. It has been found that none of the AFD ... [more ▼]

The antiferrodistortive (AFD) structural transitions of calcium titanate (CaTiO3) at ambient pressure have been extensively studied during the last few years. It has been found that none of the AFD polymorphs is polar or ferroelectric. However, it was recently shown theoretically and later experimentally confirmed that a ferroelectric transition in CaTiO3 can be induced by tensile strains. The ferroelectric instability is believed to be strongly coupled to the AFD soft modes. In this paper, we present a complete thermodynamic potential for describing the coupling between the AFD and ferroelectric phase transitions. We analyzed the dependence of transition temperatures on stress and strain condition. Based on this potential, a (001) CaTiO3 thin film diagram was constructed. The results show good agreement with available experimental observations. The strong suppression of ferroelectric transition by the AFD transition is discussed. [less ▲]

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