Successive Magnetic-Field-Induced Transitions and Colossal Magnetoelectric Effect in Ni_{3}TeO_{6}

We report the discovery of a metamagnetic phase transition in a polar antiferromagnet Ni_{3}TeO_{6} that occurs at 52 T. The new phase transition accompanies a colossal magnetoelectric effect, with a magnetic-field-induced polarization change of 0.3 μC/cm^{2}, a value that is 4 times larger than for the spin-flop transition at 9 T in the same material, and also comparable to the largest magnetically induced polarization changes observed to date. Via density-functional calculations we construct a full microscopic model that describes the data. We model the spin structures in all fields and clarify the physics behind the 52 T transition. The high-field transition involves a competition between multiple different exchange interactions which drives the polarization change through the exchange-striction mechanism. The resultant spin structure is rather counterintuitive and complex, thus providing new insights on design principles for materials with strong magnetoelectric coupling.

Mn2FeWO6 : A new Ni3TeO6-type polar and magnetic oxide

Mn(2+)2 Fe(2+)W(6+)O6 , a new polar magnetic phase, adopts the corundum-derived Ni3TeO6 -type structure with large spontaneous polarization (PS) of 67.8 μC cm(-2), complex antiferromagnetic order below ≈75 K, and field-induced first-order transition to a ferrimagnetic phase below ≈30 K. First-principles calculations predict a ferrimagnetic (udu) ground state, optimal switching path along the c-axis, and transition to a lower energy udu-udd magnetic double cell.

Adiabatic pumping of Chern-Simons axion coupling

We study the adiabatic pumping of the Chern-Simons axion (CSA) coupling along a parametric loop characterized by a nonzero second Chern number C^{(2)} from the viewpoint of the hybrid Wannier representation, in which the Wannier charge centers are visualized as sheets defined over a projected 2D Brillouin zone. We derive a new formula for the CSA coupling, expressing it as an integral involving Berry curvatures and potentials defined on the Wannier charge center sheets. We show that a loop characterized by a nonzero C^{(2)} requires a series of sheet-touching events at which 2π quanta of Berry curvature are passed from sheet to sheet, in such a way that e^{2}/h units of CSA coupling are pumped by a lattice vector by the end of the cycle. We illustrate these behaviors via explicit calculations on a model tight-binding Hamiltonian and discuss their implications.

Hyperferroelectrics: proper ferroelectrics with persistent polarization

All known proper ferroelectrics are unable to polarize normal to a surface or interface if the resulting depolarization field is unscreened, but there is no fundamental principle that enforces this behavior. In this work, we introduce hyperferroelectrics, a new class of proper ferroelectrics which polarize even when the depolarization field is unscreened, this condition being equivalent to instability of a longitudinal optic mode in addition to the transverse-optic-mode instability characteristic of proper ferroelectrics. We use first-principles calculations to show that several recently discovered hexagonal ferroelectric semiconductors have this property, and we examine its consequences both in the bulk and in a superlattice geometry.

Effective J=1/2 insulating state in Ruddlesden-Popper iridates: an LDA+DMFT study

Using ab initio methods for correlated electrons in solids, we investigate the metal-insulator transition across the Ruddlesden-Popper (RP) series of iridates and explore the robustness of the Jeff=1/2 state against band effects due to itineracy, tetragonal distortion, octahedral rotation, and Coulomb interaction. We predict the effects of epitaxial strain on the optical conductivity, magnetic moments, and Jeff=1/2 ground-state wave functions in the RP series. To describe the solution of the many-body problem in an intuitive picture, we introduce a concept of energy-dependent atomic states, which strongly resemble the atomic Jeff=1/2 states but with coefficients that are energy or time dependent. We demonstrate that the deviation from the ideal Jeff=1/2 state is negligible at short time scales for both single- and double-layer iridates, while it becomes quite significant for Sr3Ir2O7 at long times and low energy. Interestingly, Sr2IrO4 is positioned very close to the SU(2) limit, with only ∼3% deviation from the ideal Jeff=1/2 situation.

Chern insulators from heavy atoms on magnetic substrates

We propose searching for Chern insulators by depositing atomic layers of elements with large spin-orbit coupling (e.g., Bi) on the surface of a magnetic insulator. We argue that such systems will typically have isolated surface bands with nonzero Chern numbers. If these overlap in energy, a metallic surface with large anomalous Hall conductivity will result; if not, a Chern-insulator state will typically occur. Thus, our search strategy reduces to looking for examples having the Fermi level in a global gap extending across the entire Brillouin zone. We verify this search strategy and identify several candidate systems by using first-principles calculations to compute the Chern number and anomalous Hall conductivity of a large number of such systems on MnTe, MnSe, and EuS surfaces. Our search reveals several promising Chern insulators with gaps of up to 140 meV.

Orthorhombic ABC semiconductors as antiferroelectrics

We use a first-principles rational-design approach to identify a previously unrecognized class of antiferroelectric materials in the Pnma MgSrSi structure type. The MgSrSi structure type can be described in terms of antipolar distortions of the nonpolar P6(3)/mmc ZrBeSi structure type, and we find many members of this structure type are close in energy to the related polar P6(3)mc LiGaGe structure type, which includes many members we predict to be ferroelectric. We highlight known ABC combinations in which this energy difference is comparable to the antiferroelectric-ferroelectric switching barrier of PbZrO(3). We calculate structural parameters and relative energies for all three structure types, both for reported and as-yet hypothetical representatives of this class. Our results provide guidance for the experimental realization and further investigation of high-performance materials suitable for practical applications.

Hexagonal ABC semiconductors as ferroelectrics

We use a first-principles rational-design approach to identify a previously unrecognized class of ferroelectric materials in the P6(3)mc LiGaGe structure type. We calculate structural parameters, polarization, and ferroelectric well depths both for reported and as-yet hypothetical representatives of this class. Our results provide guidance for the experimental realization and further investigation of high-performance materials suitable for practical applications.

Half-Heusler semiconductors as piezoelectrics

We use a first-principles rational-design approach to demonstrate the potential of semiconducting half-Heusler compounds as a previously unrecognized class of piezoelectric materials. We perform a high-throughput scan of a large number of compounds, testing for insulating character and calculating structural, dielectric, and piezoelectric properties. Our results provide guidance for the experimental realization and characterization of high-performance materials in this class that may be suitable for practical applications.

First Principles Calculations of Surface Stress

Only recently have there been fully quantum-mechanical calculations of two-dimensional surface stress tensors. We have calculated total energies and stresses of semiconductor surfaces within the Local Density Approximation, using norm-conserving pseudopotentials. In order to hasten convergence of the stress with respect to basis size, it is useful to remove a fictitious tensile stress. We have calculated surface stress for the relaxed Si (111) 1×1 and 2×2-adatom surfaces, as well as for the relaxed Ge (111) 1×1 and 2×2-adatom surfaces. We have also calculated the surface stress for several chemisorbed systems, including Ga, Ge and As chemisorbed onto Si. We find a dramatic correlation between the electronic structure and chemistry of the surface, and its elastic properties.

Elastic Energies of Coherent Germanium Islands on Silicon

Motivated by recent observations of coherent Ge island formation during growth of Ge on Si (100), we have carried out a theoretical study of the elastic energies associated with the evolution of a uniform strained overlayer as it segregates into coherent islands. In the context of a two-dimensional model, we have explored the conditions under which coherent islands may be energetically favored over both uniform epitaxial films and dislocated islands. We find that if the interface energy (for dislocated islands) is more than about 15% of the surface energy, then there is a range of island sizes for which the coherent island structure is preferred.

Hydrogen, Acceptors, and H-Acceptor Complexes in GaN

We present ab-initio calculations on energetics and geometries of atomic hydrogen, of several candidate acceptors, and of H-acceptor complexes in wurtzite GaN For the H-Mg complex in Mg-doped GaN, we calculate the vibrational frequencies of H. Hydrogen is found to be a negative-U center. H-acceptor complex formation is always exothermic. Substitutional Be has a low formation energy and a shallow impurity level, which makes it a good candidate for p-doping in MBE growth. CN appears not to be shallow. Atomic hydrogen incorporation in undoped GaN is disfavored in an H2 atmosphere; it becomes favorable in p and n-type conditions in atomic H environments.

Structural and Electronic Properties of AlN, GaN And InN, and Band Offsets at AlN/GaN (1010) and (0001) Interfaces

Ab initio local-density-functional calculations are presented for bulk A1N, GaN, and InN in the wurtzite, zincblende, and rocksalt structures. Structural transition pressures and deformation potentials of electronic gaps are investigated. In addition, we study the band offset at the polar (0001) and non-polar (1010) AIN/GaN interfaces. Within AIN-on-GaN epitaxial conditions, we obtain valence-band offset values close to 0.7 eV for both interfaces. From the macroscopic field appearing along the growth direction of the polar interface (tentatively attributed to AIN macroscopic polarization), an estimate of the macroscopic dielectric constant of GaN is extracted. All calculations employed conjugate-gradient total-energy minimizations, ultrasoft pseudopotentials, and plane waves at 25 Ryd cutoff.

Offsets and Polarization at Strained AlN/GaN Polar Interfaces

The strain induced by lattice mismatch at the interface is responsible for the different value of the band discontinuities observed recently for the AlN/GaN (AlN on GaN) and the GaN/AlN (GaN on AlN) polar (0001) interface. We present a first-principles calculation of valence band offsets, interface dipoles, strain-induced piezoelectric fields, relaxed geometric structure, and formation energies. Our results confirm the existence of a large forward-backward asymmetry for this interface.

First-principles modeling of multiferroic RMn2O5

We investigate the phase diagrams of RMn(2)O(5) via a first-principles effective-Hamiltonian method. We are able to reproduce the most important features of the complicated magnetic and ferroelectric phase transitions. The calculated polarization as a function of temperature agrees very well with experiments. The dielectric-constant step at the commensurate-to-incommensurate magnetic phase transition is well reproduced. The microscopic mechanisms for the phase transitions are discussed.

Enhancement of ferroelectricity at metal-oxide interfaces

The development of ultrathin ferroelectric capacitors for use in memory applications has been hampered by depolarization effects arising from the electrode-film interfaces. These can be characterized in terms of a reduced interface capacitance, or equivalently an 'effective dead layer' in contact with the electrode. Here, by performing first-principles calculations on four capacitor structures based on BaTiO(3) and PbTiO(3), we determine the intrinsic interfacial effects responsible for destabilizing the ferroelectric state in ultrathin-film devices. Although it has been widely believed that these are governed by the electronic screening properties at the interface, we show that they also depend crucially on the local chemical environment through the force constants of the metal oxide bonds. In particular, in the case of interfaces formed 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 its 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.

Magnetoelectric polarizability and axion electrodynamics in crystalline insulators

The orbital motion of electrons in a three-dimensional solid can generate a pseudoscalar magnetoelectric coupling theta, a fact we derive for the single-particle case using a recent theory of polarization in weakly inhomogeneous materials. This polarizability theta is the same parameter that appears in the "axion electrodynamics" Lagrangian DeltaL_{EM}=(thetae;{2}/2pih)E.B, which is known to describe the unusual magnetoelectric properties of the three-dimensional topological insulator (theta=pi). We compute theta for a simple model that accesses the topological insulator and discuss its connection to the surface Hall conductivity. The orbital magnetoelectric polarizability can be generalized to the many-particle wave function and defines the 3D topological insulator, like the integer quantum Hall effect, in terms of a topological ground-state response function.

Electric polarization in a Chern insulator

We extend the Berry-phase concept of polarization to insulators having a nonzero value of the Chern invariant. The generalization to such Chern insulators requires special care because of the partial occupation of chiral edge states. We show how the integrated bulk current arising from an adiabatic evolution can be related to a difference of bulk polarizations. We also show how the surface charge can be related to the bulk polarization, but only with a knowledge of the wave vector at which the occupancy of the edge state is discontinuous. Furthermore, we present numerical calculations on a model Hamiltonian to provide additional support for our analytic arguments.

Predicting polarization and nonlinear dielectric response of arbitrary perovskite superlattice sequences

We carry out first-principles calculations of the nonlinear dielectric response of short-period ferroelectric superlattices. We compute and store not only the total polarization, but also the Wannier-based polarizations of individual atomic layers, as a function of the electric displacement field, and use this information to construct a model capable of predicting the nonlinear dielectric response of an arbitrary superlattice sequence. We demonstrate the successful application of our approach to superlattices composed of SrTiO3, CaTiO3, and BaTiO3 layers.

First principles study of improper ferroelectricity in TbMnO3

We carry out a first-principles theoretical study of the magnetically induced polarization in orthorhombic TbMnO3, a prototypical material in which a cycloidal-spin structure generates an electric polarization via the spin-orbit interaction. We compute both the electronic and the lattice-mediated contributions to the polarization and find that the latter is strongly dominant. We analyze the spin-orbit induced forces and lattice displacements from both atomic and mode-decomposition viewpoints, and show that a simple model based on nearest Mn-Mn neighbor Dzyaloshinskii-Moriya interactions is not able to account fully for the results. The direction and magnitude of our computed polarization are in good agreement with experiment.

Suppressed dependence of polarization on epitaxial strain in highly polar ferroelectrics

A combined experimental and computational investigation of coupling between polarization and epitaxial strain in highly polar ferroelectric PbZr(0.2)Ti(0.8)O3 (PZT) thin films is reported. A comparison of the properties of relaxed (tetragonality c/a approximately 1.05) and highly strained (c/a approximately 1.09) epitaxial films shows that polarization, while being amongst the highest reported for PZT or PbTiO3 in either film or bulk forms P(r) approximately 82 microC/cm(2)), is almost independent of the epitaxial strain. We attribute this behavior to a suppressed sensitivity of the A-site cations to epitaxial strain in these Pb-based perovskites, where the ferroelectric displacements are already large, contrary to the case of less polar perovskites, such as BaTiO3. In the latter case, the A-site cation (Ba) and equatorial oxygen displacements can lead to substantial polarization increases.

Wannier-based definition of layer polarizations in perovskite superlattices

In insulators, the method of Marzari and Vanderbilt [Phys. Rev. B 56, 12 847 (1997)] can be used to generate maximally localized Wannier functions whose centers are related to the electronic polarization. In the case of layered insulators, this approach can be adapted to provide a natural definition of the local polarization associated with each layer, based on the locations of the nuclear charges and one-dimensional Wannier centers comprising each layer. Here, we use this approach to compute and analyze layer polarizations of ferroelectric perovskite superlattices, including changes in layer polarizations induced by sublattice displacements (i.e., layer-decomposed Born effective charges) and local symmetry breaking at the interfaces. The method provides a powerful tool for analyzing the polarization-related properties of complex layered oxide systems.