arXiv subjects

Nicola A. Spaldin

Publications and source records attributed to Nicola A. Spaldin.

At least 145 records · Page 8Linked to original sources

arXiv2009

Electron-lattice instabilities suppress cuprate-like electronic structures in SrFeO$_3$/SrTiO$_3$ superlattices

Using {\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 affects the electronic properties of SrFeO$_3$: For single SrFeO$_3$ layers constrained to their bulk cubic structure, the Fermi surface is two-dimensional, nested and resembles the hole-doped superconducting cuprates. A Jahn-Teller instability couples to an octahedral tilt mode, however, to remove this degeneracy resulting in insulating superlattices.

cond-mat.mtrl-sci↗

arXiv2009

First-principles modeling of ferroelectric capacitors via constrained-D calculations

First-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 and accurately rationalized by using a finite-field density-functional theory formalism in which the fundamental electrical variable is the displacement field D. By performing calculations on model Pt/BaTiO3/Pt and Au/BaZrO3/Au capacitors we demonstrate how the interface-specific and bulk-specific properties can be identified and rigorously separated. Then, we show how the electrical properties of capacitors of arbitrary thickness and geometry (symmetric or asymmetric) can be readily reconstructed by using such information. Finally, we show how useful observables such as polarization and dielectric, piezoelectric and electrostrictive coefficients are easily evaluated as a byproduct of the above procedure. We apply this methodology to elucidate the relationship between chemical bonding, Schottky barriers and ferroelectric polarization at simple-metal/oxide interfaces. We find that BO2-electrode interfaces behave analogously to a layer of linear dielectric put in series with a bulk-like perovskite film, while a significant non-linear effect occurs at AO-electrode interfaces.

cond-mat.mtrl-sci↗

arXiv2009

Strain-induced ferroelectricity in simple rocksalt binary oxides

The 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 ferroelectric perovskite oxides, however, their practical applications remain limited and the emergence of ferroelectricity and related functional properties in simple binary oxides seems so unlikely that it was never previously considered. Here, we show using first-principles density functional calculations that ferroelectricity can be easily induced in simple alkaline earth binary oxides such as barium oxide (BaO) using appropriate epitaxial strains. Going beyond the fundamental discovery, we show that the functional properties (polarization, dielectric constant and piezoelectric response) of such strained binary oxides are comparable in magnitude to those of typical ferroelectric perovskite oxides, so making them of direct interest for applications. Finally, we show that magnetic binary oxides such as EuO, with the same rocksalt structure, behave similarly to the alkaline earth oxides, suggesting a route to new multiferroics combining ferroelectric and magnetic properties.

cond-mat.mtrl-sci↗

arXiv2009

Non-$d^0$ Mn-driven ferroelectricity in antiferromagnetic BaMnO$_3$

Using 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 covalent bond formation, and energy-raising Coulombic repulsions usually only favors off-centering on the perovskite $B$-site for non-magnetic $d^0$ ions. We explain this tendency for ferroelectric off-centering by analyzing the changes in electronic structure between the centrosymmetric and polar states, and by calculating the Born effective charges; we find anomalously large values for Mn and O consistent with our calculated polarization of 12.8 $μ$C/cm$^2$. Finally, we suggest possible routes by which the perovskite phase may be stabilized over the usual hexagonal phase, to enable a practical realization of a single-phase multiferroic.

cond-mat.mtrl-sci↗

arXiv2008

Structural effects on the spin-state transition in epitaxially strained LaCoO$_3$ films

Using 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 to stabilize a non-zero spin state, additional heteroepitaxial symmetry constraints -- in particular the suppression of octahedral rotations -- stabilize a ferromagnetic intermediate-spin state. By comparing with experimental data for the bulk material, we calculate an upper bound on the Hubbard U value, and describe the role that the on-site Coulomb interaction plays in determining the spin-state configuration.

cond-mat.mtrl-sci↗

arXiv2008

Enhancement of ferroelectricity at metal/oxide interfaces

By 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 show that, contrary to the established interpretation, the interfacial capacitance depends crucially on the local chemical environment of the interface through the force constants of the metal-oxide bonds, and is not necessarily related to the bulk screening properties of the electrode material. In particular, in the case of interfaces between AO-terminated perovskites and simple metals, we demonstrate a novel mechanism of "interfacial ferroelectricity" that produces an overall enhancement of the ferroelectric instability of the film, rather than a suppression as is usually assumed. The resulting "negative dead layer" suggests a route to thin-film ferroelectric devices that are free of deleterious size effects.

cond-mat.mtrl-sci↗

arXiv2008

Electric displacement as the fundamental variable in electronic-structure calculations

Finite-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 to fix the electric field E or the macroscopic polarization P in first-principles calculations, both methods lack the ease of use and conceptual clarity of standard zero-field calculations. Here we develop a new formalism in which the electric displacement D, rather than E or P, is the fundamental electrical variable. Fixing D has the intuitive interpretation of imposing open-circuit electrical boundary conditions, which is particularly useful in studying ferroelectric systems. Furthermore, the analogy to open-circuit capacitors suggests an appealing reformulation in terms of free charges and potentials, which dramatically simplifies the treatment of stresses and strains. Using PbTiO3 as an example, we show that our technique allows full control over the electrical variables within the density functional formalism.

cond-mat.mtrl-sci↗

arXiv2008

Superexchange-Driven Magnetoelectricity in Magnetic Vortices

We 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 Kagomé lattices provide a realization of this concept; our {\it ab initio} calculations for such a model structure yield a magnetoelectric coefficient that is 30 times larger than that of prototypical single phase magnetoelectrics. Finally, we identify the design rules required to obtain such a response in a practical material.

cond-mat.mtrl-sci↗

arXiv2008

Multiplets Matter: The Electronic Structure of Rare-Earth Semiconductors and Semimetals

We 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 particular on the rare-earth semimetal erbium arsenide (ErAs), for which effective one-electron approaches fail to provide a consistent picture of both high and low-energy electronic states. We treat the many-body states of the Er 4f shell within an atomic approximation in the framework of dynamical mean-field theory. Our results for the magnetic-field dependence of the 4f local moment, the influence of multiplets on the photoemission spectrum, and the exchange splitting of the Fermi surface pockets as measured from Shubnikov-de Haas oscillations, are found to be in good agreement with experimental results.

cond-mat.str-el↗

arXiv2008

Electronic properties of bulk and thin film SrRuO$_3$: a search for the metal-insulator transition

We 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 quite large, many experimental results have suggested that electron-electron correlations play an important role in determining this material's electronic structure. In order to investigate the importance of correlation, we use two approaches which go beyond the conventional local density approximation to density functional theory (DFT): the local spin density approximation + Hubbard $U$ (LSDA+$U$) and the pseudopotential self-interaction correction (pseudo-SIC) methods. We find that the details of the electronic structure predicted with the LSDA do not agree with the experimental spectroscopic data for bulk and thin film SrRuO$_3$. Improvement is found by including electron-electron correlations, and we suggest that bulk orthorhombic SrRuO$_3$ is a {\it weakly strongly-correlated} ferromagnet whose electronic structure is best described by a 0.6 eV on-site Hubbard term, or equivalently with corrections for the self-interaction error. We also perform {\it ab initio} transport calculations that confirm that SrRuO$_3$ has a negative spin polarization at the Fermi level, due to the position of the minority Ru 4$d$ band center. Even with correlations included in our calculations we are unable to reproduce the experimentally observed metal-insulator transition, suggesting that the electronic behavior of SrRuO$_3$ ultra-thin films might be dominated by extrinsic factors such as surface disorder and defects.

cond-mat.mtrl-sci↗

arXiv2008

Towards a microscopic theory of toroidal moments in bulk periodic crystals

We 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 basic definitions for finite systems and address the question of how to generalize these definitions to the bulk periodic case. We define the toroidization as the toroidal moment per unit cell volume, and we show that periodic boundary conditions lead to a multivaluedness of the toroidization, which suggests that only differences in toroidization are meaningful observable quantities. Our analysis bears strong analogy to the modern theory of electric polarization in bulk periodic systems, but we also point out some important differences between the two cases. We then discuss the instructive example of a one-dimensional chain of magnetic moments, and we show how to properly calculate changes of the toroidization for this system. Finally, we evaluate and discuss the toroidization (in the local dipole limit) of four important example materials: BaNiF_4, LiCoPO_4, GaFeO_3, and BiFeO_3.

cond-mat.str-el↗

arXiv2007

Tri-layer superlattices: A route to magnetoelectric multiferroics?

We explore computationally the formation of tri-layer superlattices as an alternative approach for combining ferroelectricity with magnetism to form magnetoelectric multiferroics. We find that the contribution to the superlattice polarization from tri-layering is small compared to typical polarizations in conventionalferroelectrics, and the switchable ferroelectric component is negligible. In contrast, we show that epitaxial strain and ``negative pressure'' can yield large, switchable polarizations that are compatible with the coexistence of magnetism, even in materials with no active ferroelectric ions.

cond-mat.mtrl-sci↗

arXiv2007

Self-interaction correction with Wannier functions

We describe the behavior of the Perdew-Zunger self-interaction-corrected local density approximation (SIC-LDA) functional when implemented in a plane-wave pseudopotential formalism with Wannier functions. Prototypical semiconductors and wide-bandgap oxides show a large overcorrection of the LDA bandgap. Application to transition-metal oxides and elements with d electrons is hindered by a serious breaking of the spherical symmetry, which appears even in a closed shell free atom. Our results indicate that, when all spherical approximations are lifted, the general applicability of orbital-dependent potentials is very limited and should be reconsidered in favor of rotationally invariant functionals.

cond-mat.other↗

arXiv2006

Magnetic interactions in transition metal doped ZnO : An abinitio study

We 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 types (Cr, Mn, Fe, Co, Ni and Cu) substituted at Zn sites, (ii) substitutional magnetic transition metal ions combined with additional Cu and Li dopants, (iii) substitutional magnetic transition metal ions combined with oxygen vacancies and (iv) pairs of magnetic ion types (Co and Fe, Co and Mn, etc.). Extensive convergence tests indicate that the calculated magnetic ground state is unusually sensitive to the k-point mesh and energy cut-off, the details of the geometry optimizations and the choice of the exchange-correlation functional. We find that ferromagnetic coupling is sometimes favorable for single type substitutional transition metal ions within the local spin density approximation. However, the nature of magnetic interactions changes when correlations on the transition-metal ion are treated within the more realistic LSDA + \textit{U} method, often disfavoring the ferromagnetic state. The magnetic configuration is sensitive to the detailed arrangement of the ions and the amount of lattice relaxation, except in the case of oxygen vacancies when an antiferromagnetic state is always favored.

cond-mat.mtrl-sci↗

arXiv2006

Origin of ferroelectricity in the multiferroic barium fluorides BaMF4

We 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 in these systems results from the "freezing in" of a single unstable polar phonon mode. In contrast to the case of the standard perovskite ferroelectrics, this structural distortion is not accompanied by charge transfer between cations and anions. Thus, the ferroelectric instability in the multiferroic barium fluorides arises solely due to size effects and the special geometrical constraints of the underlying crystal structure.

cond-mat.mtrl-sci↗

arXiv2006

BaNiF4: an electric field-switchable weak antiferromagnet

We 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 "weak" antiferromagnetic order parameter whose orientation is determined by the polar structural distortion and can be reversed by switching the ferroelectric polarization with an electric field. Our results point the way to a more general coupling mechanism between structural distortions and magnetic order parameters in magnetoelectric multiferroics which can be exploited in the design of electric field-switchable magnets.

cond-mat.mtrl-sci↗

arXiv2006

Lattice relaxation, electronic screening, and spin and orbital phase diagram of LaTiO$_3$/SrTiO$_3$ superlattices

The 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 Å) distortions of the TiO$_6$ octahedra in the near-La region. The resulting screening changes the Ti $d$-electron density substantially. Tight-binding fits to the relaxed-lattice band structure, combined with Hartree-Fock calculations of the resulting model, reveal a novel phase with $xy$ orbital order, which does not occur in bulk LaTiO$_3$, or in the hypothetical unrelaxed structure.

cond-mat.str-el↗

arXiv2005

Recent progress in first-principles studies of magnetoelectric multiferroics

Materials 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 we discuss recent progress in the study of such magnetoelectric multiferroics which has been achieved using computational first-principles methods based on density functional theory. In particular, we show how first-principles methods have been successfully used to explain various properties of multiferroic materials and to predict novel effects and new systems that exhibit multiferroic properties.

cond-mat.mtrl-sci↗