arXiv subjects

Claude Ederer

Publications and source records attributed to Claude Ederer.

At least 55 records · Page 3Linked to original sources

arXiv2019

Energetics of the coupled electronic-structural transition in the rare-earth nickelates

Rare-earth nickelates exhibit a metal-insulator transition accompanied by a structural distortion that breaks the symmetry between formerly equivalent Ni sites. The quantitative theoretical description of this coupled electronic-structural instability is extremely challenging. Here, we address this issue by simultaneously taking into account both structural and electronic degrees of freedom using a charge self-consistent combination of density functional theory and dynamical mean-field theory, together with screened interaction parameters obtained from the constrained random phase approximation. Our total energy calculations show that the coupling to an electronic instability towards a charge disproportionated insulating state is crucial to stabilize the structural distortion, leading to a clear first order character of the coupled transition. The decreasing octahedral rotations across the series suppress this electronic instability and simultaneously increase the screening of the effective Coulomb interaction, thus weakening the correlation effects responsible for the metal-insulator transition. Our approach allows to obtain accurate values for the structural distortion and thus facilitates a comprehensive understanding, both qualitatively and quantitatively, of the complex interplay between structural properties and electronic correlation effects across the nickelate series.

cond-mat.str-el↗

arXiv2019

Hund excitations and the efficiency of Mott solar cells

We study the dynamics of photo-induced charge carriers in realistic models of LaVO3 and YTiO3 polar heterostructures. It is shown that two types of impact ionization processes contribute to the carrier multiplication in these strongly correlated multi-orbital systems: The first mechanism involves local spin state transitions, while the second mechanism involves the scattering of high kinetic energy carriers. Both processes act on the 10 fs timescale and play an important role in the harvesting of high energy photons in solar cell applications. As a consequence, the optimal gap size for Mott solar cells is substantially smaller than for semiconductor devices.

cond-mat.str-el↗

arXiv2018

First-Principles-Based Strain and Temperature Dependent Ferroic Phase Diagram of SrMnO$_3$

Perovskite structure SrMnO$_3$ is a rare example of a multiferroic material where strain-tuning and/or cation substitution could lead to coinciding magnetic and ferroelectric ordering temperatures, which would then promise strong magnetoelectric coupling effects. Here, we establish the temperature and strain dependent ferroic phase diagram of SrMnO$_3$ using first-principles-based effective Hamiltonians. All parameters of these Hamiltonians are calculated using density functional theory, i.e., no fitting to experimental data is required. Temperature dependent properties are then obtained from Monte Carlo and molecular dynamics simulations. We observe a sequence of several magnetic transitions under increasing tensile strain, with a moderate variation of the corresponding critical temperatures. In contrast, the ferroelectric Curie temperature increases strongly after its onset around 2.5\,\% strain, and indeed crosses the magnetic transition temperature just above 3\,\% strain. Our results indicate pronounced magnetoelectric coupling, manifested in dramatic changes of the magnetic ordering temperatures and different magnetic ground states as function of the ferroelectric distortion. In addition, coexisting ferroelectric and ferromagnetic order is obtained for strains above 4\,\%. Our calculated phase diagram suggests the possibility to control the magnetic properties of SrMnO$_3$ through an applied electric field, significantly altering the magnetic transition temperatures, or even inducing transitions between different magnetic states.

cond-mat.mtrl-sci↗

arXiv2018

Metal-insulator transition in CaVO$_3$ thin films: interplay between epitaxial strain, dimensional confinement, and surface effects

We use density functional theory plus dynamical mean-field theory (DFT+DMFT) to study multiple control parameters for tuning the metal-insulator transition (MIT) in CaVO$_3$ thin films. We focus on separating the effects resulting from substrate-induced epitaxial strain from those related to the reduced thickness of the film. We show that tensile epitaxial strain of around 3-4% is sufficient to induce a transition to a paramagnetic Mott-insulating phase. This corresponds to the level of strain that could be achieved on a SrTiO$_3$ substrate. Using free-standing slab models, we then demonstrate that reduced film thickness can also cause a MIT in CaVO$_3$, however, only for thicknesses of less than 4 perovskite units. Our calculations indicate that the MIT in such ultra-thin films results mainly from a surface-induced crystal-field splitting between the $t_{2g}$-orbitals, favoring the formation of an orbitally-polarized Mott insulator. This surface-induced crystal-field splitting is of the same type as the one resulting from tensile epitaxial strain, and thus the two effects can also cooperate. Furthermore, our calculations confirm an enhancement of correlation effects at the film surface, resulting in a reduced quasiparticle spectral weight in the outermost layer, whereas bulk-like properties are recovered within only a few layers away from the surface.

cond-mat.str-el↗

arXiv2017

Interplay between breathing mode distortion and magnetic order in rare-earth nickelates $R$NiO$_3$ within DFT+$U$

We present a systematic density functional theory (DFT) plus Hubbard $U$ study of structural trends and the stability of different magnetically ordered states across the rare-earth nickelate series, $R$NiO$_3$, with $R$ from Lu to La. In particular, we investigate how the magnetic order, the change of the rare-earth ion, and the Hubbard interaction $U$ are affecting the bond-length disproportionation between the nickel sites. Our results show that structural parameters can be obtained that are in very good agreement with present experimental data, and that DFT+$U$ is in principle able to capture the most important structural trends across the nickelate series. However, the amplitude of the bond-length disproportionation depends very strongly on the specific value used for the Hubbard $U$ parameter and also on the type of magnetic order imposed in the calculation. Regarding the relative stability of different magnetic orderings, a realistic antiferromagnetic order, consistent with the experimental observations, is favored for small $U$ values, and becomes more and more favorable compared to the ferromagnetic state towards the end of the series (i.e., towards $R$=Pr). Nevertheless, it seems that the stability of the ferromagnetic state is generally overestimated within the DFT+$U$ calculations. Our work provides a profound starting point for more detailed experimental investigations, and also for future studies using more advanced computational techniques such as, e.g., DFT combined with dynamical mean-field theory.

cond-mat.mtrl-sci↗

arXiv2017

The impact of hysteresis on the electrocaloric effect at first-order phase transitions

We study the impact of thermal hysteresis at the first-order structural/ferroelectric phase transitions on the electrocaloric response in bulk BaTiO$_3$ by performing molecular dynamics simulations for a first-principles-based effective Hamiltonian. We demonstrate that the electrocaloric response can conceptually be separated in two contributions: a transitional part, stemming from the discontinuous jump in entropy at the first order phase transition, and a configurational part, due to the continuous change of polarization and entropy within each phase. This latter part increases with the strength of the applied field, but for small fields it is very small. In contrast, we find a large temperature change of $\sim 1$ K resulting from the transition entropy, which is essentially independent of the field strength. However, due to the coexistence region close to the first order phase transition, this large electrocaloric response depends on the thermal history of the sample and is generally not reversible. We show that this irreversibility can be overcome by using larger fields.

cond-mat.mtrl-sci↗

arXiv2017

The electrocaloric effect in BaTiO$_3$ at all three ferroelectric transitions: anisotropy and inverse caloric effects

We study the electrocaloric (EC) effect in bulk BaTiO$_3$ (BTO) using molecular dynamics simulations of a first principles-based effective Hamiltonian, combined with direct measurements of the adiabatic EC temperature change in BTO single crystals. We examine in particular the dependence of the EC effect on the direction of the applied electric field at all three ferroelectric transitions, and we show that the EC response is strongly anisotropic. Most strikingly, an inverse caloric effect, i.e., a temperature increase under field removal, can be observed at both ferroelectric-ferroelectric transitions for certain orientations of the applied field. Using the generalized Clausius-Clapeyron equation, we show that the inverse effect occurs exactly for those cases where the field orientation favors the higher temperature/higher entropy phase. Our simulations show that temperature changes of around 1 K can in principle be obtained at the tetragonal-orthorhombic transition close to room temperature, even for small applied fields, provided that the applied field is strong enough to drive the system across the first order transition line. Our direct EC measurements for BTO single crystals at the cubic-tetragonal and at the tetragonal-orthorhombic transitions are in good qualitative agreement with our theoretical predictions, and in particular confirm the occurrence of an inverse EC effect at the tetragonal-orthorhombic transition for electric fields applied along the [001] pseudo-cubic direction.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic order in 4-layered Aurivillius Phases

We determine the viability of 4-layered Aurivillius phases to exhibit long-range magnetic order above room temperature. We use Monte Carlo simulations to calculate transition temperatures for an effective Heisenberg model containing a minimal set of required couplings. The magnitude of the corresponding coupling constants has been determined previously from electronic structure calculations for $\mathrm{Bi_5FeTi_3O_{15}}$, for which we obtain a transition temperature far below room temperature. We analyze the role of further neighbor interactions within our Heisenberg model, in particular that of the second-nearest-neighbor coupling within the perovskite-like layers of the Aurivillius structure, as well as that of the weak inter-layer coupling, in order to identify the main bottleneck for achieving higher magnetic transition temperatures. Based on our findings, we show that the most promising strategy to obtain magnetic order at higher temperatures is to increase the concentration of magnetic cations within the perovskite-like layers, and we propose candidate compounds where magnetic order could be achieved above room temperature.

cond-mat.mtrl-sci↗

arXiv2016

Tuning the caloric response of BaTiO$_3$ by tensile epitaxial strain

We investigate the effect of epitaxial strain on the electrocaloric effect (ECE) in BaTiO$_3$ by means of ab initio based molecular dynamics simulations. We show that tensile strain can be used to optimize the operation range for ferroic cooling. Strain in the range of $\leq 1$ % can be used to shift the operation temperature by several hundreds of Kelvin both to higher and lower temperatures, depending on the direction of the external field. In addition, the transformation between multi-domain and mono-domain states, induced by an in-plane electric field, results in an additional peak of the adiabatic temperature change at lower temperatures, and a broad temperature interval where the caloric response scales linearly with the applied field strength, even up to very high fields.

cond-mat.mtrl-sci↗

arXiv2016

Tuning the metal-insulator transition in d^1 and d^2 perovskites by epitaxial strain: a first principles-based study

We investigate the effect of epitaxial strain on the Mott metal-insulator transition (MIT) in perovskite systems with d^1 and d^2 electron configuration of the transition metal (TM) cation. We first discuss the general trends expected from the changes in the crystal-field splitting and in the hopping parameters that are induced by epitaxial strain. We argue that the strain-induced crystal-field splitting generally favors the Mott-insulating state, whereas the strain-induced changes in the hopping parameters favor the metallic state under compressive strain and the insulating state under tensile strain. Thus, the two effects can effectively cancel each other under compressive strain, while they usually cooperate under tensile strain, in this case favoring the insulating state. We then validate these general considerations by performing electronic structure calculations for several d^1 and d^2 perovskites, using a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We isolate the individual effects of strain-induced changes in either hopping or crystal-field by performing DMFT calculations where we fix one type of parameter to the corresponding unstrained DFT values. These calculations confirm our general considerations for SrVO_3 (d^1) and LaVO_3 (d^2), whereas the case of LaTiO_3 (d^1) is distinctly different, due to the strong effect of the octahedral tilt distortion in the underlying perovskite crystal structure. Our results demonstrate the possibility to tune the electronic properties of correlated TM oxides by using epitaxial strain, which allows to control the strength of electronic correlations and the vicinity to the Mott MIT.

cond-mat.str-el↗

arXiv2016

Controlling the cation distribution and electric polarization with epitaxial strain in Aurivillius-phase Bi$_5$FeTi$_3$O$_{15}$

This work explores the impact of in-plane bi-axial (epitaxial) strain on the cation distribution and electric polarization of the Aurivillius-phase compound Bi$_5$FeTi$_3$O$_{15}$ using first-principles electronic structure calculations. Our calculations indicate that the site preference of the Fe$^{3+}$ cation can be controlled via epitaxial strain. Tensile strain enhances the preference for the inner sites within the perovskite-like layers of the Aurivillius-phase structure, whereas compressive strain favors occupation of the outer sites within the perovskite-layers, i.e., the sites close to the Bi$_2$O$_2$ layer. Controlling the distribution of the magnetic cations offers the possibility to control magnetic order in this magnetically dilute system. Furthermore, the magnitude of the electric polarization is strongly strain-dependent, increasing under tensile strain and decreasing under compressive strain. Analysis of the Born effective charges reveals strongly anomalous charges, both of the Bi$^{3+}$ cations and the Ti$^{4+}$ cations.

cond-mat.mtrl-sci↗

arXiv2016

The potentially multiferroic Aurivillius phase Bi$_5$FeTi$_3$O$_{15}$: cation site preference, electric polarization, and magnetic coupling from first principles

We study the structural, ferroelectric, and magnetic properties of the potentially multiferroic Aurivillius phase material Bi$_5$FeTi$_3$O$_{15}$ using first principles electronic structure calculations. Calculations are performed both with PBE and PBEsol exchange correlation functionals. We conclude that PBE systematically overestimates the lattice constants and the magnitude of the ferroelectric distortion, whereas PBEsol leads to good agreement with available experimental data. We then assess a potential site preference of the Fe$^{3+}$ cation by comparing 10 different distributions of the perovskite $B$-sites. We find a slight preference for the "inner" site, consistent with recent experimental observations. We obtain a large value of $\sim$55 $μ$C/cm$^2$ for the spontaneous electric polarization, which is rather independent of the specific Fe distribution. Finally, we calculate the strength of the magnetic coupling constants and find strong antiferromagnetic coupling between Fe$^{3+}$ cations in nearest neighbor positions, whereas the coupling between further neighbors is rather weak. This poses the question whether magnetic long range order can occur in this system in spite of the low concentration of magnetic ions.

cond-mat.mtrl-sci↗

arXiv2015

Combined first-principles and model Hamiltonian study of the perovskite series RMnO3 (R = La, Pr, Nd, Sm, Eu and Gd)

We merge advanced ab initio schemes (standard density functional theory, hybrid functionals and the GW approximation) with model Hamiltonian approaches (tight-binding and Heisenberg Hamiltonian) to study the evolution of the electronic, magnetic and dielectric properties of the manganite family RMnO3 (R = La, Pr, Nd, Sm, Eu and Gd). The link between first principles and tight-binding is established by downfolding the physically relevant subset of 3d bands with e_g character by means of maximally localized Wannier functions (MLWFs) using the VASP2WANNIER90 interface. The MLWFs are then used to construct a tight-binding Hamiltonian. The dispersion of the TB e_g bands at all levels are found to match closely the MLWFs. We provide a complete set of TB parameters which can serve as guidance for the interpretation of future studies based on many-body Hamiltonian approaches. In particular, we find that the Hund's rule coupling strength, the Jahn-Teller coupling strength, and the Hubbard interaction parameter U remain nearly constant for all the members of the RMnO3 series, whereas the nearest neighbor hopping amplitudes show a monotonic attenuation as expected from the trend of the tolerance factor. Magnetic exchange interactions, computed by mapping a large set of hybrid functional total energies onto an Heisenberg Hamiltonian, clarify the origin of the A-type magnetic ordering observed in the early rare-earth manganite series as arising from a net negative out-of-plane interaction energy. The obtained exchange parameters are used to estimate the Neel temperature by means of Monte Carlo simulations. The resulting data capture well the monotonic decrease of the ordering temperature down the R series, in agreement with experiments.

cond-mat.mtrl-sci↗

arXiv2015

Structural and electronic properties of epitaxially-strained LaVO_3 from density functional theory and dynamical mean-field theory

The effect of epitaxial strain on the structural and electronic properties of LaVO_3 is investigated through density functional theory (DFT) and dynamical mean field theory (DMFT). Two different growth orientations of the crystal are considered, one preserving the bulk Pbnm space-group symmetry and another giving rise to a symmetry lowering to P2_1/m. In the nonmagnetic DFT structures, the two growth orientations are equally favored for all tensile strains considered here, as well as for compressive strains weaker than -3%. For stronger compressive strains, the P2_1/m orientation is favored and shows a complete suppression of octahedral tilts along the out-of-plane direction. Magnetically-ordered structures do not show a complete tilt suppression, but the trend points to a similar reduction of the out-of-plane V-O-V bond angles under compressive strain. Our DMFT calculations show that, in accord with room-temperature experiments, the bulk paramagnetic Mott-insulating state of LaVO_3 is robust against epitaxial strains attainable in thin films, since the suppression of orbital fluctuations counteracts the effect of bandwidth increase with compressive strain. Under stronger compressive strains, the straightening of the V-O-V bonds in the P2_1/m geometry interferes with the suppression of orbital fluctuations and hence perturbs the Mott phase more strongly, albeit not enough to achieve a metallic phase.

cond-mat.mtrl-sci↗

arXiv2015

Strain-Induced Magnetic Anisotropy in Epitaxial Thin Films of the Spinel CoCr$_2$O$_4$

We 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 reported for the related compound CoFe$_2$O$_4$. We use density functional theory calculations within the LSDA+U approximation to reproduce and explain the observed behavior. Using second-order perturbation theory, we analyse the anisotropy tensor of the Co$^{2+}$ ions in both octahedral and tetrahedral coordination, allowing us to extend our results to spinels with general arrangements of Co$^{2+}$ ions.

cond-mat.mtrl-sci↗

arXiv2015

Ab initio phase diagram of BaTiO$_3$ under epitaxial strain revisited

We revisit the phase diagram of BaTiO$_3$ under biaxial strain using a first principles-based effective Hamiltonian approach. We show that, in addition to the tetragonal ($c$), quasi-rhombohedral ($r$), and quasi-orthorhombic ($aa$) ferroelectric phases, that have been discussed previously, there are temperature and strain regions, in particular under tensile strain, where the system decomposes into multi-domain structures. In such cases, the strained system, at least on a local level, recovers the same phase sequence as the unclamped bulk material. Furthermore, we extend these results from the case of "uniform" biaxial strain to the situation where the two in-plane lattice constants are strained differently and show that similar considerations apply in this case.

cond-mat.mtrl-sci↗

arXiv2015

First principles-based calculation of the electrocaloric effect in BaTiO$_3$: comparison between direct and indirect methods

We use molecular dynamics simulations for a first principles-based effective Hamiltonian to calculate two important quantities characterizing the electrocaloric effect in BaTiO$_3$, the adiabatic temperature change $ΔT$ and the isothermal entropy change $ΔS$, for different electric field strengths. We compare direct and indirect methods to obtain $ΔT$ and $ΔS$, and we confirm that both methods indeed lead to identical result provided that the system does not actually undergo a first order phase transition. We also show that a large electrocaloric response is obtained for electric fields beyond the critical field strength for the first order phase transition. Furthermore, our work fills several gaps regarding the application of the first principles-based effective Hamiltonian approach, which represents a very attractive and powerful method for the quantitative prediction of electrocaloric properties. In particular, we discuss the importance of maintaining thermal equilibrium during the field ramping when calculating $ΔT$ using the direct method within a molecular dynamics approach.

cond-mat.mtrl-sci↗

arXiv2014

Biquadratic and ring exchange interactions in orthorhombic perovskite manganites

We 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$_3$. Our motivation is to construct a model Hamiltonian (excluding effects due to spin-orbit coupling), which can provide an accurate description of the magnetism in these materials. First, we consider TbMnO$_3$, which exhibits a spiral magnetic order at low temperatures. We map the exchange couplings in this compound onto a Heisenberg Hamiltonian and observe a clear deviation from the Heisenberg-like behavior. We consider first the coupling between magnetic and orbital degrees of freedom as a potential source of non-Heisenberg behavior in TbMnO$_3$, but conclude that it does not explain the observed deviation. We find that higher order magnetic interactions (biquadratic and four-spin ring couplings) should be taken into account for a proper treatment of the magnetism in TbMnO$_3$ as well as in the other representatives of the o-$R$MnO$_3$ series with small radii of the $R$ cation.

cond-mat.mtrl-sci↗