Igor Kornev

Ultrathin films of ferroelectric solid solutions under a residual depolarizing field.

A first-principles-derived approach is developed to study the effects of depolarizing electric fields on the properties of Pb(Zr,Ti)O3 ultrathin films for different mechanical boundary conditions. A rich variety of ferroelectric phases and polarization patterns is found, depending on the interplay between strain and the amount of screening of surface charges. Examples include triclinic phases, monoclinic states with in-plane and/or out-of-plane components of the polarization, homogeneous and inhomogeneous tetragonal states, as well as peculiar laminar nanodomains.

Phase diagram of pb(Zr,Ti)O3 solid solutions from first principles.

A first-principles-derived scheme that incorporates ferroelectric and antiferrodistortive degrees of freedom is developed to study finite-temperature properties of Pb(Zr1-xTix)O3 solid solution near its morphotropic phase boundary. The use of this numerical technique (i) resolves controversies about the monoclinic ground state for some Ti compositions, (ii) leads to the discovery of an overlooked phase, and (iii) yields three multiphase points that are each associated with four phases. Additional neutron diffraction measurements strongly support some of these predictions.

Original properties of dipole vortices in zero-dimensional ferroelectrics

The very recent use of first-principles-based simulations to investigate zero-dimensional ferroelectrics has led to the discovery of electric vortices, as well as of many original properties associated with these vortices. These original properties are of fundamental importance and of high technological promise, and are reviewed here.

Atomistic treatment of depolarizing energy and field in ferroelectric nanostructures

An atomistic approach allowing an accurate and efficient treatment of depolarizing energy and field in any low-dimensional ferroelectric structure is developed. Application of this approach demonstrates the limits of the widely used continuum model (even) for simple test cases. Moreover, implementation of this approach within a first-principles-based model reveals an unusual phase transition---from a state exhibiting a spontaneous polarization to a phase associated with a toroid moment of polarization---in a ferroelectric nanodot for a critical value of the depolarizing field.

Phase diagrams of epitaxial BaTiO3 ultrathin films from first principles

Using a first-principles-based scheme, we determine the qualitative and quantitative effects of surface∕interface, thickness and electrical boundary conditions on the temperature-misfit strain phase diagrams of epitaxial (001) BaTiO3 ultrathin films. The microscopic reasons leading to such effects are also revealed.

Combined theoretical and experimental study of the low-temperature properties of BaZrO{sub 3}

Low temperature properties of BaZrO3 are revealed by combining experimental techniques (X-ray diffraction, neutron scattering and dielectric measurements) with theoretical first-principles-based methods (total energy and linear response calculations within density functional theory, and effective Hamiltonian approaches incorporating/neglecting zero-point phonon vibrations). Unlike most of the perovskite systems, BaZrO3 does not undergo any (long-range-order) structural phase transition and thus remains cubic and paraelectric down to 2 K, even when neglecting zero-point phonon vibrations. On the other hand, these latter pure quantum effects lead to a negligible thermal dependency of the cubic lattice parameter below ~ 40 K. They also affect the dielectricity of BaZrO3 by inducing an overall saturation of the real part of the dielectric response, for temperatures below ~ 40 K. Two fine structures in the real part, as well as in the imaginary part, of dielectric response are further observed around 50-65 K and 15 K, respectively. Microscopic origins (e.g., unavoidable defects and oxygen octahedra rotation occurring at a local scale) of such anomalies are suggested. Finally, possible reasons for the facts that some of these dielectric anomalies have not been previously reported in the better studied KTaO3 and SrTiO3 incipient ferroelectrics are also discussed.

High-pressure phases in highly piezoelectric PbZr0.52Ti0.48O3

Two novel room-temperature phase transitions are observed, via synchrotron x-ray diffraction and Raman spectroscopy, in the PbZr0.52Ti0.48O3 alloy under hydrostatic pressures up to 16 GPa. A monoclinic (M)-to-rhombohedral (R1) phase transition takes place around 2–3 GPa, while this R1 phase transforms into another rhombohedral phase, R2, at ≈6–7 GPa. First-principles calculations assign the R3m and R3c symmetry to R1 and R2, respectively, and reveal that R2 acts as a pressure-induced structural bridge between the polar R3m and a predicted antiferrodistortive R3¯c phase.

Discovery of stable skyrmionic state in ferroelectric nanocomposites

Non-coplanar swirling field textures, or skyrmions, are now widely recognized as objects of both fundamental interest and technological relevance. So far, skyrmions were amply investigated in magnets, where due to the presence of chiral interactions, these topological objects were found to be intrinsically stabilized. Ferroelectrics on the other hand, lacking such chiral interactions, were somewhat left aside in this quest. Here we demonstrate, via the use of a first-principles-based framework, that skyrmionic configuration of polarization can be extrinsically stabilized in ferroelectric nanocomposites. The interplay between the considered confined geometry and the dipolar interaction underlying the ferroelectric phase instability induces skyrmionic configurations. The topological structure of the obtained electrical skyrmion can be mapped onto the topology of domain-wall junctions. Furthermore, the stabilized electrical skyrmion can be as small as a few nanometers, thus revealing prospective skyrmion-based applications of ferroelectric nanocomposites.

Thickness dependency of 180° stripe domains in ferroelectric ultrathin films: A first-principles-based study

A first-principles-based scheme is used to investigate the thickness dependency of domain width of 180° stripe domains in Pb(Zr,Ti)O3 ultrathin films. Our study shows that (1) more metastable states with energy closer to the 180° stripe domain ground state occur in thicker films, (2) the Kittel law is valid for 180° stripe domains when the film thickness is above 1.6nm, and (3) below 1.2nm, the Kittel law cannot be applied anymore due to the disappearance of domains. The thickness dependency of the domain morphology is also discussed.

Properties of Pb ( Zr , Ti ) O 3 ultrathin films under stress-free and open-circuit electrical boundary conditions

A first-principles-based scheme is developed to simulate properties of (001) PbO-terminated