arXiv subjects

Hanna V. Shevliakova

Publications and source records attributed to Hanna V. Shevliakova.

11 recordsLinked to original sources

arXiv2026

Flexo-Strain Engineering of Phonons and Ferrons in Thin Films of Van der Waals Ferrielectrics

The influence of the flexoelectric coupling on the fluctuations of electric polarization and elastic strains can lead to the principal changes of the dispersion law of soft optical and acoustic phonons and ferrons in a bulk van der Waals ferrielectric. Since the size, gradient and strain effects determine phase diagrams and polarization behavior in thin films, it is reasonable to assume that the flexocoupling and mismatch strains should have a strong influence on the dispersion of phonons and ferrons in thin ferroelectric films. Using the Landau-Ginzburg-Devonshire approach, in this work we reveal that the dispersion of soft optical and acoustic phonons and ferrons is strongly dependent on the sign and magnitude of elastic strains, which originate from the lattice constants mismatch in thin strained films of van der Waals ferrielectric CuInP2S6. In particular, the frequency of acoustic phonons and ferrons approaches zero at nonzero wavevectors k>k_cr, where the critical value of the wavevector k_cr is determined by the mismatch strain, flexoelectric coupling strength and temperature. Zeroing of the acoustic phonon frequency, that appears with increase of tensile strains, indicates a possible emergence of a spatially modulated incommensurate polar phase induced by the flexo-strain effects. Analytical results, derived in this work, open the way for flexo-strain engineering of soft phonon and ferron dispersion in thin films of van der Waals ferrielectrics.

cond-mat.mtrl-sci↗

arXiv2024

Strain-polarization coupling in the low-dimensional Van der Waals Ferrielectrics

Using the Landau-Ginzburg-Devonshire phenomenological approach we explore the strain-polarization coupling in the low-dimensional van der Waals ferrielectrics. We evolve the analytical model of the piezoelectric susceptibility of the material in response to the periodic strain modulation, such as caused by a surface acoustic wave. Numerical calculations are performed for recently discovered and poorly studied ultrathin layers of the van der Waals ferrielectric CuInP2S6, which are very promising candidates for advanced nanoelectronics. We obtained that the temperature dependences of the dielectric and piezoelectric susceptibilities, and elastic compliance of the ultrathin CuInP2S6 layer have a sharp maximum at the temperature of CuInP2S6 paraelectric-ferrielectric phase transition near 320 K. The magnitudes of the dielectric and piezoelectric susceptibilities, and elastic compliance depend significantly on the modulation period of the surface acoustic wave. Obtained results explore the potential of ultrathin CuInP2S6 layers for application in piezoelectric and straintronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Phase diagrams and polarization reversal in nanosized Hf$_x$Zr$_{1-x}$O$_{2-y}$

To describe the polar properties of the nanosized HfxZr1-xO2-y, we evolve the "effective" Landau-Ginzburg-Devonshire (LGD) model based on the parametrization of the Landau expansion coefficients for the polar and antipolar orderings. We have shown that the effective LGD model can predict the influence of screening conditions and size effects on phase diagrams, polarization reversal and structural properties of the nanosized HfxZr1-xO2-y of various shape and sizes. To verify the model, we use available experimental results for HfxZr1-xO2 thin films and oxygen-deficient HfO2-y nanoparticles prepared at different annealing conditions. X-ray diffraction, which was used to determine the phase composition of the HfO2-y nanoparticles, revealed the formation of the ferroelectric orthorhombic phase in them. Micro-Raman spectroscopy was used to explore the correlation of lattice dynamics and structural changes appearing in dependence on the oxygen vacancies concentration in the HfO2-y nanoparticles. Since our approach allows to determine the conditions (shape, sizes, Zr content and/or oxygen vacancies amount) for which the nanosized HfxZr1-xO2-y are ferroelectrics or antiferroelectrics, we hope that obtained results are useful for creation of next generation of Si-compatible ferroelectric gate oxide nanomaterials.

cond-mat.mes-hall↗

arXiv2023

Size-Induced High Electrocaloric Response of Dense Ferroelectric Nanocomposites

Analytical results obtained within Landau-Ginzburg-Devonshire approach and effective media models, predict that the synergy of size effects and Vegard stresses can significantly enhance the electrocaloric cooling (up to 7 times) of the BaTiO3 nanoparticles in comparison with a bulk BaTiO3. To compare with the considered effective media models, we measured the capacitance-voltage and current-voltage characteristics of the dense nanocomposites consisting of (28-35) vol.% BaTiO3 nanoparticles incorporated in organic polymers and determined experimentally the effective dielectric permittivity and losses of the composites. Generalizing obtained analytical results, various ferroelectric nanoparticles spontaneously stressed by elastic defects, such as oxygen vacancies or any other elastic dipoles, which create a strong chemical pressure, can cause the giant electrocaloric response of dense ferroelectric nanocomposites. We have shown that the advantages of the studied lead-free dense nanocomposites are the good tunability of electrocaloric cooling temperature due to the size effects in ferroelectric nanoparticles and the easy control of the high electrocaloric cooling by electric fields. This makes the dense ferroelectric nanocomposites promising for cooling of conventional and innovative electronic elements, such as FETs with high-temperature superconductor channels.

physics.app-ph↗

arXiv2022

Size Effect of Local Current-Voltage Characteristics of MX$_2$ Nanoflakes: Local Density of States Reconstruction from Scanning Tunneling Microscopy Experiments

Local current-voltage characteristics for low-dimensional transition metal dichalcogenides (LD-TMD), as well as the reconstruction of their local density of states (LDOS) from scanning tunneling microscopy (STM) experiments is of fundamental interest and can be useful for advanced applications. Most of existing models are either hardly applicable for the LD-TMD of complex shape (e.g., those based on Simmons approach), or necessary for solving an ill-defined integral equation to deconvolute the unknown LDOS (e.g., those based on Tersoff approach). Using a serial expansion of Tersoff formulae, we propose a flexible method how to reconstruct the LDOS from local current-voltage characteristics measured in STM experiments. We established a set of key physical parameters, which characterize the tunneling current of a STM probe - sample contact and the sample LDOS expanded in Gaussian functions. Using a direct variational method coupled with a probabilistic analysis, we determine these parameters from the STM experiments for MoS2 nanoflakes with different number of layers. The main result is the reconstruction of the LDOS in a relatively wide energy range around a Fermi level, which allows insight in the local band structure of LD-TMDs. The reconstructed LDOS reveals pronounced size effects for the single-layer, bi-layer and three-layer MoS$_2$ nanoflakes, which we relate with low dimensionality and strong bending/corrugation of the nanoflakes. We hope that the proposed elaboration of the Tersoff approach allowing LDOS reconstruction will be of urgent interest for quantitative description of STM experiments, as well as useful for the microscopic physical understanding of the surface, strain and bending contribution to LD-TMDs electronic properties.

cond-mat.mtrl-sci↗

arXiv2021

Flexoelectric and piezoelectric coupling in a bended MoS$_2$ monolayer

Low-dimensional (LD) transition metal dichalcogenides (TMDs) in the form of nanoflakes, which consist of one or several layers, are the subject of intensive fundamental and applied research. Due to the size-induced transition from a bulk to nanoscale, they can be both nonpolar, piezoelectric or even ferroelectric. Also, in terms of electronic properties, they can be direct-band semiconductors, semi-metals or even metals. The tuning of the electronic properties in the LD-TMDs are commonly related with applied strains and strain gradients, which can affect strongly their polar properties via the piezoelectric and flexoelectric couplings. Using the density functional theory (DFT) and phenomenological Landau approach, we studied the bended 2H-MoS$_2$ monolayer and analyzed its flexoelectric and piezoelectric properties. The dependences of the dipole moment, strain and strain gradient on the coordinate along the layer were calculated. From these dependences the components of the flexoelectric and piezoelectric tensors have been determined and analyzed. Obtained results are useful for applications of LD-TMDs in strain engineering and flexible electronics.

cond-mat.mtrl-sci↗

arXiv2021

Chemical control of polarization in thin strained films of a multiaxial ferroelectric: phase diagrams and polarization rotation

The emergent behaviors in thin films of a multiaxial ferroelectric due to an electrochemical coupling between the rotating polarization and surface ions are explored within the framework of the 2-4 Landau-Ginzburg-Devonshire (LGD) thermodynamic potential combined with the Stephenson-Highland (SH) approach. The combined LGD-SH approach allows to describe the electrochemical switching and rotation of polarization vector in the multiaxial ferroelectric film covered by surface ions with a charge density dependent to the relative partial oxygen pressure. We calculate the phase diagrams and analyze the dependence of polarization components on the applied voltage, and discuss the peculiarities of quasi-static ferroelectric, dielectric and piezoelectric hysteresis loops in thin strained multiaxial ferroelectric films. The nonlinear surface screening by oxygen ions makes the diagrams very different from the known diagrams of e.g., strained BaTiO3 films. Quite unexpectedly we predict the appearance of the ferroelectric reentrant phases. Obtained results point on the possibility to control the appearance and features of ferroelectric, dielectric and piezoelectric hysteresis in multiaxial FE films covered by surface ions by varying their concentration via the partial oxygen pressure. The LGD-SH description of a multiaxial FE film can be further implemented within the Bayesian optimization framework, opening the pathway towards predictive materials optimization.

cond-mat.mtrl-sci↗

arXiv2020

Flexo-Elastic Control Factors of Domain Morphology in Core-Shell Ferroelectric Nanoparticles: Soft and Rigid Shells

Within the framework of the Landau-Ginzburg-Devonshire approach we explore the impact of elastic anisotropy, electrostriction, flexoelectric couplings, and mismatch strain on the domain structure morphology in ferroelectric core-shell nanoparticles of spherical shape. We perform finite element modelling (FEM) for multiaxial ferroelectric nanoparticle cores covered with an elastically-isotropic soft or elastically-anisotropic rigid paraelectric shell, with and without mismatch strains. In the case of a core covered with a soft shell, the FEM results show that at room temperature a single polarization vortex with a dipolar kernel can be stable if the electrostriction coupling is weak. With increasing anisotropic electrostriction coupling, the vortex disappears and is replaced by complex flux-closure structures, which are formed in the equatorial plane and transform into an elongated vortex with a central 180-degree domain wall near the core poles. This complex domain morphology develops in the core due to the anisotropic electrostriction, and the flexoelectric coupling leads to an additional curvature and twist of the polarization isosurfaces. In contrast to this, FEM performed for a core covered with a rigid shell shows that, at room temperature, the anisotropic elastic properties of the shell can stabilize vortex-like structures with three flux-closure domains, which gradually cross in the equatorial plane of the core and transform into 120-degree domains near the core poles. The flexoelectric coupling leads to a noticeable curling of the flux-closure domain walls. A mismatch strain compensates the curling of the flux-closure domains in the core confined by the elastically-anisotropic rigid shell. Our analysis reveals different types of topological defects, namely Bloch point structures (BPS) and Ising lines, that form in a ferroelectric core covered with a soft or rigid shell.

cond-mat.mtrl-sci↗

arXiv2020

Correlation between corrugation-induced flexoelectric polarization and conductivity of low-dimensional transition metal dichalcogenides

Tunability of polar and semiconducting properties of low-dimensional transition metal dichalcogenides (TMDs) have propelled them to the forefront of fundamental and applied physical research. These materials can vary from non-polar to ferroelectric, and from direct-band semiconductor to metallic. However, in addition to classical controls such as composition, doping, and field effect in TMDs the additional degrees of freedom emerge due to the curvature-induced electron redistribution and associated changes in electronic properties. Here we numerically explore the elastic and electric fields, flexoelectric polarization and free charge density for a TMD nanoflake placed on a rough substrate with a sinusoidal profile of the corrugation using finite element modelling. Numerical results for different flake thickness and corrugation depth yield insight into the flexoelectric nature of the out-of-plane electric polarization and establish the unambiguous correlation between the polarization and static conductivity modulation caused by inhomogeneous elastic strains coupled with deformation potential and strain gradients, which evolve in TMD nanoflake due to the adhesion between the flake surface and corrugated substrate. We revealed a pronounced maximum at the thickness dependences of the electron and hole conductivity of MoS2 and MoTe2 nanoflakes placed on a metallic substrate, which opens the way for their geometry optimization towards significant improvement their polar and electronic properties, necessary for their advanced applications in nanoelectronics and memory devices.Specifically, obtained results can be useful for elaboration of nanoscale straintronic devices based on the bended MoS2, MoTe2 and MoSTe nanoflakes, such as diodes and bipolar transistors with a bending-controllable sharpness of p-n junctions.

cond-mat.mtrl-sci↗

arXiv2020

The influence of the distribution function of ferroelectric nanoparticles sizes on their electrocaloric and pyroelectric properties

We consider a model of a nanocomposite based on non-interacting spherical single-domain ferroelectric nanoparticles of various sizes embedded in a dielectric matrix. The size distribution function of these nanoparticles is selected as a part of the Gaussian distribution from minimum to maximum radius (truncated normal distribution). For such nanocomposites, we calculate the dependences of the reversible part of the electric polarization, the electrocaloric temperature change, and the dielectric permittivity on the external electric field, which have the characteristic form of hysteresis loops. We then analyze the change in the shape of the hysteresis loops relative to the particle size distribution parameters. We demonstrate that for the same mean-square dispersion, the remanent polarization, coercive field, dielectric permittivity maximums, maximums and minimums of the electrocaloric temperature change depend most strongly on the most probable radius, moderately depend on the dispersion, and have the weakest dependency on the nanoparticle maximum radius. We calculated and analyzed the dependences of pyroelectric figures of merit on the average radius of the nanoparticles in the composite. The dependences confirm the presence of a phase transition induced by the size of the nanoparticles, which is characterized by the presence of a maxima near the critical average radius of the particles, the value of which increases with increasing dispersion of the distribution function.

cond-mat.mes-hall↗

arXiv2019

Analytical description of the size effect on pyroelectric and electrocaloric properties of ferroelectric nanoparticles

Using Landau-Ginzburg-Devonshire theory and effective medium approximation, we analytically calculate typical dependences of the pyroelectric and electrocaloric coefficients on external electric field, temperature and radius for spherical single-domain ferroelectric nanoparticles. The considered physical model corresponds to the nanocomposite with small fraction of ferroelectric nanoparticles. Within the framework of the analytical model we establish how the size changes determine the temperature and field behavior pyroelectric and electrocaloric coefficients on example of BaTiO3 nanoparticles covered by a semiconducting shell and placed in a dielectric polymer. We show that by changing the particle size one can induce maxima of the pyroelectric coefficient and electrocaloric temperature variation, control their width, height and sign. Obtained analytical expressions allow selecting the interval of particle sizes, voltage, and/or temperature for which pyroelectric energy conversion and electrocaloric coefficient are optimal for applications. The observed size effect opens the possibility to control pyroelectric and electrocaloric properties of ferroelectric nanocomposites that can be important for their advanced applications in energy convertors and cooling systems.

cond-mat.mtrl-sci↗