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Yuri A. Genenko

Publications and source records attributed to Yuri A. Genenko.

17 recordsLinked to original sources

Domain formation and correlation effects in quenched uniaxial ferroelectrics: A stochastic model perspective

The stochastic analysis of the polarization domain structures, emerging after quenching from a paraelectric to a ferroelectric state, in terms of the polarization correlation functions and their Fourier transforms is a fast and effective tool of the materials structure characterization. In spite of a significant volume of experimental data accumulated over the last three decades for the model uniaxial ferroelectric triglycine sulfate, there were no theoretical tools to comprehend these data until now. This work summarizes the recent progress in understanding of the experiments by means of the original stochastic model of polarization structure formation based on the Landau-Ginzburg-Devonshire theory and the Gauss random field concept assuming the predominance of the quenched polarization disorder over the thermal fluctuations. The system of integrodifferential equations for correlation functions of random polarization and electric field turns out to be analytically solvable. The model provides explanations to a range of experimental results on the polarization formation kinetics including the time-dependent correlation lengths and correlation functions on the macroscopic spatial and time scales. Notably, it predicts the dependence of the ferroelectric coercive field on the initial disordered state characteristics, which can be controlled by quenching parameters like the initial temperature and the cooling rate, thus paving the way for tailoring the functional properties of the material.

cond-mat.mtrl-sci

Universal kinetics of the stochastic formation of polarization domain structures in a uniaxial single-crystal ferroelectric

Initial conditions after quenching from a high-temperature paraelectric phase to a low-temperature ferroelectric phase have a substantial impact on the temporal development and formation of stable polarization domain structures which eventually determine physical properties and the functionality of ferroelectrics. Based on the recently advanced exactly solvable model of the stochastic domain structure kinetics in a uniaxial ferroelectric [Phys. Rev. B 107, 144109 (2023)], we study the effect of the magnitude of the initial disorder, its initial correlation length and polarization correlation function on the system evolution. For different shapes of the initial correlation function, the time-dependent correlation length and the two-point polarization correlation coefficient are calculated analytically demonstrating universal features and a good agreement with the available experimental data. Particularly, the magnitude of the charge density correlation function reveals a strong reduction of the bound charges at the nominally charged domain walls which was recently observed experimentally in uniaxial ferroelectrics. Consequently, the integrodifferential equations of evolution for the polarization correlation function and the mean polarization are numerically solved for different initial conditions. The temporal dependence of the polarization mean value and variance are evaluated demonstrating the bifurcation behavior depending on the applied electric field. The impact of the initial state properties on the coercive field deciding between the single- and multi-domain final states of the system is disclosed.

cond-mat.mtrl-sci

On the statistical correlations of random polarization and electric depolarization fields in ferroelectrics

A conceptual problem of the electric-field mediated polarization correlations during a stochastic formation of polarization domain structure in ferroelectrics is addressed by using an exactly solvable stochastic model of polarization development in a uniaxial ferroelectric [Phys. Rev. B 107, 144109 (2023)]. A full set of time-dependent two-point correlation coefficients between all random variables is derived analytically, evaluated numerically and presented graphically in 3D. They are particularly required for the analysis of nonlinear phenomena involving spatial dispersion like optical second harmonic generation and scattering.

cond-mat.mtrl-sci

Stochastic theory of ferroelectric domain structure formation dominated by quenched disorder

A self-consistent stochastic model of domain structure formation in a uniaxial ferroelectric, quenched from a high-temperature paraelectric phase to a low-temperature ferroelectric phase, is developed with an account of the applied electric field and the feedback effect via local depolarization fields. Both polarization and field components are considered as Gauss random variables. A system of integro-differential equations for correlation functions of all involved variables is derived and solved analytically and numerically. Phase diagram in terms of the average value and dispersion of polarization reveals different possible equilibrium states and available final single-domain and multi-domain states. The time-dependent evolution of the average polarization and dispersion discloses a bifurcation behavior and the temperature-dependent value of the electric field, deciding between the single-domain and multi-domain final states, which can be interpreted as the coercive field. Analytical and numerical results for the time-dependent correlation length and correlation functions exhibit plausible agreement with available experimental data.

cond-mat.mtrl-sci

Dynamic scaling properties of multistep polarization response in ferroelectrics

Ferroelectrics are multifunctional smart materials finding applications in sensor technology, micromechanical actuation, digital information storage etc. Their most fundamental property is the ability of polarization switching under applied electric field. In particular, understanding of switching kinetics is essential for digital information storage. In this regard, scaling properties of the temporal polarization response are well-known for 180°-switching processes in ferroelectrics characterized by a unique field-dependent local switching time. Unexpectedly, these properties were now observed in multiaxial polycrystalline ferroelectrics, exhibiting a number of parallel and sequential non-180°-switching processes with distinct switching times. This behaviour can be explained by a combination of the multistep stochastic mechanism and the inhomogeneous field mechanism models of polarization reversal. Scaling properties are predicted for polycrystalline ferroelectrics of tetragonal, rhombohedral and orthorhombic symmetries and exemplarily demonstrated by measurements of polarization kinetics in (K,Na)NbO3-based ferroelectric ceramic over a timescale of 7 orders of magnitude. Dynamic scaling properties allow insight into the microscopic switching mechanisms, on the one hand, and into statistical material characteristics, on the other hand, providing thereby the description of temporal polarization with high accuracy. The gained deeper insight into the mechanisms of multistep polarization switching is crucial for future ultrafast and multilevel digital information storage.

cond-mat.mtrl-sci

Multi-step stochastic mechanism of polarization reversal in orthorhombic ferroelectrics

A stochastic model of electric field-driven polarization reversal in orthorhombic ferroelectrics is advanced, providing a description of their temporal electromechanical response. The theory accounts for all possible parallel and sequential switching events. Application of the model to the simultaneous measurements of polarization and strain kinetics in a lead-free orthorhombic (K,Na)NbO3-based ferroelectric ceramic over a wide timescale of 7 orders of magnitude allowed identification of preferable polarization switching paths, fractions of individual switching processes, and their activation fields. Particularly, the analysis revealed substantial contributions of coherent non-180° switching events, which do not cause macroscopic strain and thus mimic 180° switching processes.

cond-mat.mtrl-sci

Mesoscopic theory of defect ordering-disordering transitions in thin oxide films

Ordering of mobile defects in functional materials can give rise to fundamentally new phases possessing ferroic and multiferroic functionalities. Here we develop the Landau theory for strain induced ordering of defects (e.g. oxygen vacancies) in thin oxide films, considering both the ordering and wavelength of possible instabilities. Using derived analytical expressions for the energies of various defect-ordered states, we calculated and analyzed phase diagrams dependence on the film-substrate mismatch strain, concentration of defects, and Vegard coefficients. Obtained results open possibilities to create and control superstructures of ordered defects in thin oxide films by selecting the appropriate substrate and defect concentration.

cond-mat.mtrl-sci

Multi-step stochastic mechanism of polarization reversal in rhombohedral ferroelectrics

A stochastic model for the field-driven polarization reversal in rhombohedral ferroelectrics is developed, providing a description of their temporal electromechanical response. Application of the model to simultaneous measurements of polarization and strain kinetics in a rhombohedral Pb(Zr,Ti)O3 ceramic over a wide time window allows identification of preferable switching paths, fractions of individual switching processes, and their activation fields. Complementary, the phenomenological Landau-Ginzburg-Devenshire theory is used to analyze the impact of external field and stress on switching barriers showing that residual mechanical stress may promote the fast switching.

cond-mat.mtrl-sci

Hierarchy of Domain Reconstruction Processes due to Charged Defect Migration in Acceptor Doped Ferroelectrics

Evolution of a stripe array of polarization domains triggered by the oxygen vacancy migration in an acceptor doped ferroelectric is investigated in a self-consistent manner. A comprehensive model based on the Landau-Ginzburg-Devonshire approach includes semiconductor features due to the presence of electrons and holes, and effects of electrostriction and flexoelectricity especially significant near the free surface and domain walls. A domain array spontaneously formed in the absence of an external field is shown to undergo a reconstruction in the course of the gradual oxygen vacancy migration driven by the depolarization fields. The charge defect accumulation near the free ferroelectric surface causes a series of phenomena: (i) symmetry breaking between the positive and negative c-domains, (ii) appearance of an effective dipole layer at the free surface followed by the formation of a surface electrostatic potential, (iii) tilting and recharging of the domain walls, especially pronounced at higher acceptor concentrations. An internal bias field determined by the gain in the free energy of the structure exhibits dependences of its amplitude on time and dopant concentration well comparable with available experimental results on aging in BaTiO3.

cond-mat.mtrl-sci

Stochastic model of dispersive multi-step polarization switching in ferroelectrics due to spatial electric field distribution

A stochastic model for polarization switching in tetragonal ferroelectric ceramics is introduced, which includes sequential 90°- and parallel 180°-switching processes and accounts for the dispersion of characteristic switching times due to a nonuniform spatial distribution of the applied field. It presents merging of the recent multistep stochastic mechanism (MSM) with the earlier nucleation limited switching (NLS) and inhomogeneous field mechanism (IFM) models. The new model provides a much better description of simultaneous polarization and strain responses over a wide time window and a deeper insight into the microscopic switching mechanisms, as is exemplarily shown by comparison with measurements on lead zirconate titanate.

cond-mat.mtrl-sci

Defect driven flexo-chemical coupling in thin ferroelectric films

Using Landau-Ginzburg-Devonshire theory, we considered the impact of the flexoelectro-chemical coupling on the size effects inpolar properties and phase transitions of thin ferroelectric films with a layer of elastic defects. We investigated a typical case, when defects fill a thin layer below the top film surface with a constant concentration creating an additional gradient of elastic fields. The defective surface of the film is not covered with an electrode, but instead with an ultra-thin layer of ambient screening charges, characterized by a surface screening length. This geometry is typical for the scanning probe piezoelectric force microscopy. Obtained results revealed an unexpectedly strong effect of the joint action of Vegard stresses and flexoelectric effect (shortly flexo-chemical coupling) on the ferroelectric transition temperature, distribution of the spontaneous polarization and elastic fields, domain wall structure and period in thin PbTiO3 films containing a layer of elastic defects. A nontrivial result is the ferroelectricity persisting at film thicknesses below 4 nm, temperatures lower than 350 K and relatively high surface screening length (~0.1 nm). The origin of this phenomenon is the re-building of the domain structure in the film (namely the cross-over from c-domain stripes to a-type closure domains) when its thickness decreases below 4 nm, conditioned by the flexoelectric coupling and facilitated by negative Vegard effect. For positive Vegard effect, thicker films exhibit the appearance of pronounced maxima on the thickness dependence of the transition temperature, whose position and height can be controlled by the defect type and concentration. The revealed features may have important implications for miniaturization of ferroelectric-based devices.

cond-mat.mtrl-sci

Tuning the Polar States of Ferroelectric Films via Surface Charges and Flexoelectricity

Using the self-consistent Landau-Ginzburg-Devonshire approach we simulate and analyze the spontaneous formation of the domain structure in thin ferroelectric films covered with the surface screening charge of the specific nature (Bardeen-type surface states). Hence we consider the competition between the screening and the domain formation as alternative ways to reduce the electrostatic energy and reveal unusual peculiarities of distributions of polarization, electric and elastic fields conditioned by the surface screening length and the flexocoupling strength. We have established that the critical thickness of the film and its transition temperature to a paraelectric phase strongly depend on the Bardeen screening length, while the flexocoupling affects the polarization rotation and closure domain structure and induces ribbon-like nano-scale domains in the film depth far from the top open surface. Hence the joint action of the surface screening (originating from e.g. the adsorption of ambient ions or surface states) and flexocoupling may remarkably modify polar and electromechanical properties of thin ferroelectric films.

cond-mat.mtrl-sci

Flexocoupling impact on the kinetics of polarization reversal

The impact of flexoelectric coupling on polarization reversal kinetics and space charge dynamics in thin films of ferroelectric-semiconductors has been theoretically studied. The relaxation-type Landau-Khalatnikov equation together with the Poisson equation and the theory of elasticity equations have been used to calculate in a self-consistent way the spatial-temporal development of ferroelectric polarization, electric potential and space charge, elastic stresses, strains and their gradients. The analysis of the obtained results reveals a moderate increase of the flexocoupling influence on the polarization, elastic strain, electric potential and space charge distribution dynamics with the decrease of a ferroelectric film thickness. In contrast, the dependence of polarization switching time on the applied electric field is strongly affected by the flexocoupling strength. The polarization reversal process consists typically of two stages, the first stage has no characteristic time, while the second one exhibits a switching time strongly dependent on the applied electric field.

cond-mat.mtrl-sci

Flexocoupling impact on the size effects of piezo- response and conductance in mixed-type ferroelectrics-semiconductors under applied pressure

Flexocoupling impact on the size effects of the spontaneous polarization, effective piezo-response, elastic strain and compliance, carrier concentration and piezo-conductance have been calculated in thin films of ferroelectric semiconductors with mixed-type conductivity under applied pressure. Analysis of the self-consistent calculation results revealed that the thickness dependences of aforementioned physical quantities, calculated at zero and nonzero flexoelectric couplings, are very similar under zero applied pressure, but become strongly different under the application of external pressure pext. At that the differences become noticeably stronger for the film surface under compression than under tension. The impact of the Vegard mechanism on the size effects is weaker in comparison with flexocoupling except for the thickness dependence of the piezo-conductance. Without flexoelectric coupling the studied physical quantities manifest conventional peculiarities that are characteristic of the size-induced phase transitions. Namely, when the film thickness h approaches the critical thickness hcr the transition to paraelectric phase occurs. The combined effect of flexoelectric coupling and external pressure induces polarizations at the film surfaces, which cause the electric built-in field that destroys the thickness-induced phase transition to paraelectric phase at h= hcr and induces the electret-like state with irreversible spontaneous polarization at h<hcr. The built-in field leads to noticeable increase of the average strain and elastic compliance under the film thickness decrease below hcr that scales as 1/h at small thicknesses h. The changes of the electron concentration by several orders of magnitude under positive or negative pressures can lead to the occurrence of high- or low-conductivity states, i.e. the nonvolatile piezo-resistive switching.

cond-mat.mtrl-sci

Surface potential at a ferroelectric grain due to asymmetric screening of depolarization fields

Nonlinear screening of electric depolarization fields, generated by a stripe domain structure in a ferroelectric grain of a polycrystalline material, is studied within a semiconductor model of ferroelectrics. It is shown that the maximum strength of local depolarization fields is rather determined by the electronic band gap than by the spontaneous polarization magnitude. Furthermore, field screening due to electronic band bending and due to presence of intrinsic defects leads to asymmetric space charge regions near the grain boundary, which produce an effective dipole layer at the surface of the grain. This results in the formation of a potential difference between the grain surface and its interior of the order of 1 V, which can be of either sign depending on defect transition levels and concentrations. Exemplary acceptor doping of BaTiO3 is shown to allow tuning of the said surface potential in the region between 0.1 and 1.3 V.

cond-mat.mtrl-sci

Space-charge mechanism of aging in ferroelectrics: an exactly solvable two-dimensional model

A mechanism of point defect migration triggered by local depolarization fields is shown to explain some still inexplicable features of aging in acceptor doped ferroelectrics. A drift-diffusion model of the coupled charged defect transport and electrostatic field relaxation within a two-dimensional domain configuration is treated numerically and analytically. Numerical results are given for the emerging internal bias field of about 1 kV/mm which levels off at dopant concentrations well below 1 mol%; the fact, long ago known experimentally but still not explained. For higher defect concentrations a closed solution of the model equations in the drift approximation as well as an explicit formula for the internal bias field is derived revealing the plausible time, temperature and concentration dependencies of aging. The results are compared to those due to the mechanism of orientational reordering of defect dipoles.

cond-mat.mtrl-sci

Drift of charged defects in local fields as aging mechanism in ferroelectrics

Point defect migration is considered as a mechanism for aging in ferroelectrics. Numerical results are given for the coupled problems of point defect migration and electrostatic energy relaxation in a 2D domain configuration. The peak values of the clamping pressure at domain walls are in the range of $10^6$ Pa, which corresponds to macroscopically observed coercive stresses in perovskite ferroelectrics. The effect is compared to mechanisms involving orientational reordering of defect dipoles in the bulk of domains. Domain clamping is significantly stronger in the drift mechanism than in the orientational picture for the same material parameters.

cond-mat.mtrl-sci