I. Ponomareva

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.

Low-Symmetry Phases in Ferroelectric Nanowires

Ferroelectric nanostructures have recently attracted much attention due to the quest of miniaturizing devices and discovering novel phenomena. In particular, studies conducted on two-dimensional and zero-dimensional ferroelectrics have revealed original properties and their dependences on mechanical and electrical boundary conditions. Meanwhile, researches aimed at discovering and understanding properties of one-dimensional ferroelectric nanostructures are scarce. The determination of the structural phase and of the direction of the polarization in one-dimensional ferroelectrics is of technological importance, since, e.g., a low-symmetry phase in which the polarization lies away from a highly symmetric direction typically generates phenomenal dielectric and electromechanical responses. Here, we investigate the phase transition sequence of nanowires made of KNbO(3) and BaTiO(3) perovskites, by combining X-ray diffraction, Raman spectroscopy, and first-principles-based calculations. We provide evidence of a previously unreported ferroelectric ground state of monoclinic symmetry and the tuning of the polarizations direction by varying factors inherent to the nanoscale.

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.

Formation mechanisms of nanocomposite layers based on multiwalled carbon nanotubes and non-stoichiometric tin oxide

Nanocomposite layers based on multiwalled carbon nanotubes (MWCNTs) and non-stoichiometric tin oxide (SnOx) have been grown by magnetron deposition and CVD methods. In the case of the CVD method, the study of the structure and phase composition of obtained nanocomposite layers has shown that a tin oxide “superlattice” is formed in the MWCNT layer volume, fixed by SnOx islands on the MWCNT surface. During magnetron deposition, the MWCNT surface is uniformly coated with tin oxide islands, which causes a change in properties of individual nanotubes. Electrical measurements have revealed the sensitivity of nanocomposite layers to (NO2)− molecule adsorption, which is qualitatively explained by a change in the conductivity of the semiconductor fraction of p-type MWCNTs.

Scaling law for electrocaloric temperature change in antiferroelectrics.

A combination of theoretical and first-principles computational methods, along with experimental evidence from the literature, were used to predict the existence of a scaling law for the electrocaloric temperature change in antiferroelectric materials. We show that the temperature change scales quadratically with electric field, allowing a simple transformation to collapse the set of ΔT(E) onto a single curve. This offers a unique method that can be used to predict electrocaloric behavior beyond the limits of present measurement ranges or in regions where data are not yet available.

Domain evolution of BaTiO3 ultrathin films under an electric field : A first-principles study

A first-principles-derived method is used to study the morphology and electric-field-induced evolution of stripe nanodomains in (001) BaTiO3 (BTO) ultrathin films, and to compare them with those in (001) Pb(Zr,Ti)O3 (PZT) ultrathin films. The BaTiO3 systems exhibit 180o periodic stripe domains at null electric field, as in PZT ultrathin films. However, the stripes alternate along [1-10] in BTO systems versus [010] in PZT systems, and no in-plane surface dipoles occur in BTO ultrathin films (unlike in PZT materials). Moreover, the evolution of the 180o stripe domains in the BaTiO3 systems, when applying and increasing an electric field along [001], involves four regions: Region I for which the magnitude of the down dipoles (i.e., those that are antiparallel to the electric field) is reduced, while the domain walls do not move; Region II in which some local down dipoles adjacent to domain walls switch their direction, resulting in zigzagged domain walls - with the overall stripe periodicity being unchanged; Region III in which nanobubbles are created, then contract along [110] and finally collapse; and Region IV which is associated with a single monodomain. Such evolution differs from that of PZT ultrathin films for which neither Region I nor zigzagged domain walls exist, and for which the bubbles contract along [100]. Discussion about such differences is provided.

Emergence of central mode in the paraelectric phase of ferroelectric perovskites

THz-range dielectric spectroscopy and first-principle-based effective-Hamiltonian molecular dynamics simulations were employed to elucidate the dielectric response in the paraelectric phase of (Ba,Sr)TiO3 solid solutions. Analysis of the resulting dielectric spectra suggests the existence of a crossover between two different regimes: a higher-temperature regime governed by the soft mode only versus a lower-temperature regime exhibiting a coupled soft mode/central mode dynamics. Interestingly, a single phenomenological coupling model can be used to adjust the THz dielectric response in the entire range of the paraelectric phase (i.e., even at high temperature). We conclude that the central peak is associated with thermally activated processes, and that it cannot be discerned anymore in the dielectric spectra when the rate of these thermally activated processes exceeds certain characteristic frequency of the system.

Electrocaloric effect in ferroelectric nanowires from atomistic simulations.

Electrocaloric effect is presently under active investigation owing to both the recent discoveries of giant electrocaloric effects and its potential for solid state cooling applications. We use first-principles-based direct simulations to predict the electrocaloric temperature change in ferroelectric ultrathin nanowires. Our findings suggest that in nanowires with axial polarization direction the maximum electrocaloric response is reduced when compared to bulk, while the room temperature electrocaloric properties can be enhanced by tuning the ferroelectric transition temperature. The potential of ferroelectric nanowires for electrocaloric cooling applications is discussed.

Pyro-paraelectric and flexocaloric effects in barium strontium titanate: A first principles approach

Inhomogeneous strain allows the manifestation of an unexplored component of stress-driven caloric effect (flexocaloric effect) and enhanced pyroelectric performance, obtainable significantly beyond the Curie point. A peak temperature change of 1.5 K (at 289 K) was predicted from first-principles-based simulations for Ba0.5Sr0.5TiO3 under the application of a strain gradient of 1.5 μm−1. Additionally, enhanced pyro-paraelectric coefficient (pyroelectric coefficient in paraelectric phase) and flexocaloric cooling 11 × 10−4 C m−2 K−1 and 1.02 K, respectively, could be obtained (at 330 K and 1.5 μm−1). A comparative analysis with prevailing literature indicates huge untapped potential and warrants further research.