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The relative energetic stability of the structural phases of common antiferromagnetic transition-metal oxides (MnO, FeO, CoO, and NiO) within the semilocal and hybrid density functionals are fraught with difficulties. In particular, MnO is known to be the most difficult case for almost all common semilocal and hybrid density approximations. Here, we show that the meta-generalized gradient approximation (meta-GGA) constructed from the cuspless hydrogen model and Pauli kinetic energy density (MGGAC) can lead to the correct ground state of MnO. The relative energy differences of zinc-blende (zb) and rock-salt (rs) structures as computed using MGGAC are found to be in nice agreement with those obtained from high-level correlation methods like the random phase approximation or quantum Monte Carlo techniques. Besides, we have also applied the onsite hybrid functionals (closely related to DFT+U ) based on GGA and meta-GGA functionals, and it is shown that a relatively high amount of Hartree-Fock exchange is necessary to obtain the correct ground-state structure. Our present investigation suggests that the semilocal MGGAC and onsite hybrids, both being computationally cheap, as methods of choice for the calculation of the relative stability of antiferromagnetic transition-metal oxides having potential applications in solid-state physics and structural chemistry.

The interface magnetization and its modulation is explored in oxide heterostructures within the framework of density functional theory. In particular, modulation in interface magnetization is investigated in prototype heterostructure SrRuO3|KNbO3(0 0 1) consisting ferromagnetic SrRuO3 and nonmagnetic KNbO3 as the thickness of KNbO3 is varied. The computed interfacial magnetization is found to alternate between a higher and a lower value with increasing KNbO3 thickness in the heterostructure. The average change in interface magnetization is computed to be ∼−60% as KNbO3 film changes from non-stoichiometric to stoichiometric and ∼+60% as it changes from stoichiometric to non-stoichiometric due to variation in thickness. The magnetic moment per interface cell changes from ∼2.27 μB for non-stoichiometric KNbO3 to ∼1.31 μB for stoichiometric KNbO3 in the heterostructure. The modulation in interface magnetization arises due to spin-dependent screening of effective charge of KNbO3 unit cells by accumulated free carriers at the interface. Large modulation in interface magnetization is also obtained for other heterostructures consisting magnetic substructure and ionized planes at the interface. In addition, the alteration in interface magnetization due to ferroelectric polarization reversal in ferromagnetic|ferroelectric SrRuO3|KNbO3 (0 0 1) superlattice is explored. The resulting interface magnetoelectric effect as well as control of interface magnetization using ionized planes may have interesting implications for technological applications.

Using density functional theory, the interface local magnetic moment and its variation are explored in SrRuO3|LaAlO3|vacuum (001) heterostructures with increasing number of LaAlO3 unit cells. The computed magnetic moment projected on interface planes is found to modulate nearly periodically as the number of LaAlO3 unit cells is increased in SrRuO3|LaAlO3|vacuum structure. The magnetic moment localized at the interface RuO2 plane shifts from a lower value of $\sim 0.7~\mu _{B}$ to a higher value of $\sim 1.35~\mu _{B}$ and changes by ~42% as LaAlO3 changes from non-stoichiometric to stoichiometric with increase in its thickness. However, the magnitude shifts from higher to lower value as LaAlO3 changes from stoichiometric to non-stoichiometric with further increase in thickness. The large variation and nearly periodic modulation of magnetic moment localized at the interface with increasing LaAlO3 thickness arise due to spin-dependent screening of effective total charge of LaAlO3 film by accumulated free carriers at the interface in heterostructure. The modulation of interface local magnetic moment using ultrathin LaAlO3 film at the oxide heterointerfaces can be exploited as an additional functionality, in conjunction with those reported recently such as control of band alignment and enhancement of the electric conductivity at the interface.

Polycrystalline AgNb1/2Ta1/2O3 powder was prepared by solid state reaction method. Preliminary x-ray diffractogram analysis of some aspects of crystal structure showed that a single phase compound formed exhibiting a monoclinic system. Impedance spectroscopy showed the presence of both bulk and grain boundary effects in the material. The relaxation behavior was studied by fitting electric modulus with Bergman function confirms us the existence of non-Debye type of relaxation the material. The ac conductivity spectrum obeyed Funke's double power law and fitting in results, the hopping parameters n1,n2 were indicating the existence of small and large range polaron hopping in the material. The band gap of the material 3.02 eV measured by using UV visible spectroscopy.

The interface magnetoelectric (ME) effect is promising for novel technological applications as it allows the control of interface magnetization at heterointerfaces by the external electric field. Here in this article, we explore the interface ME effect and its sensitivity on interface atomic structure and bonding in ferromagnetic-ferroelectric oxide hetrostructures within the framework of density functional theory. In particular, we consider Fe/AgNbO3 and SrRuO3/AgNbO3 superlattices with different possible defect-free interfaces. Our results suggest that interface magnetization and thereby interface ME effect can critically depend on interface structure. The interface magnetization is found to depend on atomic bondings which are sensitive to atomic displacements at the interface. This leads to ME coupling due to induced change in the interface magnetization as the electric polarization is reversed in the ferroelectric film. In addition, the contribution to interface ME coupling also comes from the change in exchange-splitting between spin-polarized electrons and consequently change in the interface magnetic moments due to ferroelectric polarization reversal. The change in exchange splitting, in turn, is caused by spin-dependent screening of the bound polarization charges at the interface. In Fe/AgNbO3 system, the interface ME coefficient is found to exhibit opposing trend for Fe|AgO and Fe|NbO2 interfaces. In addition to interface ME effect, the formation of ferroelectric interface domain wall (IDW) in these systems is also explored. The IDW is found to form only in SrRuO3/AgNbO3 system with SrO|NbO2 interface due to oppositely oriented strong dipole moments at the two interfaces. The strong dependence of interface ME effect on interface atomic structure may have implication for its experimental observation in oxide heterostructures. In particular, the interface defects may render the conclusive observation of this phenomenon challenging due to resulting fluctuations in the observed data.

Polycrystalline BiFe1−xScxO3 (x=0, 0.05, 0.1 and 0.15) compounds are prepared using solid state reaction. The XRD patterns show that all compounds are crystallized in rhombohedral structure with R3c space group. An induced weak ferromagnetism in Sc substituted BiFeO3 due to suppression of spiral modulated spin structure is revealed. In addition, a spin glass like behaviour is observed from the zero field cooled (ZFC) and field cooled (FC) magnetization curves in the low temperature region. Further, the coupling between the ferroelectric and (anti) ferromagnetic orders is evident from the appearance of anomaly in the dielectric data near the magnetic Neel temperature (373 °C). The reduction of oxygen vacancies due to Sc substitution is evident from the ac conductivity data and the suppressed anomaly in dielectric data at 220 °C. The temperature dependence of ac conductivity is consistent with correlated barrier hopping (CBH) model. The temperature dependent ac conductivity and activation energies indicate that electronic conduction, oxygen vacancies movement and creation of defects are the prime contributors to the ac conductivity in measured temperature regions. The improved magnetic and electrical properties due to the structural modification are prominent for novel device applications.

Lead free polycrystalline K1/2Bi1/2TiO3 (KBT) was prepared by solid state reaction method. Experimentally observed frequencies of Raman modes signified its tetragonal phase and matched reasonably well with theoretically calculated values. The relaxor nature of this material was observed in the temperature dependent real part of permittivity and dielectric loss curve. The value of the degree of diffusiveness was estimated from the modified Curie-Weiss law, which is 1.99, confirmed its relaxor behavior. The validation of this behavior was justified by the Vogel-Fulcher relation. The shoulder in the imaginary part of the modulus (M") and permittivity (Ԑ") spectra revealed the presence of polar nano regions (PNRs). The evidence of PNRs was detectable above freezing temperature which became weaker when the temperature exceeded Tm (temperature at the maximum of dielectric constant). Electric field induced polarization and strain curve showed the stabilization of long range ferroelectric order of the specimen at room temperature. Moreover, we obtained the discharge energy density and strain of 0.46 J/cm3 and 0.12%, respectively, at the maximum application of the electric field of 115 kV/cm at room temperature.

Using first-principles density functional theory, the zone-center phonon spectrum, mode oscillator strengths and Raman tensor coefficients are computed for the relaxor ferroelectric Na1/2Bi1/2TiO3 (NBT) in R3c phase. Subsequently, these quantities are used to compute the Infrared (IR) reflectivity and Raman intensity spectrum. The E symmetry modes at 246, 296, 580 cm−1 and A 1 symmetry modes at 256, 266, 568 cm−1 are found to have high oscillator strengths. The computed Raman coefficients are also found to be significant for aforementioned A 1 modes. The computed IR reflectivity and Raman spectra are expected to provide benchmark first-principles theroertical results for the symmetry assignment of experimental spectra of NBT in R3c phase.

Control of Schottky barrier height using a polar interface layer at oxide heterointerfaces offers interesting and promising applications in oxide-based electronics. Using ab initio density functional theory, the Schottky barrier heights are determined in SrRuO3 /SrTiO3(0 0 1) heterojunctions with interfacial polar layers such as (LaO)+, (AlO2)−, etc. The Schottky barriers at these heterointerfaces are found to modulate significantly depending on the charge of the interface layer. Large shifts in Schottky barrier height due to polar layer insertions are explained using a micro-capacitor model. Further, the ionic and electronic contributions to the Schottky barrier height at the SrRuO3/SrTiO3 interface are determined and analyzed vis-a-vis basic assumptions of empirical models based on metal-induced gap states (MIGS) and bond polarization theory. In addition, the interface electronic structure and distribution of interface MIGS in SrRuO3/SrTiO3 heterostructures are determined. Furthermore, the electronic structures for SrO- and RuO2-terminated SrRuO3(0 0 1) and SrO- and TiO2-terminated SrTiO3(0 0 1) surfaces are explored and compared to those for SrRuO3/SrTiO3 heterostructures. The modulations in workfunctions of SrO- and RuO2-terminated SrRuO3(0 0 1) surfaces due to polar (LaO)+ and (AlO2)− surface monolayers are also examined and discussed.

Based on first-principles and model calculations we investigate the effect of polar interfaces on the ferroelectric stability of thin-film ferroelectrics. As a representative model, we consider a TiO2-terminated BaTiO3 film with LaO monolayers at the two interfaces that serve as doping layers. We find that the polar interfaces create an intrinsic electric field that is screened by the electron charge leaking into the BaTiO3 layer. The amount of the leaking charge is controlled by the boundary conditions which are different for three heterostructures considered, namely Vacuum/LaO/BaTiO3/LaO, LaO/BaTiO3, and SrRuO3/LaO/BaTiO3/LaO. The intrinsic electric field forces ionic displacements in BaTiO3 to produce the electric polarization directed into the interior of the BaTiO3 layer. This creates a ferroelectric dead layer near the interfaces that is non-switchable and thus detrimental to ferroelectricity. Our first-principles and model calculations demonstrate that the effect is stronger for a larger effective ionic charge at the interface and longer screening length due to a stronger intrinsic electric field that penetrates deeper into the ferroelectric. The predicted mechanism for a ferroelectric dead layer at the interface controls the critical thickness for ferroelectricity in systems with polar interfaces., 33 Pages, 5 figures

The discovery of a two-dimensional electron gas (2DEG) at the interface between two insulating oxides has recently stimulated intense research activity in this field. The 2DEG has unique properties that are promising for applications in all-oxide electronic devices. For such applications it is desirable to have the ability to control 2DEG properties by external stimulus. Here we employ first-principles calculations to investigate ${\text{KNbO}}_{3}/A{\text{TiO}}_{3}(001)$ ($A=\text{Sr}$, Pb, and Ba) heterostructures where perovskites ferroelectrics, ${\text{KNbO}}_{3}$, ${\text{PbTiO}}_{3}$, and ${\text{BaTiO}}_{3}$, are used as oxide constituents to create the interface 2DEG. Our results suggest that the polar ${({\text{NbO}}_{2})}^{+}/{(A\text{O})}^{0}$ interface in these heretostructures favors the formation of 2DEG similar to that at the ${(\text{LaO})}^{+}/{({\text{TiO}}_{2})}^{0}$ interface in a ${\text{LaAlO}}_{3}/{\text{SrTiO}}_{3}$ heterostructure. We predict that the presence of spontaneous ferroelectric polarization which can be switched between two stable states allows modulations of the carrier density and consequently the conductivity of the 2DEG. The effect occurs due to the screening charge at the interface that counteracts the depolarizing electric field and depends on polarization orientation. The magnitude of the effect of polarization on the 2DEG properties strongly depends on contrast between polarizations of the two constituents of the heterostructure: the larger is the difference in the two polarizations, the bigger is the effect. For a sufficiently large polarization difference, we predict a metal-insulator transition at the interface driven by polarization reversal. This behavior is found for the ${\text{KNbO}}_{3}/{\text{PbTiO}}_{3}$ interface and for the ${\text{KNbO}}_{3}/{\text{BaTiO}}_{3}$ interface when polarizations of ${\text{KNbO}}_{3}$ and ${\text{BaTiO}}_{3}$ are antiparallel. The proposed concept of ferroelectrically controlled interface conductivity may be very interesting for memory and logic applications and we hope that our predictions will stimulate experimental studies in this field.

First-principles calculations predict the existence of a spin-polarized two-dimensional electron gas (2DEG) at the LaO/EuO interface in a ${\text{LaAlO}}_{3}/\text{EuO}(001)$ heterostructure. This polar interface favors electron doping into the $\text{Eu-}5d$ conduction bands resulting in a 2DEG formed at the interface. Due to the exchange splitting of the $\text{Eu-}5d$ states, the 2DEG is spin polarized below the Curie temperature of EuO. The predicted mechanism for the formation of a spin-polarized 2DEG at the interface between polar and ferromagnetic insulators may provide a robust magnetism of the 2DEG which is interesting for spintronics applications.

We report a study of the atomic and electronic structures of GaAs(001)/NiPtGe(001) interfaces. By using density functional theory, we study the dependence of the Schottky barrier on the interface stoichiometry. The calculated $p$-type Schottky barrier heights vary by as much as 0.18 eV around the average value of 0.5 eV, which corresponds to a strongly pinned interface. We relate the As-Ge bonds at the interface with a strong Fermi level pinning.

We present a comprehensive theoretical study of the electronic structure, elastic properties, surface energies, and work functions of $\mathrm{NiGe}$ and $\mathrm{PtGe}$ within the framework of density functional theory (DFT). Our calculated lattice constants are within 1--2% of recently reported experimental values. Calculated work functions for the (001) surfaces of $\mathrm{NiGe}$ and $\mathrm{PtGe}$ are 4.57 and $4.83\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, respectively, suggesting that both metals and their alloys can be used as self-aligned contacts to $p$-type germanium. We identify the growth conditions necessary to stabilize this orientation. We report on an unusual surface reconstruction of the $\mathrm{NiGe}(101)\text{--}\mathrm{Ge}$-terminated surface.

We present a comparative study of the anisotropy in the elastic properties of the C49, C54 and C40 phases of TiSi2, as well as orthorhombic TiSi and hexagonal Ti5Si3. The elastic constants, elastic moduli, Debye temperature and sound velocities are computed within the framework of density functional theory. The computed values of the elastic constants and moduli are found to be in excellent agreement with available experimental values. The average elastic moduli, such as Young's modulus, shear modulus, bulk modulus and Poisson's ratio, of polycrystalline aggregates are computed using the computed elastic constants of single crystals. The anisotropy in elastic properties is analyzed using estimates of shear anisotropic factors, bulk modulus anisotropic factors and variations in Young's and bulk moduli in different crystallographic directions. Among the Ti–Si phases, the computed directional Young's modulus profiles of C49 TiSi2 and C40 TiSi2 are found to be quite similar to those of bulk Si and Ti, respectively. In addition to the elastic properties, the electronic structure of five Ti–Si phases is studied. The density of states and planar charge density profiles reveal mixed covalent–metallic bonding in all Ti–Si phases.

We perform a comprehensive study of the structural, lattice dynamical and ferroic properties of the room temperature phase of AgNbO3 using a combination of neutron diffraction, Raman spectroscopy and first-principles density functional theory. The Rietveld analysis of neutron diffraction data indicates that AgNbO3 at room temperature crystallizes in an orthorombic structure with noncentrosymmetric Pmc21 space group symmetry. The zone-center phonon mode frequencies of Pmc21 and Pbcm phases of AgNbO3 are computed and are found to be in good agreement with experimentally obtained Raman mode frequencies. Computed frequencies for both space groups are found to be very close. However, computed modes with frequencies 488 and 846 cm −1 are found to be Raman-active for the Pmc21 phase while Raman inactive for Pbcm phase. The computed potential energy surfaces and Born effective charges of the ions suggest that the primary contribution to ferroic properties of AgNbO3 come from Nb and O ions only. Furthermore the calculated potential energy profiles suggest competing ferroic and antiferrodistortive instabilities in AgNbO3 are primarily due to the off-centering of Nb ions and rotations of NbO6 octahedra, respectively.

We present a comprehensive theoretical study of work functions and surface energies of $\mathrm{PtSi}$ and a calculation of the Schottky-barrier height at the $\mathrm{Si}(001)∕\mathrm{PtSi}(001)$ interface within the framework of the density functional theory. The $p$-type Schottky-barrier height of $0.28\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is found in good agreements with predictions of a simple metal-induced gap states theory and available experiments. This low barrier suggests $\mathrm{PtSi}$ as a low contact resistance junction metal for complimentary metal-oxide-semiconductor technology. We correlate the work function with the $\mathrm{PtSi}$ surface orientation and estimate surface energies of different surfaces. The low energy terminations correspond well to grain orientations reported for thin $\mathrm{PtSi}$ films.

Ferromagnetic hexagonal $\ensuremath{\alpha}$-MnAs has a first-order phase transition to $\ensuremath{\beta}$-MnAs at about 40 \ifmmode^\circ\else\textdegree\fi{}C. Because it shows no long-range antiferromagnetic order, most workers assume that it is paramagnetic in spite of the fact that it does not have a Curie-Weiss magnetic susceptibility. With the aid of density functional calculations, we show that it is antiferromagnetic and explain the lack of observed long-range order.

Metal silicides are highly important materials due to their fascinating properties and diverse applications in microelectronics. In this work, we have studied the Gruneisen parameter, thermal expansion and other thermodynamic properties of nickel-disilicide (NiSi2) using the density-functional theoretical framework. The Gruneisen parameter dispersions, thermal expansion coefficients, heat capacities and bulk modulus are calculated from volume dependence of phonon frequencies. The frequencies of zone-centre phonon modes are calculated to be 311 cm−1 (infrared active) and 323 cm−1 (Raman active) and are in good agreement with reported Raman scattering data. The thermodynamic Gruneisen parameter and Debye temperature are calculated to be 1.85 K and 535 K respectively. The calculated linear thermal expansion coefficient (~12.1 × 10−6 K−1) is found to be in good agreement with its recently reported experimental value. The theoretical results are interesting and are expected to stimulate experimental investigations of thermal properties of NiSi2 and other silicides.