arXiv subjects

Claude Ederer

Publications and source records attributed to Claude Ederer.

At least 73 records · Page 4Linked to original sources

arXiv2014

Electrocaloric effect in BaTiO$_3$: a first-principles-based study on the effect of misfit strain

We address the question of how the electrocaloric effect in epitaxial thin films of the prototypical ferroelectric BaTiO$_3$ is affected by the clamping to the substrate and by substrate-induced misfit strain. We use molecular dynamics simulations and a first-principles-based effective Hamiltonian to calculate the adiabatic temperature change $ΔT$ under different epitaxial constraints. Our results demonstrate that, consistent with phenomenological theory, clamping by the substrate reduces the maximum $ΔT$ compared to bulk BaTiO$_3$. On the other hand, compressive misfit-strain leads to a strong increase of $ΔT$ and shifts the maximum of the electrocaloric effect to higher temperatures. A rather small compressive strain of $-0.75$% is sufficient to obtain a $ΔT$ that is larger than the corresponding bulk value.

cond-mat.mtrl-sci↗

arXiv2014

On the calculation of crystal field parameters using Wannier functions

We 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 different levels of hybridization. We demonstrate this method using SrVO$_3$ as a generic example of a transition metal oxide. We then calculate trends in the crystal field splitting for two series of hypothetical tetragonally distorted perovskite oxides and discuss the relation between the calculated "electro-static" contribution to the crystal field and the simple point charge model. Finally, we apply our method to the charge disproportionated 5$d$ electron system CsAuCl$_3$. We show that the negative charge transfer energy in this material leads to a reversal of the $p$-$d$ ligand contribution to the crystal field splitting such that the $e_g$ states of the nominally Au$^{3+}$ cation are energetically lower than the corresponding $t_{2g}$ states.

cond-mat.mtrl-sci↗

arXiv2014

Strain-induced insulator-to-metal transition in LaTiO3 within DFT+DMFT

We present results of combined density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations, which show that the Mott insulator LaTiO3 undergoes an insulator-to-metal transition under compressive epitaxial strain of about -2%. This transition is driven by strain-induced changes in the crystal-field splitting between the Ti t2g orbitals, which in turn are intimately related to the collective tilts and rotations of the oxygen octahedra in the orthorhombically distorted Pbnm perovskite structure. An accurate treatment of the underlying crystal structure is therefore crucial for a correct description of the observed metal-insulator transition. Our theoretical results are consistent with recent experimental observations, and demonstrate that metallic behavior in heterostructures of otherwise insulating materials can emerge also from mechanisms other than genuine interface effects.

cond-mat.str-el↗

arXiv2013

Spin-filtering efficiency of ferrimagnetic spinels CoFe2O4 and NiFe2O4

We assess the potential of the ferrimagnetic spinel ferrites CoFe2O4 and NiFe2O4 to act as spin filtering barriers in magnetic tunnel junctions. Our study is based on the electronic structure calculated by means of first-principles density functional theory within different approximations for the exchange correlation energy. We show that, in agreement with previous calculations, the densities of states suggest a lower tunneling barrier for minority spin electrons, and thus a negative spin-filter effect. However, a more detailed analysis based on the complex band-structure reveals that both signs for the spin-filtering efficiency are possible, depending on the band alignment between the electrode and the barrier materials and depending on the specific wave-function symmetry of the relevant bands within the electrode.

cond-mat.mtrl-sci↗

arXiv2012

First-principles investigation of incipient ferroelectric trends of rutile TiO$_2$ in bulk and at the (110) surface

The ferroelectric trends of rutile (TiO$_2$) in bulk and at the (110) surface are investigated by means of ab initio density functional theory. We discuss the underlying mechanism of the incipient ferroelectric behavior of rutile in terms of Born effective charges, which we decompose in individual contributions by means of maximally localized Wannier functions. We show that a ferroelectric phase can be stabilized for a variety of different lattice distortions which all enlarge the shortest Ti-O distance, even if the longer apical Ti-O bond is simultaneously shortened. At the (110) surface, the ferroelectric trends are modified compared to the bulk, but nevertheless ferroelectric phases with large polarization even in the topmost surface layer can be stabilized by uniaxial strain.

cond-mat.mtrl-sci↗

arXiv2012

Rubidium superoxide: a p-electron Mott insulator

Rubidium superoxide, RbO_2, is a rare example of a solid with partially-filled electronic p states, which allows to study the interplay of spin and orbital order and other effects of strong electronic correlations in a material that is quite different from the conventional d or f electron systems. Here we show, using a combination of density functional theory (DFT) and dynamical mean-field theory, that at room temperature RbO_2 is indeed a paramagnetic Mott insulator. We construct the metal-insulator phase diagram as a function of temperature and Hubbard interaction parameters U and J. Due to the strong particle-hole asymmetry of the RbO_2 band-structure, we find strong differences compared to a simple semi-elliptical density of states, which is often used to study the multiband Hubbard model. In agreement with our previous DFT study, we also find indications for complex spin and orbital order at low temperatures.

cond-mat.str-el↗

arXiv2012

First-principles calculation of magnetoelastic coefficients and magnetostriction in the spinel ferrites CoFe2O4 and NiFe2O4

We present calculations of magnetostriction constants for the spinel ferrites CoFe2O4 and NiFe2O4 using density functional theory within the GGA+U approach. Special emphasis is devoted to the influence of different possible cation distributions on the B site sublattice of the inverse spinel structure on the calculated elastic and magnetoelastic constants. We show that the resulting symmetry-lowering has only a negligible effect on the elastic constants of both systems as well as on the magnetoelastic response of NiFe2O4, whereas the magnetoelastic response of CoFe2O4 depends more strongly on the specific cation arrangement. In all cases our calculated magnetostriction constants are in good agreement with available experimental data. Our work thus paves the way for more detailed first-principles studies regarding the effect of stoichiometry and cation inversion on the magnetostrictive properties of spinel ferrites.

cond-mat.mtrl-sci↗

arXiv2011

First-principles study of the influence of (110) strain on the ferroelectric trends of TiO$_2$

We investigate the impact of uniaxial strain on atomic shifts, dipolar interactions, polarization and electric permittivity in TiO$_2$ (rutile) by using two different implementations of density functional theory. It is shown that calculations using the Vienna ab inito simulation package (VASP) and the plane-wave self-consistent field method (PWscf) yield qualitatively the same atomic relaxations and ferroelectric trends under strain. The phonon dispersion curves of unstrained and strained TiO$_2$ (rutile) obtained by employing the linear response method confirm previous calculations of the giant LO-TO splitting and the appearance of soft polar modes. A second order phase transition into a ferroelectric phase with polarization along (110) appears under expansive strain in (110) direction.

cond-mat.mtrl-sci↗

arXiv2011

Effect of epitaxial strain on the cation distribution in spinel ferrites CoFe2O4 and NiFe2O4: a density functional theory study

The effect of epitaxial strain on the cation distribution in spinel ferrites CoFe2O4 and NiFe2O4 is investigated by GGA+U total energy calculations. We obtain a very strong (moderate) tendency for cation inversion in NiFe2O4 (CoFe2O4), in agreement with experimental bulk studies. This preference for the inverse spinel structure is reduced by tensile epitaxial strain, which can lead to strong sensitivity of the cation distribution on specific growth conditions in thin films. Furthermore, we obtain significant energy differences between different cation arrangements with the same degree of inversion, providing further evidence for recently proposed short range B site order in NiFe2O4.

cond-mat.mtrl-sci↗

arXiv2011

Effect of Hubbard U on the construction of low energy Hamiltonians for LaMnO_3 via maximally localized Wannier functions

We use maximally localized Wannier functions to construct tight-binding (TB) parameterizations for the e_g bands of LaMnO_3 based on first principles electronic structure calculations. We compare two different ways to represent the relevant bands around the Fermi level: i) a d-p model that includes atomic-like orbitals corresponding to both Mn(d) and O(p) states in the TB basis, and ii) an effective e_g model that includes only two e_g-like Wannier functions per Mn site. We first establish the effect of the Jahn-Teller distortion within the d-p model, and then compare the TB representations for both models obtained from GGA+U calculations with different values of the Hubbard parameter U. We find that in the case of the d-p model the TB parameters are rather independent on the specific value of U, if compared with the mean-field approximation of an appropriate multi-band Hubbard Hamiltonian. In contrast, the U dependence of the TB parameters for the effective e_g model cannot easily be related to a corresponding mean-field Hubbard model, and therefore these parameters depend critically on the specific value of U, and more generally on the specific exchange-correlation functional, used in the electronic structure calculation.

cond-mat.str-el↗

arXiv2010

Mechanism of ferroelectric instabilities in non d^0 perovskites: LaCrO_3 versus CaMnO_3

The incompatibility of partial d occupation on the perovskite B-site with the standard charge transfer mechanism for ferroelectricity has been a central paradigm in multiferroics research. Nevertheless, it was recently shown by density functional theory calculations that CaMnO_3 exhibits a polar instability that even dominates over the octahedral tilting for slightly enlarged unit cell volume. Here, we present similar calculations for LaCrO_3, which has the same d^3 B-site electron configuration as CaMnO_3. We find that LaCrO_3 exhibits a very similar, albeit much weaker, polar instability as CaMnO_3. In addition, while the Born effective charge (BEC) of the Mn^{4+} cation in CaMnO_3 is highly anomalous, the BEC of Cr^{3+} in LaCrO_3 is only slightly enhanced. By decomposing the BECs into contributions of individual Wannier functions we show that the ferroelectric instabilities in both systems can be understood in terms of charge transfer between TM d and O p states, analogously to the standard d^0 perovskite ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2010

Epitaxial strain effects in the spinel ferrites CoFe2O4 and NiFe2O4 from first principles

The inverse spinels CoFe2O4 and NiFe2O4, which have been of particular interest over the past few years as building blocks of artificial multiferroic heterostructures and as possible spin-filter materials, are investigated by means of density functional theory calculations. We address the effect of epitaxial strain on the magneto-crystalline anisotropy and show that, in agreement with experimental observations, tensile strain favors perpendicular anisotropy, whereas compressive strain favors in-plane orientation of the magnetization. Our calculated magnetostriction constants $λ_{100}$ of about -220 ppm for CoFe2O4 and -45 ppm for NiFe2O4 agree well with available experimental data. We analyze the effect of different cation arrangements used to represent the inverse spinel structure and show that both LSDA+U and GGA+U allow for a good quantitative description of these materials. Our results open the way for further computational investigations of spinel ferrites.

cond-mat.mtrl-sci↗

arXiv2009

Calculation of model Hamiltonian parameters for LaMnO_3 using maximally localized Wannier functions

Maximally localized Wannier functions (MLWFs) based on Kohn-Sham band-structures provide a systematic way to construct realistic, materials specific tight-binding models for further theoretical analysis. Here, we construct MLWFs for the Mn e_g bands in LaMnO_3, and we monitor changes in the MLWF matrix elements induced by different magnetic configurations and structural distortions. From this we obtain values for the local Jahn-Teller and Hund's rule coupling strength, the hopping amplitudes between all nearest and further neighbors, and the corresponding reduction due to the GdFeO_3-type distortion. By comparing our results with commonly used model Hamiltonians for manganites, where electrons can hop between two "e_g-like" orbitals located on each Mn site, we find that the most crucial limitation of such models stems from neglecting changes in the underlying Mn(d)-O(p) hybridization.

cond-mat.str-el↗

arXiv2009

Correlation effects in p-electron magnets: the case of RbO_2

We present results of GGA+U calculations for the "d^0 magnet" RbO_2, where magnetic properties are due to partially filled oxygen p orbitals. We show that on-site interactions on the oxygen sites lead to a strong tendency towards the formation of an orbitally polarized insulating state, in contrast to the half-metallic behavior predicted for this class of compounds within pure LDA/GGA. The obtained energy differences between different orbitally ordered configurations are sizeable, indicating an orbital ordering temperature higher than the antiferromagnetic Neel temperature of ~15 K. Our results demonstrate the importance of correlation effects in p electron magnets such as RbO_2.

cond-mat.str-el↗

arXiv2009

First Principles Studies of Multiferroic Materials

Multiferroics, materials where spontaneous long-range magnetic and dipolar orders coexist, represent an attractive class of compounds, which combine rich and fascinating fundamental physics with a technologically appealing potential for applications in the general area of spintronics. Ab-initio calculations have significantly contributed to recent progress in this area, by elucidating different mechanisms for multiferroicity and providing essential information on various compounds where these effects are manifestly at play. In particular, here we present examples of density-functional theory investigations for two main classes of materials: a) proper multiferroics (where ferroelectricity is driven by hybridization or purely structural effects), with BiFeO_3 as prototype material, and b) improper multiferroics (where ferroelectricity is driven by correlation effects and is strongly linked to electronic degrees of freedom such as spin, charge, or orbital ordering), with rare-earth manganites as prototypes.

cond-mat.mtrl-sci↗

arXiv2009

Toroidal moments as indicator for magneto-electric coupling: the case of BiFeO_3 versus FeTiO_3

In this paper we present an analysis of the magnetic toroidal moment and its relation to the various structural modes in R3c-distorted perovskites with magnetic cations on either the perovskite A or B site. We evaluate the toroidal moment in the limit of localized magnetic moments and show that the full magnetic symmetry can be taken into account by considering small induced magnetic moments on the oxygen sites. Our results give a transparent picture of the possible coupling between magnetization, electric polarization, and toroidal moment, thereby highlighting the different roles played by the various structural distortions in multiferroic BiFeO_3 and in the recently discussed isostructural material FeTiO_3, which has been predicted to exhibit electric field-induced magnetization switching.

cond-mat.mtrl-sci↗

arXiv2008

Electric-field switchable magnetization via the Dzyaloshinskii-Moriya interaction: FeTiO_3 versus BiFeO_3

In this article we review and discuss a mechanism for coupling between electric polarization and magnetization that can ultimately lead to electric-field switchable magnetization. The basic idea is that a ferroelectric distortion in an antiferromagnetic material can "switch on" the Dzyaloshinskii-Moriya interaction which leads to a canting of the antiferromagnetic sublattice magnetizations, and thus to a net magnetization. This magnetization M is coupled to the polarization P via a trilinear free energy contribution of the form P(M x L), where L is the antiferromagnetic order parameter. In particular, we discuss why such an invariant is present in R3c FeTiO_3 but not in the isostructural multiferroic BiFeO_3. Finally, we construct symmetry groups that in general allow for this kind of ferroelectrically-induced weak ferromagnetism.

cond-mat.mtrl-sci↗

arXiv2008

Structural distortions and model Hamiltonian parameters: from LSDA to a tight-binding description of LaMnO_3

The physics of manganites is often described within an effective two-band tight-binding (TB) model for the Mn e_g electrons, which apart from the kinetic energy includes also a local "Hund's rule" coupling to the t_{2g} core spin and a local coupling to the Jahn-Teller (JT) distortion of the oxygen octahedra. We test the validity of this model by comparing the energy dispersion calculated for the TB model with the full Kohn-Sham band-structure calculated within the local spin-density approximation (LSDA) to density functional theory. We analyze the effect of magnetic order, JT distortions, and "GdFeO_3-type" tilt-rotations of the oxygen octahedra. We show that the hopping amplitudes are independent of magnetic order and JT distortions, and that both effects can be described with a consistent set of model parameters if hopping between both nearest and next-nearest neighbors is taken into account. We determine a full set of model parameters from the density functional theory calculations, and we show that both JT distortions and Hund's rule coupling are required to obtain an insulating ground state within LSDA. Furthermore, our calculations show that the "GdFeO_3-type" rotations of the oxygen octahedra lead to a substantial reduction of the hopping amplitudes but to no significant deviation from the simple TB model.

cond-mat.str-el↗