Results for "Nicola A. Spaldin"

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Towards a microscopic theory of toroidal moments in bulk periodic crystalsJun 13 2007Jun 03 2008We present a theoretical analysis of magnetic toroidal moments in periodic systems, in the limit in which the toroidal moments are caused by a time and space reversal symmetry breaking arrangement of localized magnetic dipole moments. We summarize the ... More
Weak ferromagnetism and magnetoelectric coupling in bismuth ferriteJun 30 2004We analyze the coupling between the ferroelectric and magnetic order parameters in the magnetoelectric multiferroic BiFeO_3 using density functional theory within the local spin density approximation and the LSDA+U method. We show that weak ferromagnetism ... More
Interplay between strain, defect charge state and functionality in complex oxidesMay 09 2016We use first-principles calculations to investigate the interplay between strain and the charge state of point defect impurities in complex oxides. Our work is motivated by recent interest in using defects as active elements to provide novel functionality ... More
Magnetic interactions in transition metal doped ZnO : An abinitio studyMay 22 2006We calculate the nature of magnetic interactions in transition-metal doped ZnO using the local spin density approximation and LSDA+\textit{U} method of density functional theory. We investigate the following four cases: (i) single transition metal ion ... More
Competition and cooperation between antiferrodistortive and ferroelectric instabilities in SrTiO$_3$Oct 23 2013We use density functional theory to explore the interplay between octahedral rotations and ferroelectricity in the model compound SrTiO$_3$. We find that over the experimentally relevant range octahedral rotations suppress ferroelectricity as is generally ... More
Magnetoelectric multipoles in metalsApr 17 2018We demonstrate computationally the existence of magnetoelectric multipoles, arising from the second order term in the multipole expansion of a magnetization density in a magnetic field, in noncentrosymmetric magnetic metals. While magnetoelectric multipoles ... More
Strong-correlation effects in Born effective chargesMar 16 2003Large values of Born effective charges are generally considered as reliable indicators of the genuine tendency of an insulator towards ferroelectric instability. However, these quantities can be very much influenced by strong electron correlation and ... More
Recent progress in first-principles studies of magnetoelectric multiferroicsDec 15 2005Materials that combine magnetic and ferroelectric properties have generated increasing interest over the last few years, due to both their diverse properties and their potential utility in new types of magnetoelectric device applications. In this review ... More
Effect of epitaxial strain on the spontaneous polarization of thin film ferroelectricsJul 31 2005We investigate the variation of the spontaneous ferroelectric polarization with epitaxial strain for BaTiO$_3$, PbTiO$_3$, LiNbO$_3$, and BiFeO$_3$ using first principles calculations. We find that while the strain dependence of the polarization is very ... More
Origin of ferroelectricity in the multiferroic barium fluorides BaMF4May 01 2006We present a first principles study of the series of multiferroic barium fluorides with the composition BaMF4, where M is Mn, Fe, Co, or Ni. We discuss trends in the structural, electronic, and magnetic properties, and we show that the ferroelectricity ... More
Influence of strain and oxygen vacancies on the magnetoelectric properties of multiferroic bismuth ferriteFeb 15 2005The dependencies on strain and oxygen vacancies of the ferroelectric polarization and the weak ferromagnetic magnetization in the multiferroic material bismuth ferrite, BiFeO_3, are investigated using first principles density functional theory calculations. ... More
BaNiF4: an electric field-switchable weak antiferromagnetFeb 16 2006Feb 23 2006We show that in the antiferromagnetic ferroelectric BaNiF4 the Dzyaloshinskii-Moriya interaction leads to a small canting of the magnetic moments, away from the easy axis, resulting in a noncollinear magnetic structure. The canting corresponds to an additional ... More
Effects of intense optical phonon pumping on the structure and electronic properties of yttrium barium copper oxideJul 05 2016Jul 18 2016We investigate the structural modulations induced by optical excitation of a polar phonon mode in YBa$_2$Cu$_3$O$_7$, using first-principles calculations based on density functional theory. We focus on the intense-excitation regime in which we expect ... More
Electric displacement as the fundamental variable in electronic-structure calculationsJun 04 2008Nov 05 2008Finite-field calculations in periodic insulators are technically and conceptually challenging, due to fundamental problems in defining polarization in extended solids. While significant progress has been made recently with the establishment of techniques ... More
Strain-induced heteronuclear charge disproportionation in EuMnO$_3$Oct 22 2018Charge disproportionation transitions in complex oxides most commonly link high-temperature phases containing identical cations with the same oxidation state and crystallographic site, to low-temperature phases in which charge transfer between these ions ... More
First-principles modeling of ferroelectric capacitors via constrained-D calculationsAug 13 2009First-principles modeling of ferroelectric capacitors presents several technical challenges, due to the coexistence of metallic electrodes, long-range electrostatic forces and short-range interface chemistry. Here we show how these aspects can be efficiently ... More
Strain-induced ferroelectricity in simple rocksalt binary oxidesJun 23 2009The 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 ... More
Origin and evolution of ferroelectricity in the layered rare-earth-titanate, $R_2$Ti$_2$O$_{7}$, Lichtenberg phasesMar 28 2019We report a systematic first-principles study based on density functional theory (DFT) of the structural and ferroelectric properties of the $R_2$Ti$_2$O$_{7}$ perovskite-related oxides with $R=$ La, Ce, Pr, and Nd. We show that, in all cases, the ferroelectric ... More
Structure and properties of functional oxide thin films: Insights from electronic-structure calculationsMar 22 2011The confluence of state-of-the-art electronic-structure computations and modern synthetic materials growth techniques is proving indispensable in the search for and discovery of new functionalities in oxide thin films and heterostructures. Here, we review ... More
Effect of epitaxial strain on cation and anion vacancy formation in MnOJun 02 2015Biaxial strain in coherent epitaxial thin films can have a pronounced effect on the point defect profile in the film material. Detailed fundamental knowledge of the interaction of strain with point defects is crucial in understanding the stoichiometry ... More
Ab initio investigation of FeAs/GaAs heterostructures for potential spintronic and superconducting applicationsAug 15 2011Ultra-thin FeAs is of interest both as the active component in the newly identified pnictide superconductors, and in spintronic applications at the interface between ferromagnetic Fe and semiconducting GaAs. Here we use first-principles density functional ... More
Electron-lattice instabilities suppress cuprate-like electronic structures in SrFeO$_3$/SrTiO$_3$ superlatticesAug 24 2009Using {\it ab initio} density functional theory we explore the behavior of thin layers of metallic $d^4$ SrFeO$_3$ confined between the $d^0$ dielectric SrTiO$_3$ in a superlattice geometry. We find the presence of insulating SrTiO$_3$ spacer layers strongly ... More
Structural effects on the spin-state transition in epitaxially strained LaCoO$_3$ filmsAug 15 2008Dec 23 2008Using density functional theory within the LSDA + U method, we investigate the effect of strain on the spin state and magnetic ordering in perovskite lanthanum cobaltite, LaCoO3. We show that, while strain-induced changes in lattice parameters are insufficient ... More
Self-interaction effects in (Ga,Mn)As and (Ga,Mn)NFeb 10 2003Jul 02 2004The electronic structures of Mn-doped zincblende GaAs and wurtzite GaN are calculated using both standard local-density functional theory (LSDA), and a novel pseudopotential self-interaction-corrected approach (pseudo-SIC), able to account for the effects ... More
A density functional theory study of FeAs comparing LDA+U, GGA+U and hybrid functionalsJan 10 2014We use density functional theory within the local density approximation (LDA), LDA+U, generalized gradient approximation (GGA), GGA+U, and hybrid-functional methods to calculate the properties of iron monoarsenide. FeAs, which forms in the MnP structure, ... More
First principles study of the multiferroics BiFeO$_{3}$, Bi$_{2}$FeCrO$_{6}$, and BiCrO$_{3}$: Structure, polarization, and magnetic ordering temperatureAug 15 2005Aug 22 2005We present results of an {\it ab initio} density functional theory study of three bismuth-based multiferroics, BiFeO$_{3}$, Bi$_{2}$FeCrO$_{6}$, and BiCrO$_{3}$. We disuss differences in the crystal and electronic structure of the three systems, and we ... More
Enhancement of ferroelectricity at metal/oxide interfacesNov 05 2008By performing first-principles calculations on four capacitor structures based on BaTiO3 and PbTiO3, we determine the intrinsic interfacial effects that are responsible for the destabilization of the polar state in thin-film ferroelectric devices. We ... More
Orbital magnetic moments of phononsDec 13 2018In ionic materials, circularly polarized phonons carry orbital magnetic moments that arise from circular motions of the ions, and which interact with other magnetic moments or fields. Here, we calculate the orbital magnetic moments of phonons in 35 different ... More
First-principles calculation of the bulk magnetoelectric monopolization: Berry phase and Wannier function approachesMar 10 2016We present a formalism to calculate the macroscopic magnetoelectric monopolization from first principles within the density functional theory framework. An expression for the monopolization in the case of insulating collinear magnetism is derived first ... More
On the relationship between topological and geometric defectsMar 15 2017The study of topology in solids is undergoing a renaissance following renewed interest in the properties of ferroic domain walls as well as recent discoveries regarding topological insulators and skyrmionic lattices. Each of these systems possess a property ... More
Substrate coherency-driven octahedral rotations in perovskite oxide filmsMay 26 2010We perform first-principles density functional calculations to explore the role of substrate proximity effects on the octahedral rotation patterns in perovskite oxide superlattices. With cubic perovskite SrFeO$_3$ as our model film and tetragonal SrTiO$_3$ ... More
A self-interaction corrected pseudopotential scheme for magnetic and strongly-correlated systemsMar 03 2003Local-spin-density functional calculations may be affected by severe errors when applied to the study of magnetic and strongly-correlated materials. Some of these faults can be traced back to the presence of the spurious self-interaction in the density ... More
Induced magnetoelectric response in Pnma perovkitesAug 30 2011We use symmetry analysis to show that the G, C and A-type antiferromagnetic Pnma perovskites can exhibit magnetoelectric (ME) responses when a ferroelectric instability is induced with epitaxial strain. Using first-principles calculations we compute the ... More
Issues with J-dependence in the LSDA+U method for non-collinear magnetsNov 03 2010We re-examine the commonly used density functional theory plus Hubbard \textit{U} (DFT$+U$) method for the case of non-collinear magnets. While many studies neglect to explicitly include the exchange correction parameter \textit{J}, or consider its exact ... More
Superexchange-Driven Magnetoelectricity in Magnetic VorticesOct 02 2008We demonstrate that magnetic vortices in which spins are coupled to polar lattice distortions via superexchange exhibit an unusually large linear magnetoelectric response. We show that the periodic arrays of vortices formed by frustrated spins on Kagom\'e ... More
DFT+DMFT study of oxygen vacancies in a Mott InsulatorJan 07 2019May 08 2019Oxygen vacancies are a common source of excess electrons in complex oxides. In Mott insulators these additional electrons can induce a metal-insulator transition (MIT), fundamentally altering the electronic properties of the system. Here we study the ... More
A density-functional study of charge doping in WO3Feb 12 2004Feb 16 2004The addition of electron donors to the vacant A site of defect-perovskite structure tungsten trioxide causes a series of structural and chemical phase transitions; for instance, in the well-known case of the sodium tungsten bronzes (NaxWO3). Here we calculate ... More
The importance of the electronic contribution to linear magnetoelectricityDec 07 2009Sep 07 2010We demonstrate that the electronic contribution to the linear magnetoelectric response, usually omitted in first-principles studies, can be comparable in magnitude to that mediated by lattice distortions, even for materials in which responses are strong. ... More
Ab initio study of the ferroelectric strain dependence and 180$^\circ$ domain walls in the barium metal fluorides BaMgF$_4$ and BaZnF$_4$Oct 30 2015Nov 02 2015We investigate the strain dependence of the ferroelectric polarization and the structure of the ferroelectric domain walls in the layered perovskite-related barium fluorides, BaMF$_4$ (M=Mg, Zn). The unusual "geometric ferroelectricity" in these materials ... More
Microscopic theory of temperature-dependent magnetoelectric effect in Cr2O3Apr 12 2010We calculate the temperature-dependent magnetoelectric response of Cr2O3 from first principles. The form of the dominant magnetoelectric coupling is determined using symmetry arguments, its strength is found using ab initio methods, and the temperature ... More
DFT+DMFT study of oxygen vacancies in a Mott InsulatorJan 07 2019Oxygen vacancies are a common source of excess electrons in complex oxides. In Mott insulators these additional electrons can induce a metal-insulator transition (MIT), fundamentally altering the electronic properties of the system. Here we study the ... More
Polarization, piezoelectric constants and elastic constants of ZnO, MgO and CdOJul 08 2005Jul 11 2005We report first principles density functional calculations of the spontaneous polarizations, piezoelectric constants and elastic constants for the II-VI wurtzite structure ZnO, MgO and CdO. Using our self interaction corrected implementation of density ... More
Landau theory of topological defects in multiferroic hexagonal manganitesApr 18 2012Topological defects in ordered states with spontaneously broken symmetry often have unusual physical properties, such as fractional electric charge or a quantised magnetic field-flux, originating from their non-trivial topology. Coupled topological defects ... More
Biquadratic and ring exchange interactions in orthorhombic perovskite manganitesDec 11 2014We use ab initio electronic structure calculations within the generalized gradient approximation (GGA+U) to density functional theory (DFT) to determine the microscopic exchange interactions in the series of orthorhombic rare-earth manganites, o-$R$MnO$_3$. ... More
On the calculation of crystal field parameters using Wannier functionsMay 15 2014We discuss the calculation of crystal field splittings using Wannier functions and show how the ligand field contributions can be separated from the bare Coulomb contribution to the crystal field by constructing sets of Wannier functions incorporating ... More
Four-spin ring interaction as a source of unconventional magnetic orders in orthorhombic perovskite manganitesMar 25 2018Apr 12 2018We use ab initio electronic structure calculations in combination with Monte Carlo simulations to investigate the magnetic and ferroelectric properties of bulk orthorhombic HoMnO$_3$ and ErMnO$_3$. Our goals are to explain the inconsistencies in the measured ... More
Lattice relaxation, electronic screening, and spin and orbital phase diagram of LaTiO$_3$/SrTiO$_3$ superlatticesJan 04 2006The effects of lattice relaxation in LaTiO$_3$/SrTiO$_3$ superlattices are investigated using a combination of LDA$+U$ density functional theory, and Hartree-Fock effective Hamiltonian calculations. We find noticeable ($\sim 0.1$--0.2 \AA) distortions ... More
Band alignment at metal/ferroelectric interfaces: insights and artifacts from first principlesMar 02 2011Based on recent advances in first-principles theory, we develop a general model of the band offset at metal/ferroelectric interfaces. We show that, depending on the polarization of the film, a pathological regime might occur where the metallic carriers ... More
Non-$d^0$ Mn-driven ferroelectricity in antiferromagnetic BaMnO$_3$Jan 21 2009Using first-principles density functional theory we predict a ferroelectric ground state -- driven by off-centering of the magnetic Mn$^{4+}$ ion -- in perovskite-structure BaMnO$_3$. Our finding is surprising, since the competition between energy-lowering ... More
Interplay between strain and oxygen vacancies in lanthanum aluminateFeb 07 2012We evaluate the interplay between epitaxial strain and oxygen vacancy formation in the perovskite-structure oxide lanthanum aluminate, LaAlO$_3$. Using density functional theory within the GGA$+U$ approximation we calculate the dependence of the oxygen ... More
Multiferroic Quantum CriticalityNov 21 2017Oct 01 2018The zero-temperature limit of a continuous phase transition is marked by a quantum critical point, which can generate exotic physics that extends to elevated temperatures. Magnetic quantum criticality is now well known, and has been explored in systems ... More
Strain-Induced Magnetic Anisotropy in Epitaxial Thin Films of the Spinel CoCr$_2$O$_4$Aug 10 2015We show that the magnetic anisotropy in spinel-structure CoCr$_2$O$_4$ thin films exhibits a strain dependence in which compressive strain induces an out-of-plane magnetic easy axis and tensile strain an in-plane easy axis, exactly opposite to the behavior ... More
Strain-induced structural instability in FeRhMar 06 2016We perform density functional calculations to investigate the structure of the inter-metallic alloy FeRh under epitaxial strain. Bulk FeRh exhibits a metamagnetic transition from a low-temperature antiferromagnetic (AFM) phase to a ferromagnetic (FM) ... More
Coupling and competition between ferroelectricity, magnetism, strain and oxygen vacancies in AMnO3 perovskitesJun 16 2016We use first-principles calculations based on density functional theory to investigate the interplay between oxygen vacancies, A-site cation size / tolerance factor, epitaxial strain, ferroelectricity and magnetism in the perovskite manganite series, ... More
Oxygen vacancies in bulk and at neutral domain walls in hexagonal YMnO$_3$Jul 04 2018We use density functional calculations to investigate the accommodation and migration of oxygen vacancies in bulk hexagonal YMnO$_3$, and to study interactions between neutral ferroelectric domain walls and oxygen vacancies. Our calculations show that ... More
Strain controlled oxygen vacancy formation and ordering in CaMnO$_3$Mar 19 2013We use first-principles calculations to investigate the stability of bi-axially strained \textit{Pnma} perovskite CaMnO$_3$ towards the formation of oxygen vacancies. Our motivation is provided by promising indications that novel material properties can ... More
Observation of persistent centrosymmetricity in the hexagonal manganite familySep 07 2011Jul 19 2012The controversy regarding the ferroelectric behavior of hexagonal InMnO$_3$ is resolved by using a combination of x-ray diffraction (XRD), piezoresponse force microscopy (PFM), second harmonic generation (SHG), and density functional theory (DFT). While ... More
Multiferroic magnetic spirals induced by random magnetic exchangesOct 03 2016Multiferroism can originate from the breaking of inversion symmetry caused by magnetic-spiral order. The usual mechanism for stabilizing a magnetic spiral is competition between magnetic exchange interactions differing by their range and sign, such as ... More
Linear Magnetoelectric Effect by Orbital MagnetismJul 12 2012Aug 01 2012We use symmetry analysis and first principles calculations to show that the linear magnetoelectric effect can originate from the response of orbital magnetic moments to the polar distortions induced by an applied electric field. Using LiFePO4 as a model ... More
The macroscopic monopolization in diagonal magnetoelectricsJun 23 2013Jun 29 2013We develop the formalism of the macroscopic monopolization -- that is the monopole moment per unit volume -- in periodic solids, and discuss its relationship to the diagonal magnetoelectric effect. For the series of lithium transition metal phosphate ... More
Multiferroic magnetic spirals induced by random magnetic exchangesOct 03 2016Nov 14 2016Multiferroism can originate from the breaking of inversion symmetry caused by magnetic-spiral order. The usual mechanism for stabilizing a magnetic spiral is competition between magnetic exchange interactions differing by their range and sign, such as ... More
Multiplets Matter: The Electronic Structure of Rare-Earth Semiconductors and SemimetalsJul 22 2008We demonstrate that a theoretical framework fully incorporating intra-atomic correlations and multiplet structure of the localized 4f states is required in order to capture the essential physics of rare-earth semiconductors and semimetals. We focus in ... More
Electronic properties of bulk and thin film SrRuO$_3$: a search for the metal-insulator transitionJun 08 2008We calculate the properties of the 4$d$ ferromagnet SrRuO$_3$ in bulk and thin film form with the aim of understanding the experimentally observed metal to insulator transition at reduced thickness. Although the spatial extent of the 4$d$ orbitals is ... More
Strong coupling of Jahn-Teller distortion to oxygen-octahedron rotation and functional properties in epitaxially-strained orthorhombic LaMnO$_3$Jul 12 2013First-principles calculations reveal a large cooperative coupling of Jahn-Teller (JT) distortion to oxygen-octahedron rotations in perovskite LaMnO$_3$. The combination of the two distortions is responsible for stabilizing the strongly orthorhombic $A$-AFM ... More
Multiferroic magnetic spirals induced by random magnetic exchangesOct 03 2016Oct 19 2016Multiferroism can originate from the breaking of inversion symmetry caused by magnetic-spiral order. The usual mechanism for stabilizing a magnetic spiral is competition between magnetic exchange interactions differing by their range and sign, such as ... More
Chemical control of polar behavior in bicomponent short-period superlatticesApr 29 2010Using first-principles density functional calculations, we study the interplay of ferroelectricity and polar discontinuities in a range of 1-1 oxide superlattices, built out of ferroelectric and paraelectric components. Studies have been carried out for ... More
Quantum critical origin of the superconducting dome in SrTiO$_3$Jul 29 2015We investigate the origin of superconductivity in doped SrTiO$_3$ (STO) using a combination of density functional and strong coupling theories within the framework of quantum criticality. Our density functional calculations of the ferroelectric soft mode ... More
A first principles study of wurtzite-structure MnONov 24 2003We present results of a density functional theory study of MnO in the wurtzite structure. Our motivation is provided by recent experiments reporting ferromagnetism in Mn-doped wurtzite structure ZnO. We find that wurtzite MnO a) is not strongly energetically ... More
Non collinear magnetism and single ion anisotropy in multiferroic perovskitesJun 04 2012The link between the crystal distortions of the perovskite structure and the magnetic exchange interaction, the single-ion anisotropy (SIA) and the Dzyaloshinsky-Moriya (DM) interaction are investigated by means of density-functional calculations. Using ... More
Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganitesJul 30 2017The improper ferroelectricity in YMnO$_3$ and related hexagonal manganites is known to cause topologically protected ferroelectric domains that give rise to rich and diverse physical phenomena. The local structure and structural coherence across the transition, ... More
Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganitesJul 30 2017Apr 11 2018The improper ferroelectricity in YMnO$_3$ and other related multiferroic hexagonal manganites are known to cause topologically protected ferroelectric domains that give rise to rich and diverse physical phenomena. The local structure and structural coherence ... More
A dynamical magnetic field accompanying the motion of ferroelectric domain wallsApr 12 2019The recently proposed dynamical multiferroic effect describes the generation of magnetization from temporally varying electric polarization. Here, we show that the effect can lead to a magnetic field at moving ferroelectric domain walls, where the rearrangement ... More
Anisotropic conductance at improper ferroelectric domain wallsDec 21 2011Transition metal oxides hold great potential for the development of new device paradigms because of the field-tunable functionalities driven by their strong electronic correlations, combined with their earth abundance and environmental friendliness. Recently, ... More
First-principles study of ferroelectric domain walls in multiferroic bismuth ferriteSep 29 2009We present a first-principles density functional study of the structural, electronic and magnetic properties of the ferroelectric domain walls in multiferroic BiFeO3. We find that domain walls in which the rotations of the oxygen octahedra do not change ... More
Ultrafast Structure Switching through Nonlinear PhononicsJul 06 2016We describe an ultrafast coherent control of the transient structural distortion arising from nonlinear phononics in ErFeO$_3$. Using density functional theory, we calculate the structural properties as input to an anharmonic phonon model that describes ... More
Correlated local dipoles in PbTeSep 08 2017Nov 25 2017We present a combined single-crystal x-ray diffuse scattering and ab-initio molecular dynamics study of lead telluride, PbTe. Well-known for its thermoelectric and narrow-gap semiconducting properties, PbTe recently achieved further notoriety following ... More
Emphanitic anharmonicity in PbSe at high temperature and the anomalous electronic properties in the PbQ (Q=S, Se, Te) systemMay 03 2018The temperature dependence of the local structure of PbSe has been investigated using pair distribution function (PDF) analysis of x-ray and neutron powder diffraction data and density functional theory (DFT) calculations. Observation of non-Gaussian ... More
Strain and ferroelectric soft mode induced superconductivity in strontium titanateDec 22 2017We investigate the effects of strain on superconductivity with particular reference to SrTiO$_3$. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of ... More
A high-energy density antiferroelectric made by interfacial electrostatic engineeringDec 22 2018Dielectric capacitors hold a tremendous advantage for energy storage due to their fast charge/discharge times and stability in comparison to batteries and supercapacitors. A key limitation to today's dielectric capacitors, however, is the low storage ... More
Electric and magnetic polarizabilities of hexagonal Ln2CuTiO6 (Ln=Y, Dy, Ho, Er and Yb)Jun 02 2010Sep 24 2010We investigated the rare-earth transition metal oxide series, Ln2CuTiO6 (Ln=Y, Dy, Ho, Er and Yb), crystallizing in the hexagonal structure with non-centrosymmetric P63cm space group for possible occurrences of multiferroic properties. Our results show ... More
Relationship between crystal structure and multiferroic orders in orthorhombic perovskite manganitesMay 06 2018Jun 08 2018We use resonant and non-resonant X-ray diffraction measurements in combination with first-principles electronic structure calculations and Monte Carlo simulations to study the relationship between crystal structure and multiferroic orders in the orthorhombic ... More
Dynamical MultiferroicityDec 19 2016Apr 10 2017An appealing mechanism for inducing multiferroicity in materials is the generation of electric polarization by a spatially varying magnetization that is coupled to the lattice through the spin-orbit interaction. Here we describe the reciprocal effect, ... More
Carrier-mediated magnetoelectricity in complex oxide heterostructuresJun 14 2007While tremendous success has been achieved to date in creating both single phase and composite magnetoelectric materials, the quintessential electric-field control of magnetism remains elusive. In this work, we demonstrate a linear magnetoelectric effect ... More
Geometric Ferroelectricity in Fluoro-PerovskitesNov 10 2013We used first-principles calculations to investigate the existence and origin of the ferroelectric in- stability in the ABF3 fluoro-perovskites. We find that many fluoro-perovskites have a ferroelectric instability in their high symmetry cubic structure, ... More
Quasi-static magnetoelectric quadrupoles as the order parameter for the pseudo-gap phase in cuprate superconductorsOct 16 2015Dec 14 2015We introduce a mechanism in which coupling between fluctuating spin magnetic dipole moments and polar optical phonons leads to a non-zero ferroic ordering of quasi-static magnetoelectric multipoles. Using first-principles calculations within the LSDA$+U$ ... More
Stress-induced R-MA-MC-T symmetry changes in BiFeO3 filmsOct 01 2010The recent discovery of a stress-induced structural phase transition in the single-component perovskite BiFeO3 has revived interest in this lead-free ferroelectric. The coexistence of different phases may lead to large piezoelectric coefficients, a property ... More
Dynamic multiferroicity of a ferroelectric quantum critical pointAug 16 2018Feb 13 2019Quantum matter hosts a large variety of phases, some coexisting, some competing; when two or more orders occur together, they are often entangled and cannot be separated. Dynamical multiferroicity, where fluctuations of electric dipoles lead to magnetisation, ... More
Magnetophononics: ultrafast spin control through the latticeJul 11 2017Mar 05 2018Using a combination of first-principles and magnetization-dynamics calculations, we study the effect of the intense optical excitation of phonons on the magnetic behavior in insulating magnetic materials. Taking the prototypical magnetoelectric \CrO\ ... More
Ferrodistortive instability at the (001) surface of half-metallic manganitesMay 02 2007We present the structure of the fully relaxed (001) surface of the half-metallic manganite La0.7Sr0.3MnO3, calculated using density functional theory within the generalized gradient approximation (GGA). Two relevant ferroelastic order parameters are identified ... More
First-principles study of spontaneous polarization in multiferroic BiFeO$_3$Jul 26 2004The ground-state structural and electronic properties of ferroelectric BiFeO$_3$ are calculated using density functional theory within the local spin-density approximation and the LSDA+U method. The crystal structure is computed to be rhombohedral with ... More
From multiferroics to cosmology: Scaling behaviour and beyond in the hexagonal manganitesApr 17 2012We show that the improper ferroelectric phase transition in the multiferroic hexagonal manganites displays the same symmetry-breaking characteristics as those proposed in early-universe theories. We present an analysis of the Kibble-Zurek theory of topological ... More
A Bespoke Single-Band Hubbard Model MaterialAug 18 2015Aug 29 2015The Hubbard model, which augments independent-electron band theory with a single parameter to describe electron-electron correlations, is widely regarded to be the `standard model' of condensed matter physics. The model has been remarkably successful ... More
Mott Transition of MnO under Pressure: Comparison of Correlated Band TheoriesMay 17 2006Oct 04 2006The electronic structure, magnetic moment, and volume collapse of MnO under pressure are obtained from four different correlated band theory methods; local density approximation + Hubbard U (LDA+U), pseudopotential self-interaction correction (pseudo-SIC), ... More
Quantifying octahedral rotations in strained perovskite oxide filmsFeb 05 2010Epitaxial strain is a proven route to enhancing the properties of complex oxides, however, the details of how the atomic structure accommodates strain are poorly understood due to the difficulty of measuring the oxygen positions in thin films. We present ... More
Magnetoelectro-elastic control of magnetism in an artificial multiferroicSep 11 2015We study the coexistence of strain- and charge-mediated magnetoelectric coupling in a cobalt (0-7 nm) wedge on ferroelectric [Pb(Mg$_{1/3}$/Nb$_{2/3}$)O$_{3}$]$_{0.68}$-[PbTiO$_{3}$]$_{0.32}$ (011) using surface-sensitive x-ray magnetic circular dichroism ... More
Incommensurate magnetic structure, Fe/Cu chemical disorder and magnetic interactions in the high-temperature multiferroic YBaCuFeO5Jan 08 2015Motivated by the recent observations of incommensurate magnetic order and electric polarization in YBaCuFeO5 up to temperatures TN2 as high as 230K [1,2] we report here for the first time a model for the incommensurate magnetic structure of this material ... More
Nonequilibrium chiral perturbation theory and disoriented chiral condensatesOct 28 1999We analyse the extension of Chiral Perturbation Theory to describe a meson gas out of thermal equilibrium. For that purpose, we let the pion decay constant be a time-dependent function and work within the Schwinger-Keldysh contour technique. A useful ... More
An overview of mixed finite elements for the analysis of inelastic bidimensional structuresJul 09 2018As inelastic structures are ubiquitous in many engineering fields, a central task in computational mechanics is to develop accurate, robust and efficient tools for their analysis. Motivated by the poor performances exhibited by standard displacement-based ... More
Nonequilibrium Chiral Dynamics and Effective LagrangiansSep 14 2001We review our recent work on Chiral Lagrangians out of thermal equilibrium, which are introduced to analyse the pion gas formed after a Relativistic Heavy Ion Collision. Chiral Perturbation Theory is extended by letting $\fpi$ be time dependent and allows ... More
Nonequilibrium chiral perturbation theoryFeb 15 1999We explore the extension of chiral perturbation theory out of thermal equilibrium. The pion decay constant becomes then a time-dependent function and we work within the Schwinger-Keldysh contour technique. A useful connection with curved space-time QFT ... More